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'''Thorium''' is a [[chemical element]]; it has [[chemical symbol|symbol]] '''Th''' and [[atomic number]] 90. Thorium is a weakly [[radioactive]] light silver metal which [[tarnish]]es olive grey when it is exposed to air, forming [[thorium dioxide]]; it is moderately soft, [[malleable]], and has a high [[melting point]]. Thorium is an electropositive [[actinide]] whose chemistry is dominated by the +4 [[oxidation state]]; it is quite reactive and can ignite in air when finely divided.

All known thorium [[isotope]]s are unstable. The most stable isotope, [[thorium-232|<sup>232</sup>Th]], has a [[half-life]] of 14.05&nbsp;billion years, or about the [[age of the universe]]; it decays very slowly via [[alpha decay]], starting a [[decay chain]] named the [[thorium series]] that ends at stable <sup>208</sup>[[Lead|Pb]]. On Earth, thorium and [[uranium]] are the only elements with no stable or nearly-stable isotopes that still occur naturally in large quantities as [[primordial nuclide|primordial elements]].{{efn|[[Bismuth]] is very slightly radioactive, but its half-life (1.9{{e|19}} years) is so long that its decay is negligible even over geological timespans.}} Thorium is estimated to be over three times as [[natural abundance|abundant]] as uranium in the Earth's crust, and is chiefly refined from [[monazite]] sands as a by-product of extracting [[Rare-earth element|rare-earth elements]].

Thorium was discovered in 1828 by the Swedish chemist [[Jöns Jacob Berzelius]], who named it after [[Thor]], the [[list of Germanic deities|Norse god]] of thunder and war. Its first applications were developed in the late 19th century. Thorium's radioactivity was widely acknowledged during the first decades of the 20th century. In the second half of the 20th century, thorium was replaced in many uses due to concerns about its radioactivity properties.

Thorium is still used as an alloying element in [[TIG welding]] electrodes but is slowly being replaced in the field with different compositions. It was also material in high-end optics and scientific instrumentation, used in some broadcast vacuum tubes, and as the light source in [[gas mantle]]s, but these uses have become marginal. It has been suggested as a replacement for uranium as nuclear fuel in [[nuclear reactor]]s, and several [[thorium fuel cycle#Thorium-fueled reactors|thorium reactors]] have been built. Thorium is also used in strengthening [[magnesium]], coating [[tungsten]] wire in electrical and welding equipment, controlling the grain size of tungsten in [[electric lamps]], high-temperature crucibles, and glasses including camera and scientific instrument lenses. Other uses for thorium include heat-resistant ceramics, [[aircraft engines]], and in [[light bulbs]]. Ocean science has utilised <sup>231</sup>[[Protactinium|Pa]]/<sup>230</sup>Th isotope ratios to understand the ancient ocean.<ref>{{cite journal | last1 = Negre | first1 = César | display-authors = etal | year = 2010| title = Reversed flow of Atlantic deep water during the Last Glacial Maximum | url = https://www.pure.ed.ac.uk/ws/files/11751410/ReversedATlantic_Deep.pdf| journal = Nature | volume = 468 | issue = 7320| pages = 84–8 | doi = 10.1038/nature09508 | pmid = 21048764 | bibcode = 2010Natur.468...84N }}</ref>

==Bulk properties==
Thorium is a moderately soft, [[paramagnetism|paramagnetic]], bright silvery radioactive actinide metal that can be bent or shaped. In the [[periodic table]], it lies to the right of [[actinium]], to the left of [[protactinium]], and below [[cerium]]. Pure thorium is very [[Ductility|ductile]] and, as normal for metals, can be [[cold-rolling#Cold rolling|cold-rolled]], [[Swaging|swaged]], and [[Drawing (manufacturing)|drawn]].{{sfn|Wickleder|Fourest|Dorhout|2006|pp=61–63}} At room temperature, thorium metal has a [[Cubic crystal system|face-centred cubic]] crystal structure; it has two other forms, one at high temperature (over 1360&nbsp;°C; body-centred cubic) and one at high pressure (around 100&nbsp;GPa; [[tetragonal crystal system|body-centred tetragonal]]).{{sfn|Wickleder|Fourest|Dorhout|2006|pp=61–63}}

Thorium metal has a [[bulk modulus]] (a measure of resistance to compression of a material) of 54&nbsp;[[gigapascal|GPa]], about the same as [[tin]]'s (58.2&nbsp;GPa). [[Aluminium]]'s is 75.2&nbsp;GPa; copper's 137.8&nbsp;GPa; and mild steel's is 160–169&nbsp;GPa.<ref>{{cite book |last1=Gale |first1=W. F. |last2=Totemeier |first2=T. C. |title=Smithells Metals Reference Book |year=2003 |publisher=[[Butterworth-Heinemann]] |isbn=978-0-08-048096-1 |language=en |pages=15-2–15-3}}</ref> Thorium is about as hard as soft [[steel]], so when heated it can be rolled into sheets and pulled into wire.<ref name="Yu. D. Tretyakov">{{cite book|editor-first=Yu. D. |editor-last=Tretyakov|title= Non-organic chemistry in three volumes| publisher= Academy|date= 2007|volume= 3|series= Chemistry of transition elements|isbn= 978-5-7695-2533-9}}</ref>

Thorium is nearly half as dense as [[uranium]] and [[plutonium]] and is harder than both.<ref name="Yu. D. Tretyakov" /> It becomes [[superconductor|superconductive]] below 1.4&nbsp;[[kelvin|K]].{{sfn|Wickleder|Fourest|Dorhout|2006|pp=61–63}} Thorium's [[melting point]] of 1750&nbsp;°C is above both those of actinium (1227&nbsp;°C) and protactinium (1568&nbsp;°C). At the start of [[period 7 element|period 7]], from [[francium]] to thorium, the melting points of the elements increase (as in other periods), because the number of delocalised electrons each atom contributes increases from one in francium to four in thorium, leading to greater attraction between these electrons and the metal ions as their charge increases from one to four. After thorium, there is a new downward trend in melting points from thorium to [[plutonium]], where the number of f-electrons increases from about 0.4 to about 6: this trend is due to the increasing hybridisation of the 5f and 6d orbitals and the formation of directional bonds resulting in more complex crystal structures and weakened metallic bonding.<ref name="Yu. D. Tretyakov" /><ref name="Johansson" /> (The f-electron count for thorium metal is a non-integer due to a 5f–6d overlap.)<ref name="Johansson">{{cite journal |last1=Johansson |first1=B. |last2=Abuja |first2=R. |last3=Eriksson |first3=O. |last4=Wills |first4=J. M. |display-authors=3 |year=1995 |title=Anomalous fcc crystal structure of thorium metal. |journal=Physical Review Letters |volume=75 |issue=2 |pages=280–283 |doi=10.1103/PhysRevLett.75.280 |pmid=10059654 |bibcode=1995PhRvL..75..280J |url=https://zenodo.org/record/1233903 |archive-date=8 March 2023 |access-date=24 August 2019 |archive-url=https://web.archive.org/web/20230308191836/https://zenodo.org/record/1233903 |url-status=live }}</ref> Among the actinides up to [[californium]], which can be studied in at least milligram quantities, thorium has the highest melting and boiling points and second-lowest density; only actinium is lighter. Thorium's boiling point of 4788&nbsp;°C is the fifth-highest among all the elements with known boiling points.{{efn|Behind [[osmium]], [[tantalum]], [[tungsten]], and [[rhenium]];{{sfn|Wickleder|Fourest|Dorhout|2006|pp=61–63}} higher boiling points are speculated to be found in the 6d transition metals, but they have not been produced in large enough quantities to test this prediction.{{Fricke1975}}}}

The properties of thorium vary widely depending on the degree of impurities in the sample. The major impurity is usually [[thorium dioxide]] {{chem2|ThO2}}); even the purest thorium specimens usually contain about a tenth of a per cent of the dioxide.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=61–63}} Experimental measurements of its density give values between 11.5&nbsp;and&nbsp;11.66&nbsp;g/cm<sup>3</sup>: these are slightly lower than the theoretically expected value of 11.7&nbsp;g/cm<sup>3</sup> calculated from thorium's [[lattice parameter]]s, perhaps due to microscopic voids forming in the metal when it is cast.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=61–63}} These values lie between those of its neighbours actinium (10.1&nbsp;g/cm<sup>3</sup>) and protactinium (15.4&nbsp;g/cm<sup>3</sup>), part of a trend across the early actinides.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=61–63}}

Thorium can form [[alloy]]s with many other metals. Addition of small proportions of thorium improves the mechanical strength of [[magnesium]], and thorium-aluminium alloys have been considered as a way to store thorium in proposed future thorium nuclear reactors. Thorium forms [[eutectic mixture]]s with [[chromium]] and uranium, and it is completely [[miscibility|miscible]] in both solid and liquid [[state of matter|states]] with its lighter [[congener (chemistry)|congener]] cerium.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=61–63}}

==Isotopes==
{{Main|Isotopes of thorium}}
There are seven naturally occurring isotopes of Thorium but none are stable. <sup>232</sup>Th is one of the two nuclides beyond bismuth (the other being [[uranium-238|<sup>238</sup>U]]) that have half-lives measured in billions of years; its half-life is 14.05&nbsp;billion&nbsp;years, about three times the [[age of the Earth]], and slightly longer than the [[age of the universe]]. Four-fifths of the thorium present at Earth's formation has survived to the present.<ref name="NUBASE">{{cite journal |last1=Audi |first1=G. |last2=Bersillon |first2=O. |last3=Blachot |first3=J. |last4=Wapstra |first4=A.H. |title=The Nubase evaluation of nuclear and decay properties |journal=Nuclear Physics A |date=December 2003 |volume=729 |issue=1 |pages=3–128 |doi=10.1016/j.nuclphysa.2003.11.001 |bibcode=2003NuPhA.729....3A |url=http://hal.in2p3.fr/in2p3-00014184/file/democrite-00014184.pdf |archive-date=16 April 2023 |access-date=10 October 2021 |archive-url=https://web.archive.org/web/20230416024455/http://hal.in2p3.fr/in2p3-00014184/file/democrite-00014184.pdf |url-status=live }}</ref><ref>{{CIAAW2003}}</ref><ref>{{cite journal |last1=Wieser |first1=M. E. |title=Atomic weights of the elements 2005 (IUPAC Technical Report) |journal=Pure and Applied Chemistry |date=1 January 2006 |volume=78 |issue=11 |pages=2051–2066 |doi=10.1351/pac200678112051 |s2cid=94552853 |doi-access=free }}</ref> <sup>232</sup>Th is the only isotope of thorium occurring in quantity in nature.<ref name="NUBASE" /> Its stability is attributed to its closed [[nuclear shell|nuclear subshell]] with 142 neutrons.<ref>{{cite book |last=Nagy |first=S. |date=2009 |title=Radiochemistry and Nuclear Chemistry |volume=2 |publisher=EOLSS Publications |page=374 |isbn=978-1-84826-127-3}}</ref><ref>{{cite book |last=Griffin |first=H. C. |date=2010 |title=Handbook of Nuclear Chemistry |publisher=[[Springer Science+Business Media]] |page=668 |isbn=978-1-4419-0719-6 |editor1-last=Vértes |editor1-first=A. |editor2-last=Nagy |editor2-first=S. |editor3-last=Klencsár |editor3-first=Z. |editor4-last=Lovas |editor4-first=R. G. |editor5-last=Rösch |editor5-first=F. |display-editors=3 |chapter=Natural Radioactive Decay Chains}}</ref> Thorium has a characteristic terrestrial isotopic composition, with [[standard atomic weight|atomic weight]] {{val|232.0377|0.0004}}.{{CIAAW2021}} It is one of only four radioactive elements (along with bismuth, protactinium and uranium) that occur in large enough quantities on Earth for a standard atomic weight to be determined.{{CIAAW2021}}

Thorium nuclei are susceptible to [[alpha decay]] because the strong nuclear force cannot overcome the electromagnetic repulsion between their protons.<ref name="beiser">{{cite book|title=Concepts of Modern Physics|chapter-url=http://phy240.ahepl.org/Concepts_of_Modern_Physics_by_Beiser.pdf|year=2003|publisher=[[McGraw-Hill Education]]|isbn=978-0-07-244848-1|chapter=Nuclear Transformations|pages=432–434|edition=6|first=A.|last=Beiser|access-date=4 July 2016|archive-date=4 October 2016|archive-url=https://web.archive.org/web/20161004204701/http://phy240.ahepl.org/Concepts_of_Modern_Physics_by_Beiser.pdf|url-status=dead}}</ref> The alpha decay of <sup>232</sup>Th initiates the 4''n'' [[decay chain]] which includes isotopes with a [[mass number]] divisible by 4 (hence the name; it is also called the thorium series after its progenitor). This chain of consecutive alpha and [[beta decay]]s begins with the decay of <sup>232</sup>Th to <sup>228</sup>Ra and terminates at <sup>208</sup>Pb.<ref name="NUBASE" /> Any sample of thorium or its compounds contains traces of these daughters, which are isotopes of [[thallium]], [[lead]], bismuth, polonium, [[radon]], [[radium]], and actinium.<ref name="NUBASE" /> Natural thorium samples can be chemically purified to extract useful daughter nuclides, such as <sup>212</sup>Pb, which is used in [[nuclear medicine]] for [[cancer therapy]].<ref>{{cite press release |url=http://us.areva.com/EN/home-2564/areva-inc-areva-med-launches-production-of-lead212-at-new-facility.html |title=AREVA Med launches production of lead-212 at new facility |publisher=[[Areva]] |year=2013 |access-date=1 January 2017 |archive-date=19 December 2020 |archive-url=https://web.archive.org/web/20201219225628/http://us.areva.com/EN/home-2564/areva-inc-areva-med-launches-production-of-lead212-at-new-facility.html |url-status=dead }}</ref><ref>{{cite web|url=http://minerals.usgs.gov/minerals/pubs/commodity/thorium/myb1-2011-thori.pdf|title=Mineral Yearbook 2012|publisher=[[United States Geological Survey]]|access-date=30 September 2017|archive-date=11 April 2013|archive-url=https://web.archive.org/web/20130411121452/http://minerals.usgs.gov/minerals/pubs/commodity/thorium/myb1-2011-thori.pdf|url-status=live}}</ref> <sup>227</sup>Th (alpha emitter with an 18.68 days half-life) can also be used in cancer treatments such as [[Targeted alpha-particle therapy|targeted alpha therapies]].<ref>{{cite journal |last1=Ramdahl |first1=Thomas |last2=Bonge-Hansen |first2=Hanne T. |last3=Ryan |first3=Olav B. |last4=Larsen |first4=Åsmund |last5=Herstad |first5=Gunnar |last6=Sandberg |first6=Marcel |last7=Bjerke |first7=Roger M. |last8=Grant |first8=Derek |last9=Brevik |first9=Ellen M. |last10=Cuthbertson |first10=Alan S. |title=An efficient chelator for complexation of thorium-227 |journal=Bioorganic & Medicinal Chemistry Letters |date=September 2016 |volume=26 |issue=17 |pages=4318–4321 |doi=10.1016/j.bmcl.2016.07.034 |pmid=27476138 |doi-access=free }}</ref><ref>{{cite journal |last1=Deblonde |first1=Gauthier J.-P. |last2=Lohrey |first2=Trevor D. |last3=Booth |first3=Corwin H. |last4=Carter |first4=Korey P. |last5=Parker |first5=Bernard F. |last6=Larsen |first6=Åsmund |last7=Smeets |first7=Roger |last8=Ryan |first8=Olav B. |last9=Cuthbertson |first9=Alan S. |last10=Abergel |first10=Rebecca J. |title=Solution Thermodynamics and Kinetics of Metal Complexation with a Hydroxypyridinone Chelator Designed for Thorium-227 Targeted Alpha Therapy |journal=Inorganic Chemistry |date=19 November 2018 |volume=57 |issue=22 |pages=14337–14346 |doi=10.1021/acs.inorgchem.8b02430 |pmid=30372069 |osti=1510758 |s2cid=53115264 |url=https://escholarship.org/uc/item/7nz4j81s |archive-date=11 May 2021 |access-date=3 February 2019 |archive-url=https://web.archive.org/web/20210511140331/https://escholarship.org/uc/item/7nz4j81s |url-status=live }}</ref><ref>{{cite journal |last1=Captain |first1=Ilya |last2=Deblonde |first2=Gauthier J.-P. |last3=Rupert |first3=Peter B. |last4=An |first4=Dahlia D. |last5=Illy |first5=Marie-Claire |last6=Rostan |first6=Emeline |last7=Ralston |first7=Corie Y. |last8=Strong |first8=Roland K. |last9=Abergel |first9=Rebecca J. |title=Engineered Recognition of Tetravalent Zirconium and Thorium by Chelator–Protein Systems: Toward Flexible Radiotherapy and Imaging Platforms |journal=Inorganic Chemistry |date=21 November 2016 |volume=55 |issue=22 |pages=11930–11936 |doi=10.1021/acs.inorgchem.6b02041 |pmid=27802058 |osti=1458481 |url=http://www.escholarship.org/uc/item/2nx8r6pz |archive-date=29 April 2021 |access-date=3 February 2019 |archive-url=https://web.archive.org/web/20210429025503/https://escholarship.org/uc/item/2nx8r6pz |url-status=live }}</ref> <sup>232</sup>Th also very occasionally undergoes [[spontaneous fission]] rather than alpha decay, and has left evidence of doing so in its minerals (as trapped [[xenon]] gas formed as a fission product), but the [[partial half-life]] of this process is very large at over 10<sup>21</sup>&nbsp;years and alpha decay predominates.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=53–55}}<ref>{{cite journal |last1=Bonetti |first1=R. |last2=Chiesa |first2=C. |first3=A. |last3=Guglielmetti |first4=R. |last4=Matheoud |first5=G. |last5=Poli |first6=V. L. |last6=Mikheev |first7=S. P. |last7=Tretyakova |display-authors=3 |year=1995 |title=First observation of spontaneous fission and search for cluster decay of <sup>232</sup>Th |journal=Physical Review C |volume=51 |issue=5 |pages=2530–2533 |doi=10.1103/PhysRevC.51.2530|pmid=9970335 |bibcode=1995PhRvC..51.2530B}}</ref>

[[File:Decay Chain Thorium.svg|thumb|upright=1.25|alt=Ball-and-arrow presentation of the thorium decay series|The 4''n'' [[decay chain]] of <sup>232</sup>Th, commonly called the "thorium series"]]
In total, 32 [[radioisotope]]s have been characterised, which range in mass number from 207<ref name="Yang207">{{cite journal |title=New isotope <sup>207</sup>Th and odd-even staggering in α-decay energies for nuclei with ''Z''&nbsp;>&nbsp;82 and ''N''&nbsp;<&nbsp;126 |last=Yang |first=H. B. |display-authors=et al. |journal=Physical Review C |year=2022 |volume=105 |number=L051302 |doi=10.1103/PhysRevC.105.L051302|bibcode=2022PhRvC.105e1302Y |s2cid=248935764 }}</ref> to 238.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=53–55}} After <sup>232</sup>Th, the most stable of them (with respective half-lives) are <sup>230</sup>Th (75,380&nbsp;years), <sup>229</sup>Th (7,917&nbsp;years), <sup>228</sup>Th (1.92&nbsp;years), <sup>234</sup>Th (24.10&nbsp;days), and <sup>227</sup>Th (18.68&nbsp;days). All of these isotopes occur in nature as [[trace radioisotope]]s due to their presence in the decay chains of <sup>232</sup>Th, <sup>235</sup>U, <sup>238</sup>U, and <sup>237</sup>[[neptunium|Np]]: the last of these is long [[extinct radionuclide|extinct]] in nature due to its short half-life (2.14&nbsp;million&nbsp;years), but is continually produced in minute traces from [[neutron capture]] in uranium ores. All of the remaining thorium isotopes have half-lives that are less than thirty days and the majority of these have half-lives that are less than ten minutes.<ref name="NUBASE" /> <sup>233</sup>Th (half-life 22&nbsp;minutes) occurs naturally as the result of [[neutron activation]] of natural <sup>232</sup>Th.<ref name=4n1>{{cite journal |last1=Peppard |first1=D. F. |last2=Mason |first2=G. W. |last3=Gray |first3=P. R. |last4=Mech |first4=J. F. |title=Occurrence of the (4n + 1) series in nature |journal=Journal of the American Chemical Society |date=1952 |volume=74 |issue=23 |pages=6081–6084 |doi=10.1021/ja01143a074 |bibcode=1952JAChS..74.6081P |url=https://digital.library.unt.edu/ark:/67531/metadc172698/m2/1/high_res_d/metadc172698.pdf |archive-url=https://web.archive.org/web/20190429182951/https://digital.library.unt.edu/ark:/67531/metadc172698/m2/1/high_res_d/metadc172698.pdf |archive-date=29 April 2019 |url-status=live }}</ref> <sup>226</sup>Th (half-life 31&nbsp;minutes) has not yet been observed in nature, but would be produced by the still-unobserved [[double beta decay]] of natural <sup>226</sup>Ra.<ref name="Tretyak2002">{{Cite journal
 |last1=Tretyak |first1=V.I. 
 |last2=Zdesenko |first2=Yu.G. 
 |year=2002
 |title=Tables of Double Beta Decay Data — An Update
 |journal=[[At. Data Nucl. Data Tables]] |volume=80 |issue=1 |pages=83–116
 |doi=10.1006/adnd.2001.0873
|bibcode=2002ADNDT..80...83T }}</ref>

In deep [[seawater]]s the isotope <sup>230</sup>Th makes up to {{val|0.02|u=%}} of natural thorium.{{NUBASE2020|ref}} This is because its parent <sup>238</sup>U is soluble in water, but <sup>230</sup>Th is insoluble and precipitates into the sediment. Uranium ores with low thorium concentrations can be purified to produce gram-sized thorium samples of which over a quarter is the <sup>230</sup>Th isotope, since <sup>230</sup>Th is one of the daughters of <sup>238</sup>U.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=53–55}} The [[International Union of Pure and Applied Chemistry]] (IUPAC) reclassified thorium as a binuclidic element in 2013; it had formerly been considered a [[mononuclidic element]].<ref>"[https://www.ciaaw.org/thorium.htm Standard Atomic Weights: Thorium] {{Webarchive|url=https://web.archive.org/web/20230325161627/https://www.ciaaw.org/thorium.htm |date=25 March 2023 }}". [[CIAAW]]. 2013.</ref>

Thorium has three known [[nuclear isomer]]s (or metastable states), <sup>216m1</sup>Th, <sup>216m2</sup>Th, and <sup>229m</sup>Th. <sup>229m</sup>Th has the lowest known excitation energy of any isomer,<ref name="Ruchowska">{{cite journal|last1=Ruchowska |first1=E. |title=Nuclear structure of <sup>229</sup>Th |journal=Physical Review C|volume= 73|pages=044326|date=2006 |doi=10.1103/PhysRevC.73.044326|issue=4 |bibcode= 2006PhRvC..73d4326R |display-authors=3 |last2=Płóciennik |first2=W. A. |last3=Żylicz |first3=J. |last4=Mach |last5=Kvasil |last6=Algora |last7=Amzal |last8=Bäck |last9=Borge |last10=Boutami |last11=Butler |last12=Cederkäll |last13=Cederwall |last14=Fogelberg |last15=Fraile |last16=Fynbo |last17=Hagebø |last18=Hoff |last19=Gausemel |last20=Jungclaus |last21=Kaczarowski |last22=Kerek |last23=Kurcewicz |last24=Lagergren |last25=Nacher |last26=Rubio |last27=Syntfeld |last28=Tengblad |last29=Wasilewski |last30=Weissman|url=https://cds.cern.ch/record/974608 |hdl=10261/12130 |hdl-access=free }}</ref> measured to be {{val|7.6|0.5|u=eV}}. This is so low that when it undergoes [[isomeric transition]], the emitted gamma radiation is in the [[ultraviolet]] range.<ref name="Beck">{{cite journal |last1=Beck |first1=B. R. |title=Energy splitting in the ground state doublet in the nucleus <sup>229</sup>Th |journal=[[Physical Review Letters]] |volume=98 |pages=142501 |date=2007 |doi=10.1103/PhysRevLett.98.142501 |pmid=17501268 |bibcode=2007PhRvL..98n2501B |issue=14 |display-authors=3 |last2=Becker |first2=J. A. |last3=Beiersdorfer |first3=P. |last4=Brown |last5=Moody |last6=Wilhelmy |last7=Porter |last8=Kilbourne |last9=Kelley |url=https://zenodo.org/record/1233955 |archive-date=13 April 2023 |access-date=24 August 2019 |archive-url=https://web.archive.org/web/20230413111140/https://zenodo.org/record/1233955 |url-status=live }}</ref><ref name="nuclear_clock">{{cite journal |journal=[[Nature (journal)|Nature]]
| volume=533 |issue=7601 |pages=47–51| year=2016| title= Direct detection of the <sup>229</sup>Th nuclear clock transition| first1=L. |last1=von der Wense| first2=B. |last2=Seiferle| first3=M. |last3=Laatiaoui| first4=Jürgen B. |last4=Neumayr| first5=Hans-Jörg |last5=Maier| first6=Hans-Friedrich |last6=Wirth| first7=Christoph |last7=Mokry| first8=Jörg |last8=Runke| first9=Klaus |last9=Eberhardt| first10=Christoph E. |last10=Düllmann| first11=Norbert G. |last11=Trautmann| first12=Peter G. |last12=Thirolf| display-authors=3| doi=10.1038/nature17669| pmid=27147026 | bibcode=2016Natur.533...47V|arxiv=1710.11398| s2cid=205248786 }}</ref>{{efn|Gamma rays are distinguished by their origin in the nucleus, not their wavelength; hence there is no lower limit to gamma energy derived from radioactive decay.<ref>{{cite book|last1=Feynman|first1=R.|author-link=Richard Feynman|last2=Leighton|first2=R.|last3=Sands|first3=M.|title=The Feynman Lectures on Physics|volume=1|publisher=[[Addison-Wesley]]|year=1963|pages=2–5|isbn=978-0-201-02116-5|url=https://feynmanlectures.caltech.edu/I_02.html|access-date=13 January 2018|archive-date=17 February 2021|archive-url=https://web.archive.org/web/20210217134956/https://www.feynmanlectures.caltech.edu/I_02.html|url-status=live}}</ref>}} The nuclear transition from <sup>229</sup>Th to <sup>229m</sup>Th is being investigated for a [[nuclear clock]].<ref name="nuclear_clock" />

Different isotopes of thorium are chemically identical, but have slightly differing physical properties: for example, the densities of pure <sup>228</sup>Th, <sup>229</sup>Th, <sup>230</sup>Th, and <sup>232</sup>Th are respectively expected to be 11.5, 11.6, 11.6, and 11.7&nbsp;g/cm<sup>3</sup>.<ref name="critical">{{cite web|publisher= [[Institut de radioprotection et de sûreté nucléaire]]|title= Evaluation of nuclear criticality safety data and limits for actinides in transport|page= 15|url= http://ec.europa.eu/energy/nuclear/transport/doc/irsn_sect03_146.pdf|access-date=20 December 2010 |archive-url=https://web.archive.org/web/20070710105629/http://ec.europa.eu/energy/nuclear/transport/doc/irsn_sect03_146.pdf |archive-date=10 July 2007}}</ref> The isotope <sup>229</sup>Th is expected to be [[fissionable]] with a bare [[critical mass]] of 2839&nbsp;kg, although with steel [[neutron reflector|reflectors]] this value could drop to 994&nbsp;kg.<ref name="critical" />{{efn|name="fissionable"|A ''fissionable'' nuclide is capable of undergoing fission (even with a low probability) after capturing a high-energy neutron. Some of these nuclides can be induced to fission with low-energy thermal neutrons with a high probability; they are referred to as ''fissile''. A ''fertile'' nuclide is one that could be bombarded with neutrons to produce a fissile nuclide. [[Critical mass]] is the mass of a ball of a material which could undergo a sustained [[nuclear chain reaction]].}} <sup>232</sup>Th is not fissionable, but it is [[fertile material|fertile]] as it can be converted to fissile [[uranium-233|<sup>233</sup>U]] by neutron capture and subsequent beta decay.<ref name="critical" />{{sfn|Wickleder|Fourest|Dorhout|2006|pp=52–53}}

===Radiometric dating===
Two radiometric dating methods involve thorium isotopes: [[uranium–thorium dating]], based on the decay of [[uranium-234|<sup>234</sup>U]] to <sup>230</sup>Th, and [[ionium–thorium dating]], which measures the ratio of <sup>232</sup>Th to <sup>230</sup>Th.{{efn|The name ''ionium'' for <sup>230</sup>Th is a remnant from a period when different isotopes were not recognised to be the same element and were given different names.}} These rely on the fact that <sup>232</sup>Th is a primordial radioisotope, but <sup>230</sup>Th only occurs as an intermediate decay product in the decay chain of <sup>238</sup>U.<ref name="uth" /> Uranium–thorium dating is a relatively short-range process because of the short half-lives of <sup>234</sup>U and <sup>230</sup>Th relative to the age of the Earth: it is also accompanied by a sister process involving the alpha decay of <sup>235</sup>U into <sup>231</sup>Th, which very quickly becomes the longer-lived <sup>231</sup>Pa, and this process is often used to check the results of uranium–thorium dating. Uranium–thorium dating is commonly used to determine the age of [[calcium carbonate]] materials such as [[speleothem]] or [[coral]], because uranium is more soluble in water than thorium and protactinium, which are selectively precipitated into ocean-floor [[sediment]]s, where their ratios are measured. The scheme has a range of several hundred thousand years.<ref name="uth">{{cite web |url=http://www3.nd.edu/~nsl/Lectures/phys178/pdf/chap3_6.pdf |title=3–6: Uranium Thorium Dating |publisher=Institute for Structure and Nuclear Astrophysics, [[University of Notre Dame]] |access-date=7 October 2017 |archive-date=21 April 2021 |archive-url=https://web.archive.org/web/20210421193422/https://www3.nd.edu/~nsl/Lectures/phys178/pdf/chap3_6.pdf |url-status=dead }}</ref><ref>{{cite web|last=Davis |first=O.|url=http://www.geo.arizona.edu/Antevs/ecol438/uthdating.html |title=Uranium-Thorium Dating |publisher=Department of Geosciences, [[University of Arizona]] |archive-date=28 March 2017 |archive-url=https://web.archive.org/web/20170328095352/http://www.geo.arizona.edu/Antevs/ecol438/uthdating.html |access-date=7 October 2017}}</ref> Ionium–thorium dating is a related process, which exploits the insolubility of thorium (both <sup>232</sup>Th and <sup>230</sup>Th) and thus its presence in ocean sediments to date these sediments by measuring the ratio of <sup>232</sup>Th to <sup>230</sup>Th.<ref name="rafferty2010">{{citation|title=Geochronology, Dating, and Precambrian Time: The Beginning of the World As We Know It|date=2010|last1=Rafferty|first1=J. P.|series=The Geologic History of Earth|page=150|publisher=[[Rosen Publishing]]|isbn=978-1-61530-125-6}}</ref><ref name="vertes2010">{{citation|title=Handbook of Nuclear Chemistry|date=2010|last1=Vértes|first1=A.|volume=5|page=800|edition=2nd|publisher=Springer Science+Business Media|editor1-last=Nagy|editor2-last=Klencsár|editor3-last=Lovas|editor4-last=Rösch|editor1-first=S.|editor2-first=Z.|editor3-first=R. G.|editor4-first=F.|display-editors=3|isbn=978-1-4419-0719-6}}</ref> Both of these dating methods assume that the proportion of <sup>230</sup>Th to <sup>232</sup>Th is a constant during the period when the sediment layer was formed, that the sediment did not already contain thorium before contributions from the decay of uranium, and that the thorium cannot migrate within the sediment layer.<ref name="rafferty2010" /><ref name="vertes2010" />

==Chemistry==
{{Main|Thorium compounds}}
A thorium atom has 90 electrons, of which four are [[valence electron]]s. Four [[atomic orbital]]s are theoretically available for the valence electrons to occupy: 5f, 6d, 7s, and 7p.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=59–60}} Despite thorium's position in the [[f-block]] of the periodic table, it has an anomalous [Rn]6d<sup>2</sup>7s<sup>2</sup> electron configuration in the ground state, as the 5f and 6d subshells in the early actinides are very close in energy, even more so than the 4f and 5d subshells of the lanthanides: thorium's 6d subshells are lower in energy than its 5f subshells, because its 5f subshells are not well-shielded by the filled 6s and 6p subshells and are destabilised. This is due to [[relativistic effects]], which become stronger near the bottom of the periodic table, specifically the relativistic [[spin–orbit interaction]]. The closeness in energy levels of the 5f, 6d, and 7s energy levels of thorium results in thorium almost always losing all four valence electrons and occurring in its highest possible oxidation state of +4. This is different from its lanthanide congener cerium, in which +4 is also the highest possible state, but +3 plays an important role and is more stable. Thorium is much more similar to the [[transition metal]]s zirconium and hafnium than to cerium in its ionization energies and redox potentials, and hence also in its chemistry: this transition-metal-like behaviour is the norm in the first half of the actinide series, from actinium to americium.<ref name="CottonSA2006" /><ref name="NIST">{{cite journal |last1=Martin |first1=W. C. |last2=Hagan |first2=Lucy |last3=Reader |first3=Joseph |last4=Sugar |first4=Jack |title=Ground Levels and Ionization Potentials for Lanthanide and Actinide Atoms and Ions |journal=Journal of Physical and Chemical Reference Data |date=July 1974 |volume=3 |issue=3 |pages=771–780 |doi=10.1063/1.3253147 |bibcode=1974JPCRD...3..771M }}</ref><ref name=johnson>{{cite book |last=Johnson |first=David |date=1984 |title=The Periodic Law |url=https://www.rsc.org/images/23_The_Periodic_Law_tcm18-30005.pdf |location= |publisher=The Royal Society of Chemistry |page= |isbn=0-85186-428-7 |archive-date=31 March 2022 |access-date=11 January 2024 |archive-url=https://web.archive.org/web/20220331224430/https://www.rsc.org/images/23_The_Periodic_Law_tcm18-30005.pdf |url-status=live }}</ref>

[[File:CaF2 polyhedra.png|thumb|alt=Crystal structure of fluorite|Thorium dioxide has the [[fluorite]] crystal structure. <br/> {{chem2|Th(4+)}}: <span style="color:silver; background:silver;">__</span>&nbsp;&nbsp;/&nbsp;&nbsp;{{chem2|O(2−)}}: <span style="color:#9c0; background:#9c0;">__</span>]]

Despite the anomalous electron configuration for gaseous thorium atoms, metallic thorium shows significant 5f involvement. A hypothetical metallic state of thorium that had the [Rn]6d<sup>2</sup>7s<sup>2</sup> configuration with the 5f orbitals above the [[Fermi level]] should be [[hexagonal close packed]] like the [[group 4 element]]s titanium, zirconium, and hafnium, and not face-centred cubic as it actually is. The actual crystal structure can only be explained when the 5f states are invoked, proving that thorium is metallurgically a true actinide.<ref name="Johansson" />

Tetravalent thorium compounds are usually colourless or yellow, like those of [[silver]] or lead, as the {{chem2|Th(4+)}} ion has no 5f or 6d electrons.<ref name="Yu. D. Tretyakov" /> Thorium chemistry is therefore largely that of an electropositive metal forming a single [[diamagnetic]] ion with a stable noble-gas configuration, indicating a similarity between thorium and the [[main group element]]s of the s-block.<ref name="King">{{cite book |last=King |first=R. Bruce |date=1995 |title=Inorganic Chemistry of Main Group Elements |publisher=[[Wiley-VCH]] |isbn=978-0-471-18602-1}}</ref>{{efn|Unlike the previous similarity between the actinides and the transition metals, the main-group similarity largely ends at thorium before being resumed in the second half of the actinide series, because of the growing contribution of the 5f orbitals to covalent bonding. The only other commonly-encountered actinide, uranium, retains some echoes of main-group behaviour. The chemistry of uranium is more complicated than that of thorium, but the two most common oxidation states of uranium are uranium(VI) and uranium(IV); these are two oxidation units apart, with the higher oxidation state corresponding to formal loss of all valence electrons, which is similar to the behaviour of the heavy main-group elements in the [[p-block]].<ref name="King" />}} Thorium and uranium are the most investigated of the radioactive elements because their radioactivity is low enough not to require special handling in the laboratory.{{sfn|Greenwood|Earnshaw|1997|p=1262}}

===Reactivity===
Thorium is a highly [[reactivity (chemistry)|reactive]] and electropositive metal. With a [[standard reduction potential]] of −1.90&nbsp;V for the {{chem2|Th(4+)}}/Th couple, it is somewhat more electropositive than zirconium or aluminium.{{sfn|Stoll|2005|p=6}} Finely divided thorium metal can exhibit [[pyrophoricity]], spontaneously igniting in air.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=61–63}} When heated in air, thorium [[swarf|turnings]] ignite and burn with a brilliant white light to produce the dioxide. In bulk, the reaction of pure thorium with air is slow, although corrosion may occur after several months; most thorium samples are contaminated with varying degrees of the dioxide, which greatly accelerates corrosion.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=61–63}} Such samples slowly tarnish, becoming grey and finally black at the surface.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=61–63}}

At [[standard temperature and pressure]], thorium is slowly attacked by water, but does not readily dissolve in most common acids, with the exception of [[hydrochloric acid]], where it dissolves leaving a black insoluble residue of ThO(OH,Cl)H.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=61–63}}<ref name="CRC">{{cite book| last= Hammond| first= C. R.| title= The Elements, in Handbook of Chemistry and Physics| edition= 81st| publisher= [[CRC Press]]| isbn= 978-0-8493-0485-9| date= 2004| url-access= registration| url= https://archive.org/details/crchandbookofche81lide}}</ref> It dissolves in concentrated [[nitric acid]] containing a small quantity of catalytic [[fluoride]] or [[fluorosilicate]] ions;{{sfn|Wickleder|Fourest|Dorhout|2006|pp=61–63}}<ref name="ekhyde">{{cite book|url= http://www.radiochemistry.org/periodictable/pdf_books/pdf/rc000034.pdf|author= Hyde, E. K.|title= The radiochemistry of thorium|publisher= [[National Academy of Sciences]]|date= 1960|access-date= 29 September 2017|archive-date= 5 March 2021|archive-url= https://web.archive.org/web/20210305130415/http://www.radiochemistry.org/periodictable/pdf_books/pdf/rc000034.pdf|url-status= dead}}</ref> if these are not present, [[passivation (chemistry)|passivation]] by the nitrate can occur, as with uranium and plutonium.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=61–63}}{{sfn|Greenwood|Earnshaw|1997|p=1264}}<ref>{{cite journal |last1=Moore |first1=Robert Lee |last2=Goodall |first2=C. A. |first3=J. L. |last3=Hepworth |first4=R. A. |last4=Watts |date=May 1957 |title=Nitric Acid Dissolution of Thorium. Kinetics of Fluoride-Catalyzed Reaction |journal=Industrial & Engineering Chemistry |volume=49 |issue=5 |pages=885–887 |doi=10.1021/ie50569a035}}</ref>

[[File:Kristallstruktur Uran(IV)-fluorid.png|thumb|alt=Crystal structure of thorium tetrafluoride|Crystal structure of thorium tetrafluoride<br/>{{chem2|Th(4+)}}: <span style="color:silver; background:silver;">__</span>&nbsp;&nbsp;/&nbsp;&nbsp;{{chem2|F−}}: <span style="color:#9c0; background:#9c0;">__</span>]]

===Inorganic compounds===
Most binary compounds of thorium with nonmetals may be prepared by heating the elements together.{{sfn|Greenwood|Earnshaw|1997|p=1267}} In air, thorium burns to form {{chem2|ThO2}}, which has the [[fluorite]] structure.<ref name="Yamashita">{{cite journal |title= Thermal expansions of NpO<sub>2</sub> and some other actinide dioxides |journal= J. Nucl. Mater. |volume= 245 |issue= 1 |date= 1997 |pages= 72–78 |last1= Yamashita |first1=Toshiyuki |last2= Nitani |first2=Noriko |last3= Tsuji |first3=Toshihide |last4= Inagaki |first4=Hironitsu| doi= 10.1016/S0022-3115(96)00750-7 |bibcode=1997JNuM..245...72Y}}</ref> Thorium dioxide is a [[refractory material]], with the highest melting point (3390&nbsp;°C) of any known oxide.<ref name="Emsley2011" /> It is somewhat [[hygroscopic]] and reacts readily with water and many gases;{{sfn|Wickleder|Fourest|Dorhout|2006|pp=70–77}} it dissolves easily in concentrated nitric acid in the presence of fluoride.{{sfn|Greenwood|Earnshaw|1997|p=1269}}

When heated in air, thorium dioxide emits intense blue light; the light becomes white when {{chem2|ThO2}} is mixed with its lighter homologue [[cerium dioxide]] ({{chem2|CeO2}}, ceria): this is the basis for its previously common application in [[gas mantle]]s.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=70–77}} A flame is not necessary for this effect: in 1901, it was discovered that a hot Welsbach gas mantle (using {{chem2|ThO2}} with 1% {{chem2|CeO2}}) remained at "full glow" when exposed to a cold unignited mixture of flammable gas{{which|date=November 2022}} and air.<ref name="Ivey" /> The light emitted by thorium dioxide is higher in wavelength than the [[blackbody]] emission expected from [[incandescence]] at the same temperature, an effect called [[candoluminescence]]. It occurs because {{chem2|ThO2}} : Ce acts as a catalyst for the recombination of [[free radical]]s that appear in high concentration in a flame, whose deexcitation releases large amounts of energy. The addition of 1% cerium dioxide, as in gas mantles, heightens the effect by increasing emissivity in the visible region of the spectrum; but because cerium, unlike thorium, can occur in multiple oxidation states, its charge and hence visible emissivity will depend on the region on the flame it is found in (as such regions vary in their chemical composition and hence how oxidising or reducing they are).<ref name="Ivey" />

Several binary thorium [[chalcogen]]ides and oxychalcogenides are also known with [[sulfur]], [[selenium]], and [[tellurium]].{{sfn|Wickleder|Fourest|Dorhout|2006|pp=95–97}}

All four thorium tetrahalides are known, as are some low-valent bromides and iodides:{{sfn|Wickleder|Fourest|Dorhout|2006|pp=78–94}} the tetrahalides are all 8-coordinated hygroscopic compounds that dissolve easily in polar solvents such as water.{{sfn|Greenwood|Earnshaw|1997|p=1271}} Many related polyhalide ions are also known.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=78–94}} Thorium tetrafluoride has a [[monoclinic crystal system|monoclinic]] crystal structure like those of [[zirconium tetrafluoride]] and [[hafnium tetrafluoride]], where the {{chem2|Th(4+)}} ions are coordinated with {{chem2|F−}} ions in somewhat distorted [[square antiprism]]s.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=78–94}} The other tetrahalides instead have dodecahedral geometry.{{sfn|Greenwood|Earnshaw|1997|p=1271}} Lower iodides {{chem2|ThI3}} (black) and {{chem2|ThI2}} (gold-coloured) can also be prepared by reducing the tetraiodide with thorium metal: they do not contain Th(III) and Th(II), but instead contain {{chem2|Th(4+)}} and could be more clearly formulated as [[electride]] compounds.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=78–94}} Many polynary halides with the alkali metals, [[barium]], thallium, and ammonium are known for thorium fluorides, chlorides, and bromides.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=78–94}} For example, when treated with [[potassium fluoride]] and [[hydrofluoric acid]], {{chem2|Th(4+)}} forms the complex anion {{chem2|[ThF6](2-)}} (hexafluorothorate(IV)), which precipitates as an insoluble salt, {{chem2|K2[ThF6]}} (potassium hexafluorothorate(IV)).<ref name="ekhyde" />

Thorium borides, carbides, silicides, and nitrides are refractory materials, like those of uranium and plutonium, and have thus received attention as possible [[nuclear fuel]]s.{{sfn|Greenwood|Earnshaw|1997|p=1267}} All four heavier [[pnictogen]]s ([[phosphorus]], [[arsenic]], [[antimony]], and bismuth) also form binary thorium compounds. Thorium germanides are also known.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=97–101}} Thorium reacts with hydrogen to form the thorium hydrides {{chem2|ThH2}} and {{chem2|Th4H15}}, the latter of which is superconducting below 7.5–8&nbsp;K; at standard temperature and pressure, it conducts electricity like a metal.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=64–66}} The hydrides are thermally unstable and readily decompose upon exposure to air or moisture.{{sfn|Greenwood|Earnshaw|1997|p=127}}

[[File:Uranocene-3D-balls.png|thumb|upright|alt=Structure of thorocene|Sandwich molecule structure of thorocene]]

===Coordination compounds===
In an acidic aqueous solution, thorium occurs as the tetrapositive [[aqua ion]] {{chem2|[Th(H2O)9](4+)}}, which has [[tricapped trigonal prismatic molecular geometry]]:{{sfn|Wickleder|Fourest|Dorhout|2006|pp=117–134}}<ref>{{cite journal |last=Persson |first=I. |date=2010 |title=Hydrated metal ions in aqueous solution: How regular are their structures? |journal=Pure and Applied Chemistry |volume=82 |issue=10 |pages=1901–1917 |doi=10.1351/PAC-CON-09-10-22|doi-access=free }}</ref> at pH&nbsp;&lt;&nbsp;3, the solutions of thorium salts are dominated by this cation.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=117–134}} The {{chem2|Th(4+)}} ion is the largest of the tetrapositive actinide ions, and depending on the coordination number can have a radius between 0.95&nbsp;and&nbsp;1.14&nbsp;Å.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=117–134}} It is quite acidic due to its high charge, slightly stronger than [[sulfurous acid]]: thus it tends to undergo hydrolysis and polymerisation (though to a lesser extent than {{chem2|[[iron|Fe]](3+)}}), predominantly to {{chem2|[Th2(OH)2](6+)}} in solutions with pH 3 or below, but in more alkaline solution polymerisation continues until the gelatinous hydroxide {{chem2|Th(OH)4}} forms and precipitates out (though equilibrium may take weeks to be reached, because the polymerisation usually slows down before the precipitation).{{sfn|Greenwood|Earnshaw|1997|pp=1275–1277}} As a [[HSAB theory|hard Lewis acid]], {{chem2|Th(4+)}} favours hard ligands with oxygen atoms as donors: complexes with sulfur atoms as donors are less stable and are more prone to hydrolysis.<ref name="CottonSA2006">{{cite book |last=Cotton |first=S. |year=2006 |title=Lanthanide and Actinide Chemistry|publisher=[[John Wiley & Sons]]}}</ref>

High coordination numbers are the rule for thorium due to its large size. Thorium nitrate pentahydrate was the first known example of coordination number 11, the oxalate tetrahydrate has coordination number 10, and the borohydride (first prepared in the [[Manhattan Project]]) has coordination number 14.{{sfn|Greenwood|Earnshaw|1997|pp=1275–1277}} These thorium salts are known for their high solubility in water and polar organic solvents.<ref name="Yu. D. Tretyakov" />

Many other inorganic thorium compounds with polyatomic anions are known, such as the [[perchlorate]]s, [[sulfate]]s, [[sulfite]]s, nitrates, carbonates, [[phosphate]]s, [[vanadate]]s, [[molybdate]]s, and [[chromates]], and their hydrated forms.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=101–115}} They are important in thorium purification and the disposal of nuclear waste, but most of them have not yet been fully characterised, especially regarding their structural properties.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=101–115}} For example, thorium nitrate is produced by reacting thorium hydroxide with nitric acid: it is soluble in water and alcohols and is an important intermediate in the purification of thorium and its compounds.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=101–115}} Thorium complexes with organic ligands, such as [[oxalate]], [[citrate]], and [[EDTA]], are much more stable. In natural thorium-containing waters, organic thorium complexes usually occur in concentrations orders of magnitude higher than the inorganic complexes, even when the concentrations of inorganic ligands are much greater than those of organic ligands.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=117–134}}

[[File:Thorium half sandwich.svg|thumb|upright|alt=Piano-stool molecule structure of (η8-C8H8)ThCl2(THF)2|Piano-stool molecule structure of ({{chem2|η^{8}\-C8H8)ThCl2(THF)2}}]]

In January 2021, the aromaticity has been observed in a large [[metal cluster]] anion consisting of 12 [[Bismuth|bismuth atoms]] stabilised by a center thorium cation.<ref>{{Cite web |last=Krämer2021-01-04T14:40:00+00:00 |first=Katrina |title=Heavy-metal cluster sets size record for metal aromaticity |url=https://www.chemistryworld.com/news/heavy-metal-cluster-sets-size-record-for-metal-aromaticity/4012946.article |access-date=2 July 2022 |website=Chemistry World |language=en |archive-date=4 January 2021 |archive-url=https://web.archive.org/web/20210104151533/https://www.chemistryworld.com/news/heavy-metal-cluster-sets-size-record-for-metal-aromaticity/4012946.article |url-status=live }}</ref> This compound was shown to be surprisingly stable, unlike many previous known [[Metal aromaticity|aromatic metal clusters]].

===Organothorium compounds===
Most of the work on organothorium compounds has focused on the [[cyclopentadienyl complex]]es and [[cyclooctatetraenide anion|cyclooctatetraenyls]]. Like many of the early and middle actinides (up to [[americium]], and also expected for [[curium]]), thorium forms a cyclooctatetraenide complex: the yellow {{chem2|Th(C8H8)2}}, [[thorocene]]. It is [[isotypic]] with the better-known analogous uranium compound [[uranocene]].{{sfn|Wickleder|Fourest|Dorhout|2006|pp=116–117}} It can be prepared by reacting [[potassium cyclooctatetraenide|{{chem2|K2C8H8}}]] with thorium tetrachloride in [[tetrahydrofuran]] (THF) at the temperature of [[dry ice]], or by reacting thorium tetrafluoride with {{chem2|MgC8H8}}.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=116–117}} It is unstable in air and decomposes in water or at 190&nbsp;°C.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=116–117}} [[Half sandwich compound]]s are also known, such as {{chem2|(η^{8}\-C8H8)ThCl2(THF)2}}, which has a piano-stool structure and is made by reacting thorocene with thorium tetrachloride in tetrahydrofuran.<ref name="CottonSA2006" />

The simplest of the cyclopentadienyls are {{chem2|Th(C5H5)3}} and {{chem2|Th(C5H5)4}}: many derivatives are known. The former (which has two forms, one purple and one green){{sfn|Wickleder|Fourest|Dorhout|2006|pp=116–117}} is a rare example of thorium in the formal +3 oxidation state;{{sfn|Greenwood|Earnshaw|1997|pp=1278–1280}} a formal +2 oxidation state occurs in a derivative.<ref>{{cite journal |first1=Ryan R. |last1=Langeslay |first2=Megan E. |last2=Fieser |first3=Joseph W. |last3=Ziller |first4=Philip |last4=Furche |first5=William J. |last5=Evans |title=Synthesis, structure, and reactivity of crystalline molecular complexes of the {[C<sub>5</sub>H<sub>3</sub>(SiMe<sub>3</sub>)<sub>2</sub>]<sub>3</sub>Th}<sup>1−</sup> anion containing thorium in the formal +2 oxidation state |journal=[[Chemical Science (journal)|Chemical Science]] |volume=6 |year=2015 |issue=1 |pages=517–521 |doi=10.1039/C4SC03033H|pmid=29560172 |pmc=5811171 }}</ref> The chloride derivative {{chem2|[Th(C5H5)3Cl]}} is prepared by heating thorium tetrachloride with [[limiting reagent|limiting]] {{chem2|KC5H5}} used (other univalent metal cyclopentadienyls can also be used). The [[alkyl]] and [[aryl]] derivatives are prepared from the chloride derivative and have been used to study the nature of the Th–C [[sigma bond]].{{sfn|Greenwood|Earnshaw|1997|pp=1278–1280}}

Other organothorium compounds are not well-studied. Tetrabenzylthorium, {{chem2|Th(CH2C6H5)4}}, and tetraallylthorium, {{chem2|Th(CH2CH\dCH2)4}}, are known, but their structures have not been determined. They decompose slowly at room temperature. Thorium forms the monocapped trigonal prismatic anion {{chem2|[Th(CH3)7](3−)}}, heptamethylthorate(IV), which forms the salt {{chem2|[Li(tmeda)]3[Th(CH3)7]}} (tmeda = {{chem2|(CH3)2NCH2CH2N(CH3)2}}). Although one methyl group is only attached to the thorium atom (Th–C distance 257.1&nbsp;pm) and the other six connect the lithium and thorium atoms (Th–C distances 265.5–276.5&nbsp;pm), they behave equivalently in solution. Tetramethylthorium, {{chem2|Th(CH3)4}}, is not known, but its [[adduct]]s are stabilised by [[phosphine]] ligands.<ref name="CottonSA2006" />

==Occurrence==
{{Main|Occurrence of thorium}}

===Formation===
<sup>232</sup>Th is a primordial nuclide, having existed in its current form for over ten billion years; it was formed during the [[r-process]], which probably occurs in [[supernova]]e and [[neutron star merger]]s. These violent events scattered it across the galaxy.<ref name="Cameron">{{cite journal |last1=Cameron |first1=A.G.W. |title=Abundances of the elements in the solar system |journal=Space Science Reviews |date=September 1973 |volume=15 |issue=1 |page=121 |doi=10.1007/BF00172440 |bibcode= 1973SSRv...15..121C |s2cid=120201972 }}</ref><ref>{{cite journal |last1=Frebel |first1=Anna |last2=Beers |first2=Timothy C. |title=The formation of the heaviest elements |journal=Physics Today |date=January 2018 |volume=71 |issue=1 |pages=30–37 |doi=10.1063/pt.3.3815 |arxiv=1801.01190 |bibcode=2018PhT....71a..30F |s2cid=4295865 }}</ref> The letter "r" stands for "rapid neutron capture", and occurs in core-collapse supernovae, where heavy seed nuclei such as [[iron-56|<sup>56</sup>Fe]] rapidly capture neutrons, running up against the [[neutron drip line]], as neutrons are captured much faster than the resulting nuclides can beta decay back toward stability. Neutron capture is the only way for stars to synthesise elements beyond iron because of the increased [[Coulomb barrier]]s that make interactions between charged particles difficult at high atomic numbers and the fact that fusion beyond <sup>56</sup>Fe is [[Endothermic process|endothermic]].<ref name="nucleosynthesis">{{cite journal |last1=Roederer |first1=I. U. |last2=Kratz |first2=K.-L. |first3=A. |last3=Frebel |display-authors=3 |first4=Norbert |last4=Christlieb |first5=Bernd |last5=Pfeiffer |first6=John J. |last6=Cowan |first7=Christopher |last7=Sneden |date=2009 |title=The End of Nucleosynthesis: Production of Lead and Thorium in the Early Galaxy |journal=The Astrophysical Journal |volume=698 |issue=2 |pages=1963–1980 |doi=10.1088/0004-637X/698/2/1963 |arxiv=0904.3105 |bibcode=2009ApJ...698.1963R|s2cid=14814446 }}</ref> Because of the abrupt loss of stability past <sup>209</sup>Bi, the r-process is the only process of stellar nucleosynthesis that can create thorium and uranium; all other processes are too slow and the intermediate nuclei alpha decay before they capture enough neutrons to reach these elements.<ref name="Cameron" /><ref name="B2FH">{{cite journal |last1=Burbidge |first1=E. Margaret |last2=Burbidge |first2=G. R. |last3=Fowler |first3=William A. |last4=Hoyle |first4=F. |title=Synthesis of the Elements in Stars |journal=Reviews of Modern Physics |date=1 October 1957 |volume=29 |issue=4 |pages=547–650 |doi=10.1103/RevModPhys.29.547 |bibcode=1957RvMP...29..547B |doi-access=free }}</ref><ref>{{cite book|last=Clayton|first=D. D.|author-link=Donald D. Clayton|title=Principles of Stellar Evolution and Nucleosynthesis|url=https://archive.org/details/principlesofstel00clay|url-access=registration|publisher=McGraw-Hill Education|date=1968|pages=[https://archive.org/details/principlesofstel00clay/page/577 577–591]|isbn=978-0-226-10953-4}}</ref>

=== Abundance ===
In the universe, thorium is among the rarest of the primordial elements at rank 77th in cosmic abundance<ref name=Cameron/><ref>{{Cite web |last=Helmenstine |first=Anne |date=28 June 2022 |title=Composition of the Universe – Element Abundance |url=https://sciencenotes.org/composition-of-the-universe-element-abundance/ |access-date=13 June 2024 |website=Science Notes and Projects |language=en-US |archive-date=24 May 2024 |archive-url=https://web.archive.org/web/20240524071239/https://sciencenotes.org/composition-of-the-universe-element-abundance/ |url-status=live }}</ref> because it is one of the two elements that can be produced only in the r-process (the other being uranium), and also because it has slowly been decaying away from the moment it formed. The only primordial elements rarer than thorium are [[thulium]], [[lutetium]], tantalum, and rhenium, the odd-numbered elements just before the third peak of r-process abundances around the heavy platinum group metals, as well as uranium.<ref name="Cameron" /><ref name="nucleosynthesis" />{{efn|[[Even and odd atomic nuclei|An even number of either protons or neutrons]] generally increases nuclear stability of isotopes, compared to isotopes with odd numbers. Elements with odd atomic numbers have no more than two stable isotopes; even-numbered elements have multiple stable isotopes, with tin (element 50) having ten.<ref name="NUBASE" />}} In the distant past the abundances of thorium and uranium were enriched by the decay of plutonium and curium isotopes, and thorium was enriched relative to uranium by the decay of <sup>236</sup>U to <sup>232</sup>Th and the natural depletion of <sup>235</sup>U, but these sources have long since decayed and no longer contribute.{{sfn|Stoll|2005|p=2}}

In the Earth's crust, thorium is much more abundant: with an [[Abundance of elements in Earth's crust|abundance]] of 8.1&nbsp;g/[[tonne]], it is one of the most abundant of the heavy elements, almost as abundant as lead (13&nbsp;g/tonne) and more abundant than tin (2.1&nbsp;g/tonne).{{sfn|Greenwood|Earnshaw|1997|p=1294}} This is because thorium is likely to form oxide minerals that do not sink into the core; it is classified as a [[Goldschmidt classification|lithophile]] under the [[Goldschmidt classification]], meaning that it is generally found combined with oxygen. Common thorium compounds are also poorly soluble in water. Thus, even though the [[Refractory metals|refractory elements]] have the same relative abundances in the Earth as in the Solar System as a whole, there is more accessible thorium than heavy platinum group metals in the crust.<ref name="albarede">{{cite book |title= Geochemistry: an introduction |page= 17 |publisher= [[Cambridge University Press]] |year= 2003 |isbn= 978-0-521-89148-6 |first= F. |last= Albarède}}</ref>

[[File:Evolution of Earth's radiogenic heat-no total.svg|thumb|upright=1.25|alt=Heat produced by the decay of K-40, Th-232, U-235, U-238 within the Earth over time|The [[radiogenic heat]] from the decay of <sup>232</sup>Th (violet) is a major contributor to the [[earth's internal heat budget]]. Of the four major nuclides providing this heat, <sup>232</sup>Th has grown to provide the most heat as the other ones decayed faster than thorium.<ref name="thoruranium">{{cite journal |last1=Trenn |first1=T. J. |date=1978 |title=Thoruranium (U-236) as the extinct natural parent of thorium: The premature falsification of an essentially correct theory |journal=Annals of Science |volume=35 |issue=6 |pages=581–597 |doi=10.1080/00033797800200441}}</ref><ref>{{cite journal |last1=Diamond |first1=H. |last2=Friedman |first2=A. M. |last3=Gindler |first3=J. E. |display-authors=3 |last4=Fields |first4=P. R. |date=1956 |title=Possible Existence of Cm<sup>247</sup> or Its Daughters in Nature |journal=Physical Review |volume=105 |issue=2 |pages=679–680 |doi=10.1103/PhysRev.105.679|bibcode=1957PhRv..105..679D}}</ref><ref>{{cite journal |last1=Rao |first1=M. N. |last2=Gopalan |first2=K. |date=1973 |title=Curium-248 in the Early Solar System |journal=Nature |volume=245 |issue=5424 |pages=304–307 |doi=10.1038/245304a0|bibcode=1973Natur.245..304R|s2cid=4226393 }}</ref><ref>{{cite journal |last1=Rosenblatt |first1=D. B. |date=1953 |title=Effects of a Primeval Endowment of U<sup>236</sup> |journal=Physical Review |volume=91 |issue=6 |pages=1474–1475 |doi=10.1103/PhysRev.91.1474|bibcode=1953PhRv...91.1474R}}</ref>]]

===On Earth===
Natural thorium is usually almost pure <sup>232</sup>Th, which is the longest-lived and most stable isotope of thorium, having a half-life comparable to the age of the universe.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=53–55}} Its radioactive decay is the largest single contributor to the [[Earth#Heat|Earth's internal heat]]; the other major contributors are the shorter-lived primordial radionuclides, which are <sup>238</sup>U, <sup>40</sup>K, and <sup>235</sup>U in descending order of their contribution. (At the time of the Earth's formation, <sup>40</sup>K and <sup>235</sup>U contributed much more by virtue of their short half-lives, but they have decayed more quickly, leaving the contribution from <sup>232</sup>Th and <sup>238</sup>U predominant.)<ref name="NGJuly11">{{cite journal |last1=Gando |first1=A. |last2=Gando |first2=Y. |last3=Ichimura |first3=K. |last4=Ikeda |first4=H. |last5=Inoue |first5=K. |last6=Kibe |first6=Y. |last7=Kishimoto |first7=Y. |last8=Koga |first8=M. |last9=Minekawa |first9=Y. |last10=Mitsui |first10=T. |last11=Morikawa |first11=T. |last12=Nagai |first12=N. |last13=Nakajima |first13=K. |last14=Nakamura |first14=K. |last15=Narita |first15=K. |last16=Shimizu |first16=I. |last17=Shimizu |first17=Y. |last18=Shirai |first18=J. |last19=Suekane |first19=F. |last20=Suzuki |first20=A. |last21=Takahashi |first21=H. |last22=Takahashi |first22=N. |last23=Takemoto |first23=Y. |last24=Tamae |first24=K. |last25=Watanabe |first25=H. |last26=Xu |first26=B. D. |last27=Yabumoto |first27=H. |last28=Yoshida |first28=H. |last29=Yoshida |first29=S. |last30=Enomoto |first30=S. |last31=Kozlov |first31=A. |last32=Murayama |first32=H. |last33=Grant |first33=C. |last34=Keefer |first34=G. |last35=Piepke |first35=A. |last36=Banks |first36=T. I. |last37=Bloxham |first37=T. |last38=Detwiler |first38=J. A. |last39=Freedman |first39=S. J. |last40=Fujikawa |first40=B. K. |last41=Han |first41=K. |last42=Kadel |first42=R. |last43=O’Donnell |first43=T. |last44=Steiner |first44=H. M. |last45=Dwyer |first45=D. A. |last46=McKeown |first46=R. D. |last47=Zhang |first47=C. |last48=Berger |first48=B. E. |last49=Lane |first49=C. E. |last50=Maricic |first50=J. |last51=Miletic |first51=T. |last52=Batygov |first52=M. |last53=Learned |first53=J. G. |last54=Matsuno |first54=S. |last55=Sakai |first55=M. |last56=Horton-Smith |first56=G. A. |last57=Downum |first57=K. E. |last58=Gratta |first58=G. |last59=Tolich |first59=K. |last60=Efremenko |first60=Y. |last61=Perevozchikov |first61=O. |last62=Karwowski |first62=H. J. |last63=Markoff |first63=D. M. |last64=Tornow |first64=W. |last65=Heeger |first65=K. M. |last66=Decowski |first66=M. P. |title=Partial radiogenic heat model for Earth revealed by geoneutrino measurements |journal=Nature Geoscience |date=September 2011 |volume=4 |issue=9 |pages=647–651 |doi=10.1038/ngeo1205 |bibcode=2011NatGe...4..647K |url=https://authors.library.caltech.edu/25422/ |access-date=3 February 2019 |archive-date=17 April 2023 |archive-url=https://web.archive.org/web/20230417202330/https://authors.library.caltech.edu/25422/ |url-status=dead }}</ref> Its decay accounts for a gradual decrease of thorium content of the Earth: the planet currently has around 85% of the amount present at the formation of the Earth.<ref name="Emsley2011" /> The other natural thorium isotopes are much shorter-lived; of them, only <sup>230</sup>Th is usually detectable, occurring in [[secular equilibrium]] with its parent <sup>238</sup>U, and making up at most 0.04% of natural thorium.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=53–55}}{{efn|Other isotopes may occur alongside <sup>232</sup>Th, but only in trace quantities. If the source contains no uranium, the only other thorium isotope present would be <sup>228</sup>Th, which occurs in the [[decay chain]] of <sup>232</sup>Th (the [[thorium series]]): the ratio of <sup>228</sup>Th to <sup>232</sup>Th would be under 10<sup>−10</sup>.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=53–55}} If uranium is present, tiny traces of several other isotopes will also be present: <sup>231</sup>Th and <sup>227</sup>Th from the decay chain of <sup>235</sup>U (the [[actinium series]]), and slightly larger but still tiny traces of <sup>234</sup>Th and <sup>230</sup>Th from the decay chain of <sup>238</sup>U (the [[uranium series]]).{{sfn|Wickleder|Fourest|Dorhout|2006|pp=53–55}} <sup>229</sup>Th is also been produced in the decay chain of <sup>237</sup>Np (the [[neptunium series]]): all primordial <sup>237</sup>Np is [[extinct radionuclide|extinct]], but it is still produced as a result of nuclear reactions in uranium ores.<ref>{{cite journal |last1=Peppard |first1=D. F. |last2=Mason |first2=G. W. |first3=P. R. |last3=Gray |display-authors=3 |first4=J. F. |last4=Mech |date=1952 |title=Occurrence of the (4''n'' + 1) Series in Nature |journal=Journal of the American Chemical Society |volume=74 |issue=23 |pages=6081–6084 |doi=10.1021/ja01143a074 |bibcode=1952JAChS..74.6081P |url=https://digital.library.unt.edu/ark:/67531/metadc172698/ |archive-date=28 July 2019 |access-date=3 February 2019 |archive-url=https://web.archive.org/web/20190728065436/https://digital.library.unt.edu/ark:/67531/metadc172698/ |url-status=live }}</ref> <sup>229</sup>Th is mostly produced as a [[decay product|daughter]] of artificial <sup>233</sup>U made by [[neutron irradiation]] of <sup>232</sup>Th, and is extremely rare in nature.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=53–55}}}}

Thorium only occurs as a minor constituent of most minerals, and was for this reason previously thought to be rare.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=55–56}}<ref>{{cite report |url=http://www.atsdr.cdc.gov/tfacts147.pdf |title=Thorium |author=[[Agency for Toxic Substances and Disease Registry]] |year=2016 |access-date=30 September 2017 |archive-date=12 April 2021 |archive-url=https://web.archive.org/web/20210412041611/https://www.atsdr.cdc.gov/tfacts147.pdf |url-status=dead }}</ref> In fact, it is the 37th most abundant element in the Earth's crust with an abundance of 12 parts per million.<ref>{{Cite book |last=Emsley |first=John |url=https://books.google.com/books?id=dGZaDwAAQBAJ&dq=%2237th+most+abundant+element%22+thorium&pg=PA547 |title=Nature's Building Blocks: An A-Z Guide to the Elements |date=25 August 2011 |publisher=Oxford University Press |isbn=978-0-19-257046-8 |language=en}}</ref> In nature, thorium occurs in the +4 oxidation state, together with uranium(IV), [[zirconium]](IV), hafnium(IV), and cerium(IV), and also with [[scandium]], [[yttrium]], and the trivalent lanthanides which have similar [[ionic radius|ionic radii]].{{sfn|Wickleder|Fourest|Dorhout|2006|pp=55–56}} Because of thorium's radioactivity, minerals containing it are often [[metamictization|metamict]] (amorphous), their crystal structure having been damaged by the alpha radiation produced by thorium.<ref name="Woodhead">{{cite journal |last1=Woodhead |first1=James A. |last2=Rossman |first2=George R. |last3=Silver |first3=Leon T. |title=The metamictization of zircon: Radiation dose-dependent structural characteristics |journal=American Mineralogist |date=1 February 1991 |volume=76 |issue=1–2 |pages=74–82 |url=https://pubs.geoscienceworld.org/msa/ammin/article-abstract/76/1-2/74/42514/The-metamictization-of-zircon-Radiation-dose |archive-date=13 April 2023 |access-date=10 October 2021 |archive-url=https://web.archive.org/web/20230413105622/https://pubs.geoscienceworld.org/msa/ammin/article-abstract/76/1-2/74/42514/The-metamictization-of-zircon-Radiation-dose |url-status=live }}</ref> An extreme example is [[ekanite]], {{chem2|(Ca,Fe,Pb)2(Th,U)Si8O20}}, which almost never occurs in nonmetamict form due to the thorium it contains.<ref name="ekanite">{{cite journal |last1=Szymanski |first1=J. T. |last2=Owens |first2=D. R. |last3=Roberts |first3=A. C. |last4=Ansell |first4=H. G. |last5=Chao |first5=George Y. |title=A mineralogical study and crystal-structure determination of nonmetamict ekanite, ThCa<sub>2</sub>Si<sub>8</sub>O<sub>20</sub> |journal=The Canadian Mineralogist |date=1 February 1982 |volume=20 |issue=1 |pages=65–75 |url=https://pubs.geoscienceworld.org/canmin/article/20/1/65/11549/A-mineralogical-study-and-crystal-structure |archive-date=24 October 2021 |access-date=10 October 2021 |archive-url=https://web.archive.org/web/20211024193133/https://pubs.geoscienceworld.org/canmin/article/20/1/65/11549/A-mineralogical-study-and-crystal-structure |url-status=live }}</ref>

[[Monazite]] (chiefly phosphates of various rare-earth elements) is the most important commercial source of thorium because it occurs in large deposits worldwide, principally in India, South Africa, Brazil, Australia, and [[Malaysia]]. It contains around 2.5% thorium on average, although some deposits may contain up to 20%.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=55–56}}{{sfn|Greenwood|Earnshaw|1997|p=1255}} Monazite is a chemically unreactive mineral that is found as yellow or brown sand; its low reactivity makes it difficult to extract thorium from it.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=55–56}} [[Allanite]] (chiefly silicates-hydroxides of various metals) can have 0.1–2% thorium and [[zircon]] (chiefly [[zirconium silicate]], {{chem2|ZrSiO4}}) up to 0.4% thorium.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=55–56}}

Thorium dioxide occurs as the rare mineral [[thorianite]]. Due to its being isotypic with [[uranium dioxide]], these two common actinide dioxides can form solid-state solutions and the name of the mineral changes according to the {{chem2|ThO2}} content.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=55–56}}{{efn|Thorianite refers to minerals with 75–100&nbsp;mol%&nbsp;{{chem2|ThO2}}; uranothorianite, 25–75&nbsp;mol%&nbsp;{{chem2|ThO2}}; thorian uraninite, 15–25&nbsp;mol%&nbsp;{{chem2|ThO2}}; [[uraninite]], 0–15&nbsp;mol%&nbsp;{{chem2|ThO2}}.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=55–56}}}} [[Thorite]] (chiefly [[thorium silicate]], {{chem2|ThSiO4}}), also has a high thorium content and is the mineral in which thorium was first discovered.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=55–56}} In thorium silicate minerals, the {{chem2|Th(4+)}} and {{chem2|SiO4(4-)}} ions are often replaced with {{chem2|M(3+)}} (where M = Sc, Y, or Ln) and phosphate ({{chem2|PO4(3-)}}) ions respectively.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=55–56}} Because of the great insolubility of thorium dioxide, thorium does not usually spread quickly through the environment when released. The {{chem2|Th(4+)}} ion is soluble, especially in acidic soils, and in such conditions the thorium concentration can be higher.<ref name="Emsley2011">{{cite book| pages=544–548| title=Nature's building blocks: an A–Z guide to the elements|first= J.|last=Emsley|author-link=John Emsley|publisher=[[Oxford University Press]]| isbn= 978-0-19-960563-7| date=2011}}</ref>

==History==
[[File:Mårten Eskil Winge - Tor's Fight with the Giants - Google Art Project.jpg|thumb|upright|alt=Thor raising his hammer in a battle against the giants|''[[Thor's Fight with the Giants]]'' (1872) by [[Mårten Eskil Winge]]; [[Thor]], the [[Norse god]] of thunder, raising his hammer [[Mjölnir]] in a battle against the [[Jötunn|giants]].<ref>{{Cite web|url=https://artsandculture.google.com/asset/tor-s-fight-with-the-giants/3gGd_ynWqGjGfQ?hl=en|title=Tor's Fight with the Giants |publisher=[[Google Arts & Culture]]|language=en|access-date=26 June 2016}}</ref>]]

===Erroneous report===

In 1815, the Swedish chemist [[Jöns Jacob Berzelius]] analysed an unusual sample of [[gadolinite]] from a copper mine in [[Falun]], central Sweden. He noted impregnated traces of a white mineral, which he cautiously assumed to be an earth ([[oxide]] in modern chemical nomenclature) of an unknown element. Berzelius had already discovered two elements, [[cerium]] and [[selenium]], but he had made a public mistake once, announcing a new element, ''gahnium'', that turned out to be [[zinc oxide]].<ref name="Lost" /> Berzelius privately named the putative element "thorium" in 1817<ref>{{cite book |last1=Ryabchikov |first1=D. I. |last2=Gol'braikh |first2=E. K. |date=2013 |title=The Analytical Chemistry of Thorium: International Series of Monographs on Analytical Chemistry |publisher=[[Elsevier]] |page=1 |isbn=978-1-4831-5659-0}}</ref> and its supposed oxide "thorina" after [[Thor]], the [[Norse god]] of thunder.<ref>{{cite book |last=Thomson |first=T. |date=1831 |title=A System of Chemistry of Inorganic Bodies |volume=1 |publisher=Baldwin & Cradock and [[William Blackwood]] |page=475}}</ref> In 1824, after more deposits of the same mineral in [[Vest-Agder]], Norway, were discovered, he retracted his findings, as the mineral (later named [[xenotime]]) proved to be mostly [[yttrium phosphate|yttrium orthophosphate]].{{sfn|Wickleder|Fourest|Dorhout|2006|pp=52–53}}<ref name="Lost">{{cite book|ref=Fontani|last1=Fontani|first1=M.|last2=Costa|first2=M.|last3=Orna|first3=V.|title=The Lost Elements: The Periodic Table's Shadow Side|publisher=Oxford University Press|year=2014|page=73|isbn=978-0-19-938334-4}}</ref><ref>{{cite journal |last=Berzelius |first=J. J. |author-link=Jöns Jakob Berzelius |date=1824 |title=Undersökning af några Mineralier. 1. Phosphorsyrad Ytterjord. |trans-title=Examining some minerals. 1st phosphoric yttria. |journal=Kungliga Svenska Vetenskapsakademiens Handlingar |volume=2 |pages=334–338 |language=sv}}</ref><ref name="Mindat">{{cite web |url=http://www.mindat.org/min-4333.html |title=Xenotime-(Y) |publisher=Mindat database |access-date=7 October 2017 |archive-date=16 March 2017 |archive-url=https://web.archive.org/web/20170316030444/http://www.mindat.org/min-4333.html |url-status=live }}</ref>

===Discovery===
In 1828, [[Morten Thrane Esmark]] found a black mineral on [[Løvøya, Telemark|Løvøya]] island, [[Telemark]] county, Norway. He was a Norwegian [[priest]] and amateur [[mineralogist]] who studied the minerals in Telemark, where he served as [[vicar]]. He commonly sent the most interesting specimens, such as this one, to his father, [[Jens Esmark]], a noted mineralogist and professor of mineralogy and geology at the [[Royal Frederick University]] in Christiania (today called [[Oslo]]).<ref name="snl">{{cite encyclopedia|year=2007|title=Morten Thrane Esmark|encyclopedia=[[Store norske leksikon]]<!--|editor-last=Henriksen |editor-first=P. is this correct? -->|first=R. S.|last=Selbekk|publisher=[[Kunnskapsforlaget]]|url=http://www.snl.no/Morten_Thrane_Esmark|access-date=16 May 2009|language=no|archive-date=28 April 2021|archive-url=https://web.archive.org/web/20210428144944/https://snl.no/Morten_Thrane_Esmark|url-status=live}}</ref> The elder Esmark determined that it was not a known mineral and sent a sample to Berzelius for examination. Berzelius determined that it contained a new element.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=52–53}} He published his findings in 1829, having isolated an impure sample by reducing {{chem2|K[ThF5]}} (potassium pentafluorothorate(IV)) with [[potassium]] metal.<ref name="Weeks" /><ref>{{cite journal|last=Berzelius|first=J. J.|language=de|date=1829|url=http://gallica.bnf.fr/ark:/12148/bpt6k151010.pleinepage.r=Annalen+der+Physic.f395.langFR|title=Untersuchung eines neues Minerals und einer darin erhalten zuvor unbekannten Erde|trans-title=Investigation of a new mineral and of a previously unknown earth contained therein|journal=Annalen der Physik und Chemie|volume=16|pages=385–415|doi=10.1002/andp.18290920702|bibcode=1829AnP....92..385B|issue=7|archive-date=27 April 2021|access-date=20 July 2009|archive-url=https://web.archive.org/web/20210427143538/https://gallica.bnf.fr/ark:/12148/bpt6k151010.pleinepage.r=Annalen+der+Physic.f395.langFR|url-status=live}} (modern citation: ''Annalen der Physik'', vol. 92, no. 7, pp. 385–415).</ref><ref>{{cite journal|last=Berzelius |first=J. J. |date= 1829|title=Undersökning af ett nytt mineral (Thorit), som innehåller en förut obekant jord |trans-title=Investigation of a new mineral (thorite), as contained in a previously unknown earth|journal=Kungliga Svenska Vetenskaps Akademiens Handlingar |pages=1–30 |language=sv}}</ref> Berzelius reused the name of the previous supposed element discovery<ref name="Weeks">{{cite journal |doi= 10.1021/ed009p1231|bibcode= 1932JChEd...9.1231W |title= The discovery of the elements. XI. Some elements isolated with the aid of potassium and sodium: Zirconium, titanium, cerium, and thorium |date= 1932 |last1= Weeks |first1= M. E. |author-link1=Mary Elvira Weeks| journal= Journal of Chemical Education |volume= 9 |issue= 7 |pages= 1231}}</ref><ref>{{cite journal |doi= 10.1002/ange.19020153703 |title= Die eigentlichen Thorit-Mineralien (Thorit und Orangit) |trans-title= The actual thoritic minerals (thorite and orangite) |language= de |date= 1902 |last1= Schilling |first1= J. |journal= Zeitschrift für Angewandte Chemie |volume= 15 |issue= 37 |pages= 921–929 |bibcode= 1902AngCh..15..921S |url= https://zenodo.org/record/1424433 |archive-date= 13 April 2023 |access-date= 24 August 2019 |archive-url= https://web.archive.org/web/20230413111147/https://zenodo.org/record/1424433 |url-status= live }}</ref> and named the source mineral thorite.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=52–53}}

[[File:J J Berzelius.jpg|thumb|upright|alt=Jöns Jacob Berzelius|[[Jöns Jacob Berzelius]], who first identified thorium as a new element]]

Berzelius made some initial characterisations of the new metal and its chemical compounds: he correctly determined that the thorium–oxygen mass ratio of thorium oxide was 7.5 (its actual value is close to that, ~7.3), but he assumed the new element was divalent rather than tetravalent, and so calculated that the atomic mass was 7.5 times that of oxygen (120 [[atomic mass unit|amu]]); it is actually 15 times as large.{{efn|At the time, the [[rare-earth element]]s, among which thorium was found and with which it is closely associated in nature, were thought to be divalent; the rare earths were given [[atomic weight]] values two-thirds of their actual ones, and thorium and uranium are given values half of the actual ones.}} He determined that thorium was a very [[Electronegativity#Electropositivity|electropositive]] metal, ahead of cerium and behind zirconium in electropositivity.<ref name="leach2">{{cite web |url=http://www.meta-synthesis.com/webbook/35_pt/pt_database.php?PT_id=453 |title=The Internet Database of Periodic Tables: Berzelius' Electronegativity Table |last=Leach |first=M. R. |access-date=16 July 2016 |archive-date=28 April 2021 |archive-url=https://web.archive.org/web/20210428202351/https://www.meta-synthesis.com/webbook/35_pt/pt_database.php?PT_id=453 |url-status=live }}</ref> Metallic thorium was isolated for the first time in 1914 by Dutch entrepreneurs Dirk Lely Jr. and Lodewijk Hamburger.{{efn|The main difficulty in isolating thorium lies not in its chemical electropositivity, but in the close association of thorium in nature with the rare-earth elements and uranium, which collectively are difficult to separate from each other. Swedish chemist [[Lars Fredrik Nilson]], the discoverer of scandium, had previously made an attempt to isolate thorium metal in 1882, but was unsuccessful at achieving a high degree of purity.<ref>{{cite journal |last=Nilson |first=L. F. |date=1882 |title=Über metallisches Thorium |trans-title=About metallic thorium |journal=Berichte der Deutschen Chemischen Gesellschaft |volume=15 |issue=2 |pages=2537–2547 |doi=10.1002/cber.188201502213 |language=de |url=https://zenodo.org/record/1425272 |archive-date=13 April 2023 |access-date=24 August 2019 |archive-url=https://web.archive.org/web/20230413114159/https://zenodo.org/record/1425272 |url-status=live }}</ref> Lely and Hamburger obtained 99% pure thorium metal by reducing thorium chloride with sodium metal.<ref name="Meister" /> A simpler method leading to even higher purity was discovered in 1927 by American engineers John Marden and Harvey Rentschler, involving the reduction of thorium oxide with calcium in presence of calcium chloride.<ref name="Meister">{{cite report |year=1948 |last=Meister |first=G. |url=http://www.lm.doe.gov/Considered_Sites/F/Foote_Mineral_Co_-_PA_27/PA_27-3.pdf |title=Production of Rarer Metals |publisher=[[United States Atomic Energy Commission]] |access-date=22 September 2017 |archive-date=24 February 2017 |archive-url=https://web.archive.org/web/20170224180301/https://www.lm.doe.gov/Considered_Sites/F/Foote_Mineral_Co_-_PA_27/PA_27-3.pdf |url-status=dead }}</ref>}}

===Initial chemical classification===
In the periodic table published by [[Dmitri Mendeleev]] in 1869, thorium and the rare-earth elements were placed outside the main body of the table, at the end of each vertical period after the [[alkaline earth metal]]s. This reflected the belief at that time that thorium and the rare-earth metals were divalent. With the later recognition that the rare earths were mostly trivalent and thorium was tetravalent, Mendeleev moved cerium and thorium to group IV in 1871, which also contained the modern [[carbon group]] (group 14) and titanium group (group 4), because their maximum oxidation state was +4.<ref name="leach">{{cite web |url=http://www.meta-synthesis.com/webbook//35_pt/pt_database.php |title=The Internet Database of Periodic Tables |last=Leach |first=M. R. |access-date=14 May 2012 |archive-date=24 March 2016 |archive-url=https://web.archive.org/web/20160324070522/http://www.meta-synthesis.com/webbook/35_pt/pt_database.php |url-status=live }}</ref><ref name="Jensen">{{cite journal|author1-link=William B. Jensen |last1=Jensen |first1=William B. |title=The Place of Zinc, Cadmium, and Mercury in the Periodic Table |journal=Journal of Chemical Education |date=August 2003 |volume=80 |issue=8 |pages=952 |doi=10.1021/ed080p952 |bibcode=2003JChEd..80..952J }}</ref> Cerium was soon removed from the main body of the table and placed in a separate lanthanide series; thorium was left with group 4 as it had similar properties to its supposed lighter congeners in that group, such as [[titanium]] and zirconium.<ref name="Masterton" />{{efn|Thorium also appears in the 1864 table by British chemist [[John Newlands (chemist)|John Newlands]] as the last and heaviest element, as it was initially thought that uranium was a trivalent element with an atomic weight of around 120: this is half of its actual value, since uranium is predominantly hexavalent. It also appears as the heaviest element in the 1864 table by British chemist [[William Odling]] under titanium, zirconium, and [[tantalum]]. It does not appear in the periodic systems published by French geologist [[Alexandre-Émile Béguyer de Chancourtois]] in 1862, German-American musician [[Gustav Hinrichs]] in 1867, or German chemist [[Julius Lothar Meyer]] in 1870, all of which exclude the rare earths and thorium.<ref name="leach" />}}

===First uses===
[[File:Old thorium dioxide gas mantle - oblong shape.JPG|thumb|alt=Gas mantle|[[World War II]] thorium dioxide gas mantle]]
While thorium was discovered in 1828 its first application dates only from 1885, when Austrian chemist [[Carl Auer von Welsbach]] invented the [[gas mantle]], a portable source of light which produces light from the incandescence of thorium oxide when heated by burning gaseous fuels.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=52–53}} Many applications were subsequently found for thorium and its compounds, including ceramics, carbon arc lamps, heat-resistant crucibles, and as catalysts for industrial chemical reactions such as the oxidation of ammonia to nitric acid.{{sfn|Wickleder|Fourest|Dorhout|2006|p=52}}

===Radioactivity===
Thorium was first observed to be radioactive in 1898, by the German chemist [[Gerhard Carl Schmidt]] and later that year, independently, by the Polish-French physicist [[Marie Curie]]. It was the second element that was found to be radioactive, after the 1896 discovery of radioactivity in uranium by French physicist [[Henri Becquerel]].<ref>{{cite journal|last=Curie |first=M. |author-link=Marie Curie |date=1898|title= Rayons émis par les composés de l'uranium et du thorium |trans-title=Rays emitted by compounds of uranium and thorium|journal=Comptes Rendus|volume= 126| pages= 1101–1103|ol=24166254M |language=fr}}</ref><ref>{{cite journal |last=Schmidt |first=G. C. |author-link=Gerhard Carl Schmidt |date=1898 |title=Über die vom Thorium und den Thoriumverbindungen ausgehende Strahlung |trans-title=On the radiation emitted by thorium and thorium compounds |journal=Verhandlungen der Physikalischen Gesellschaft zu Berlin |volume=17 |pages=14–16 |language=de |url=http://digital.slub-dresden.de/id507751434-18980000/24 }}</ref><ref>{{cite journal |last=Schmidt |first=G. C. |url=http://gallica.bnf.fr/ark:/12148/bpt6k153068.image.r=Annalen+der+Physic.f149.langFR |title=Über die von den Thorverbindungen und einigen anderen Substanzen ausgehende Strahlung |trans-title=On the radiation emitted by thorium compounds and some other substances |journal=Annalen der Physik und Chemie |volume=65 |issue=5 |pages=141–151 |date=1898 |language=de |bibcode=1898AnP...301..141S |doi=10.1002/andp.18983010512 |archive-date=28 April 2021 |access-date=19 July 2009 |archive-url=https://web.archive.org/web/20210428101106/https://gallica.bnf.fr/ark:/12148/bpt6k153068.image.r=Annalen+der+Physic.f149.langFR |url-status=live }} (modern citation: ''Annalen der Physik'', vol. 301, pp. 141–151 (1898)).</ref> Starting from 1899, the New Zealand physicist [[Ernest Rutherford]] and the American electrical engineer [[Robert Bowie Owens]] studied the radiation from thorium; initial observations showed that it varied significantly. It was determined that these variations came from a short-lived gaseous daughter of thorium, which they found to be a new element. This element is now named [[radon]], the only one of the rare radioelements to be discovered in nature as a daughter of thorium rather than uranium.<ref>{{cite journal|author=Rutherford, E.|author-link=Ernest Rutherford|author2=Owens, R. B.|author2-link=Robert Bowie Owens|title=Thorium and uranium radiation|journal=Trans. R. Soc. Can.|volume=2|date= 1899|pages= 9–12}}: "The radiation from thorium oxide was not constant, but varied in a most capricious manner", whereas "All the compounds of Uranium give out a radiation which is remarkably constant."</ref>

After accounting for the contribution of radon, Rutherford, now working with the British physicist [[Frederick Soddy]], showed how thorium decayed at a fixed rate over time into a series of other elements in work dating from 1900 to 1903. This observation led to the identification of the [[half-life]] as one of the outcomes of the [[alpha particle]] experiments that led to the disintegration theory of [[radioactivity]].<ref>{{cite book|last=Simmons|first=J. G.|title=The Scientific 100: A Ranking of the Most Influential Scientists, Past and Present|url=https://archive.org/details/scientific100ran00simm|url-access=registration|page=[https://archive.org/details/scientific100ran00simm/page/19 19]|date=1996|publisher=Carol|isbn=978-0-8065-2139-8}}</ref> The biological effect of radiation was discovered in 1903.<ref>{{cite web |url=https://www.nobelprize.org/nobel_prizes/themes/physics/curie/ |title=Marie and Pierre Curie and the Discovery of Polonium and Radium |last=Fröman |first=N. |date=1996 |website=nobelprize.org |publisher=[[Nobel Media AB]] |access-date=11 May 2017 |archive-date=7 August 2018 |archive-url=https://web.archive.org/web/20180807095032/https://www.nobelprize.org/nobel_prizes/themes/physics/curie/ |url-status=live }}</ref> The newly discovered phenomenon of radioactivity excited scientists and the general public alike. In the 1920s, thorium's radioactivity was promoted as a cure for [[rheumatism]], [[diabetes]], and [[sexual impotence]]. In 1932, most of these uses were banned in the United States after a federal investigation into the health effects of radioactivity.<ref name="Burns1987" /> 10,000 individuals in the United States had been injected with thorium during X-ray diagnosis; they were later found to suffer health issues such as leukaemia and abnormal chromosomes.<ref name="Emsley2011" /> Public interest in radioactivity had declined by the end of the 1930s.<ref name="Burns1987">{{cite book|last=Burns|first=M.|title=Low-Level Radioactive Waste Regulation-Science, Politics and Fear|year=1987|publisher=CRC Press|isbn=978-0-87371-026-8|pages=24–25}}</ref>

[[File:Seaborg in lab - restoration.jpg|thumb|upright|alt=Glenn T. Seaborg|[[Glenn T. Seaborg]], who settled thorium's location in the f-block]]

===Further classification===
Up to the late 19th century, chemists unanimously agreed that thorium and uranium were the heaviest members of group 4 and [[group 6 element|group 6]] respectively; the existence of the lanthanides in the sixth row was considered to be a one-off fluke. In 1892, British chemist Henry Bassett postulated a second extra-long periodic table row to accommodate known and undiscovered elements, considering thorium and uranium to be analogous to the lanthanides. In 1913, Danish physicist [[Niels Bohr]] published a [[Bohr model|theoretical model]] of the atom and its electron orbitals, which soon gathered wide acceptance. The model indicated that the seventh row of the periodic table should also have f-shells filling before the d-shells that were filled in the transition elements, like the sixth row with the lanthanides preceding the 5d transition metals.<ref name="leach" /> The existence of a second inner transition series, in the form of the actinides, was not accepted until similarities with the electron structures of the lanthanides had been established;<ref>{{cite book|last=van Spronsen |first=J. W. |year=1969 |title=The periodic system of chemical elements |publisher=Elsevier |pages=315–316 |isbn=978-0-444-40776-4}}.</ref> Bohr suggested that the filling of the 5f orbitals may be delayed to after uranium.<ref name="leach" />

It was only with the discovery of the first [[transuranic element]]s, which from plutonium onward have dominant +3 and +4 oxidation states like the lanthanides, that it was realised that the actinides were indeed filling f-orbitals rather than d-orbitals, with the transition-metal-like chemistry of the early actinides being the exception and not the rule.<ref>{{cite book |last=Rhodes |first=R. |title=The Making of the Atomic Bomb |edition=25th Anniversary |date=2012 |publisher=[[Simon & Schuster]] |isbn=978-1-4516-7761-4 |pages=221–222, 349}}</ref> In 1945, when American physicist [[Glenn T. Seaborg]] and his team had discovered the transuranic elements americium and curium, he proposed the [[actinide concept]], realising that thorium was the second member of an f-block actinide series analogous to the lanthanides, instead of being the heavier congener of [[hafnium]] in a fourth d-block row.<ref name="Masterton">{{cite book |last1=Masterton|first1=W. L. |last2=Hurley|first2=C. N.|last3=Neth|first3=E. J.|title=Chemistry: Principles and reactions|publisher=[[Cengage Learning]]|edition=7th|isbn=978-1-111-42710-8|page=173|year=2011 }}</ref>{{efn|The filling of the 5f subshell from the beginning of the actinide series was confirmed when the 6d elements were reached in the 1960s, proving that the 4f and 5f series are of equal length. [[Lawrencium]] has only +3 as an oxidation state, breaking from the trend of the late actinides towards the +2 state; it thus fits as a heavier congener of [[lutetium]]. Even more importantly, the next element, [[rutherfordium]], was found to behave like hafnium and show only a +4 state.<ref name=johnson/><ref>{{cite journal |doi= 10.1016/S0925-8388(98)00072-3 |title= Evidence for relativistic effects in the chemistry of element 104 |first9= D. |last10= Timokhin |first10= S. N. |last11= Yakushev |first11= A. B. |last12= Zvara |first12= I. |last9= Piguet |first8= V. Ya. |last8= Lebedev |first7= D. T. |last7= Jost |first6= S. |last6= Hübener |first5= M. |last5= Grantz |first4= H. W. |last4= Gäggeler |first3= B. |last3= Eichler |first2= G. V. |date= 1998 |last2= Buklanov |last1= Türler| first1= A. |journal= Journal of Alloys and Compounds |volume= 271–273 |pages= 287–291| display-authors=3}}</ref> Today, thorium's similarities to hafnium are still sometimes acknowledged by calling it a "pseudo group 4 element".<ref name="Pershina">{{cite book |last1=Kratz |first1=J. V. |last2=Nagame |first2=Y. |editor1-last=Schädel |editor1-first=M. |editor2-last=Shaughnessy |editor2-first=D. |chapter=Liquid-Phase Chemistry of Superheavy Elements |date=2014 |edition=2nd |title=The Chemistry of Superheavy Elements |publisher=Springer-Verlag |page=335 |isbn=978-3-642-37465-4 |doi=10.1007/978-3-642-37466-1 |s2cid=122675117 |chapter-url=https://cds.cern.ch/record/643991 |archive-date=17 April 2021 |access-date=21 June 2023 |archive-url=https://web.archive.org/web/20210417211550/http://cds.cern.ch/record/643991 |url-status=live }}</ref>}}

===Phasing out===
In the 1990s, most applications that do not depend on thorium's radioactivity declined quickly due to safety and environmental concerns as suitable safer replacements were found.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=52–53}}<ref name="Furuta">{{cite journal|last1=Furuta|first1=E.|last2=Yoshizawa|first2=Y.|last3=Aburai|first3=T.|date=2000|title=Comparisons between radioactive and non-radioactive gas lantern mantles|journal=J. Radiol. Prot.|volume=20|issue=4|pages=423–431|pmid=11140713|bibcode=2000JRP....20..423F|doi=10.1088/0952-4746/20/4/305|s2cid=7368077 }}</ref> Despite its radioactivity, the element has remained in use for applications where no suitable alternatives could be found. A 1981 study by the [[Oak Ridge National Laboratory]] in the United States estimated that using a thorium gas mantle every weekend would be safe for a person,<ref name="Furuta" /> but this was not the case for the dose received by people manufacturing the mantles or for the soils around some factory sites.<ref>{{cite journal|author=New Jersey Department of Health|date=1996|title=Health and hazardous waste|url=http://www.state.nj.us/health/eoh/hhazweb/hhw_no_3.pdf|journal=A Practitioner's Guide to Patients' Environmental Exposures|volume=1|issue=3|pages=1–8|archive-url=https://web.archive.org/web/20160415153459/http://www.state.nj.us/health/eoh/hhazweb/hhw_no_3.pdf|archive-date=15 April 2016}}</ref> Some manufacturers have changed to other materials, such as yttrium.<ref>{{cite journal |last1=Toepker |first1=Terrence P. |date=1996 |title=Thorium and yttrium in gas lantern mantles |journal=American Journal of Physics |volume=64 |issue=2 |pages=109 |doi=10.1119/1.18463 |bibcode=1996AmJPh..64..109T |doi-access=free }}</ref> As recently as 2007, some companies continued to manufacture and sell thorium mantles without giving adequate information about their radioactivity, with some even falsely claiming them to be non-radioactive.<ref name="Furuta" /><ref name="Poljanc">{{cite journal|last1=Poljanc|first1=K.|last2=Steinhauser|first2=G.|last3=Sterba|first3=J. H.|last4=Buchtela|first4=K.|last5=Bichler|first5=M.|display-authors=3|date=2007|title=Beyond low-level activity: on a "non-radioactive" gas mantle|journal=[[Science of the Total Environment]]|volume=374|issue=1|pages=36–42|doi=10.1016/j.scitotenv.2006.11.024|pmid=17270253|bibcode=2007ScTEn.374...36P}}</ref>

===Nuclear power===
{{Main|Thorium-based nuclear power|Thorium fuel cycle}}
[[File:Indian_Point_Nuclear_Power_Plant.jpg|left|thumb|alt=Indian Point Energy Center|The [[Indian Point Energy Center]] ([[Buchanan, New York]], United States), home of the world's first thorium reactor]]

Thorium has been used as a power source on a prototype scale. The earliest thorium-based reactor was built at the [[Indian Point Energy Center]] located in [[Buchanan, New York|Buchanan]], New York, [[United States]] in 1962.<ref>{{cite web|url=http://www.americanscientist.org/issues/feature/thorium-fuel-for-nuclear-energy/2|title=Thorium Fuel for Nuclear Energy|last=Kazimi|first=M.|date=2003|publisher=[[American Scientist]]|archive-url=https://web.archive.org/web/20170101123406/http://www.americanscientist.org/issues/feature/thorium-fuel-for-nuclear-energy/2|archive-date=1 January 2017|access-date=29 September 2017}}</ref> China may be the first to have a shot at commercialising the technology.<ref>{{cite journal |last1=Mallapaty |first1=Smriti |title=China prepares to test thorium-fuelled nuclear reactor |journal=Nature |date=9 September 2021 |volume=597 |issue=7876 |pages=311–312 |doi=10.1038/d41586-021-02459-w |pmid=34504330 |bibcode=2021Natur.597..311M |s2cid=237471852 }}</ref> The country with the largest estimated reserves of thorium in the world is [[India]], which has sparse reserves of uranium. In the 1950s, India targeted achieving energy independence with their [[India's three-stage nuclear power programme|three-stage nuclear power programme]].<ref>{{cite report|last1=Majumdar|first1=S.|last2=Purushotham|first2=D. S. C.|entry=Experience of thorium fuel development in India|title=Thorium fuel utilization: Options and trends|year=1999|publisher=[[International Atomic Energy Agency]]|access-date=7 October 2017|url=http://large.stanford.edu/courses/2012/ph241/bordia2/docs/te_1319_web.pdf|archive-date=12 April 2021|archive-url=https://web.archive.org/web/20210412041730/http://large.stanford.edu/courses/2012/ph241/bordia2/docs/te_1319_web.pdf|url-status=live}}</ref><ref name="World Nuclear Association India">{{Cite web|url=http://www.world-nuclear.org/information-library/country-profiles/countries-g-n/india.aspx|title=Nuclear Power in India|publisher=World Nuclear Association|year=2017|access-date=29 September 2017|archive-date=6 September 2016|archive-url=https://archive.today/20160906112259/http://www.world-nuclear.org/information-library/country-profiles/countries-g-n/india.aspx|url-status=live}}</ref> In most countries, uranium was relatively abundant and the progress of thorium-based reactors was slow; in the 20th century, three reactors were built in India and twelve elsewhere.<ref>{{cite web |url=http://www-pub.iaea.org/MTCD/publications/PDF/TE_1450_web.pdf |publisher=International Atomic Energy Agency |title=IAEA-TECDOC-1450 Thorium Fuel Cycle – Potential Benefits and Challenges |date=2005 |access-date=23 March 2009 |archive-date=4 August 2016 |archive-url=https://web.archive.org/web/20160804054758/http://www-pub.iaea.org/MTCD/publications/PDF/TE_1450_web.pdf |url-status=live }}</ref> Large-scale research was begun in 1996 by the [[International Atomic Energy Agency]] to study the use of thorium reactors; a year later, the [[United States Department of Energy]] started their research. [[Alvin Radkowsky]] of [[Tel Aviv University]] in [[Israel]] was the head designer of [[Shippingport Atomic Power Station]] in Pennsylvania, the first American civilian reactor to breed thorium.<ref name="asme-landmark">{{cite web|url=http://files.asme.org/ASMEORG/Communities/History/Landmarks/5643.pdf|archive-url=https://web.archive.org/web/20150717051921/http://files.asme.org/ASMEORG/Communities/History/Landmarks/5643.pdf|archive-date=17 July 2015|title=Historic Achievement Recognized: Shippingport Atomic Power Station, A National Engineering Historical Landmark|page=4|access-date=24 June 2006|author=Shippingport Atomic Power Station}}</ref> He founded a consortium to develop thorium reactors, which included other laboratories: [[Raytheon]] Nuclear Inc. and [[Brookhaven National Laboratory]] in the United States, and the [[Kurchatov Institute]] in Russia.<ref name="Inc.1997">{{cite journal |last1=Friedman |first1=John S. |title=More power to thorium? |journal=Bulletin of the Atomic Scientists |date=September 1997 |volume=53 |issue=5 |pages=19–20 |doi=10.1080/00963402.1997.11456765 |bibcode=1997BuAtS..53e..19F }}</ref>

In the 21st century, thorium's potential for reducing nuclear proliferation and its [[nuclear waste|waste]] characteristics led to renewed interest in the thorium fuel cycle.<ref>{{cite web|url=http://www-pub.iaea.org/MTCD/publications/PDF/te_1349_web.pdf|title=IAEA-TECDOC-1349 Potential of thorium-based fuel cycles to constrain plutonium and to reduce the long-lived waste toxicity|date=2002|publisher=International Atomic Energy Agency|access-date=24 March 2009|archive-date=28 April 2021|archive-url=https://web.archive.org/web/20210428163855/https://www-pub.iaea.org/MTCD/publications/PDF/te_1349_web.pdf|url-status=live}}</ref><ref>{{cite news|url=http://www.abc.net.au/news/newsitems/200604/s1616391.htm|title=Scientist urges switch to thorium|last=Evans|first=B.|date=2006|publisher=[[ABC News (Australia)|ABC News]]|archive-url=https://web.archive.org/web/20100328211103/http://www.abc.net.au/news/newsitems/200604/s1616391.htm|archive-date=28 March 2010|access-date=17 September 2011}}</ref><ref>{{cite news|url=https://www.wired.com/magazine/2009/12/ff_new_nukes/|title=Uranium is So Last Century – Enter Thorium, the New Green Nuke|last=Martin|first=R.|date=2009|magazine=[[Wired (magazine)|Wired]]|access-date=19 June 2010|archive-date=26 June 2010|archive-url=https://web.archive.org/web/20100626014207/http://www.wired.com/magazine/2009/12/ff_new_nukes/|url-status=live}}</ref> India has projected meeting as much as 30% of its electrical demands through thorium-based [[nuclear power]] by 2050. In February 2014, [[Bhabha Atomic Research Centre]] (BARC), in [[Mumbai]], India, presented their latest design for a "next-generation nuclear reactor" that burns thorium as its fuel core, calling it the [[Advanced Heavy Water Reactor]] (AHWR). In 2009, the chairman of the Indian Atomic Energy Commission said that India has a "long-term objective goal of becoming energy-independent based on its vast thorium resources."

On 16 June 2023 China's National Nuclear Safety Administration issued a licence to the Shanghai Institute of Applied Physics (SINAP) of the Chinese Academy of Sciences to begin operating the [[TMSR-LF1]], 2 MWt liquid fuel thorium-based molten salt experimental reactor which was completed in August 2021.<ref name= carpineti >Dr. Alfredo Carpineti  [https://www.iflscience.com/experimental-molten-salt-nuclear-reactor-gets-go-ahead-in-china-69417 (16 June 2023) Experimental Molten Salt Nuclear Reactor Gets Go-Ahead In China ] {{Webarchive|url=https://web.archive.org/web/20230702021045/https://www.iflscience.com/experimental-molten-salt-nuclear-reactor-gets-go-ahead-in-china-69417 |date=2 July 2023 }}</ref> China is believed to have one of the largest thorium reserves in the world. The exact size of those reserves has not been publicly disclosed, but it is estimated to be enough to meet the country's total energy needs for more than 20,000 years.<ref>{{Cite web|url=https://www.scmp.com/news/china/science/article/3224183/china-gives-green-light-nuclear-reactor-burns-thorium-fuel-could-power-country-20000-years|title=China gives green light to its first thorium-powered nuclear reactor|date=15 June 2023|website=South China Morning Post}}</ref>

===Nuclear weapons===
When gram quantities of [[plutonium]] were first produced in the [[Manhattan Project]], it was discovered that a minor isotope ([[plutonium-240|<sup>240</sup>Pu]]) underwent significant [[spontaneous fission]], which brought into question the viability of a plutonium-fuelled [[Gun-type fission weapon|gun-type nuclear weapon]]. While the [[Project Y|Los Alamos]] team began work on the [[Nuclear weapon design#Implosion-type weapon|implosion-type weapon]] to circumvent this issue, the [[Metallurgical Laboratory|Chicago team]] discussed reactor design solutions. [[Eugene Wigner]] proposed to use the <sup>240</sup>Pu-contaminated plutonium to drive the conversion of thorium into <sup>233</sup>U in a special converter reactor. It was hypothesized that the <sup>233</sup>U would then be usable in a gun-type weapon, though concerns about contamination from <sup>232</sup>U were voiced. Progress on the implosion weapon was sufficient, and this converter was not developed further, but the design had enormous influence on the development of nuclear energy. It was the first detailed description of a highly enriched water-cooled, water-moderated reactor similar to future naval and commercial power reactors.<ref name="First Nuclear Era">{{cite book |last=Weinberg |first=Alvin |author-link=Alvin Weinberg |year=1994 |title=The First Nuclear Era: The life and times of a technological fixer |pages=36–38 |location=New York |publisher=AIP Press |isbn=978-1-56396-358-2}}</ref>

During the [[Cold War]] the United States explored the possibility of using <sup>232</sup>Th as a source of <sup>233</sup>U to be used in a [[nuclear bomb]]; they fired [[Operation Teapot#MET|a test bomb]] in 1955.<ref name="World Nuclear Association Thorium">{{cite web |title=Thorium |publisher=[[World Nuclear Association]] |year=2017 |url=http://www.world-nuclear.org/information-library/current-and-future-generation/thorium.aspx |access-date=21 June 2017 |archive-date=8 May 2017 |archive-url=https://web.archive.org/web/20170508215033/http://www.world-nuclear.org/information-library/current-and-future-generation/thorium.aspx |url-status=live }}</ref> They concluded that a <sup>233</sup>U-fired bomb would be a very potent weapon, but it bore few sustainable "technical advantages" over the contemporary uranium–plutonium bombs,<ref>{{cite report |last=Woods |first=W.K. |date=1966 |title=LRL Interest in U-233 |publisher=[[Battelle Memorial Institute]] |doi=10.2172/79078 |osti=79078 |language=en |url=https://digital.library.unt.edu/ark:/67531/metadc720752/m2/1/high_res_d/79078.pdf |archive-date=10 September 2023 |access-date=24 August 2019 |archive-url=https://web.archive.org/web/20230910145607/https://digital.library.unt.edu/ark:/67531/metadc720752/m2/1/high_res_d/79078.pdf |url-status=live }}</ref> especially since <sup>233</sup>U is difficult to produce in isotopically pure form.<ref name="World Nuclear Association Thorium" />

Thorium metal was used in the [[hohlraum|radiation case]] of at least one nuclear weapon design deployed by the United States (the [[W71]]).<ref>{{cite web |title=Classification Bulletin WNP-118 |publisher=U.S. Department of Energy |date=12 March 2008 |url=https://www.osti.gov/opennet/servlets/purl/1052069/1052069.pdf |access-date=13 September 2019 |archive-date=3 February 2017 |archive-url=https://web.archive.org/web/20170203132623/https://www.osti.gov/opennet/servlets/purl/1052069/1052069.pdf |url-status=live }}</ref>

==Production==
{{See also|List of countries by thorium resources}}
<div style="float: right; margin: 2px; font-size:85%; margin-left:18px; margin-bottom:18px>
{| class="wikitable sortable collapsible" cellpadding="3" rules="all" style="background:#f9f9f9; border:1px #aaa solid"
|+'''Lower-bound estimates of thorium reserves in thousand [[tonne]]s, 2014'''<ref name="World Nuclear Association Thorium" />
! Country !! data-sort-type="number"|Reserves
|-
| [[India]] || align="right"|846
|-
| [[Brazil]] || align="right"|632
|-
| [[Australia]] || align="right"|595
|-
| [[United States]] || align="right"|595
|-
| [[Egypt]] || align="right"|380
|-
| [[Turkey]] || align="right"|374
|-
| [[Venezuela]] || align="right"|300
|-
| [[Canada]] || align="right"|172
|-
| [[Russia]] || align="right"|155
|-
| [[South Africa]] || align="right"|148
|-
| [[China]] || align="right"|100
|-
| [[Norway]] || align="right"|87
|-
| [[Greenland]] || align="right"|86
|-
| [[Finland]] || align="right"|60
|-
| [[Sweden]] || align="right"|50
|-
| [[Kazakhstan]] || align="right"|50
|-
| Other countries || align="right" |1725
|-
| align="center" |''World total'' || align="right" |6355
|}
</div>

The low demand makes working mines for extraction of thorium alone not profitable, and it is almost always extracted with the rare earths, which themselves may be by-products of production of other minerals.{{sfn|Stoll|2005|p=7}} The current reliance on monazite for production is due to thorium being largely produced as a by-product; other sources such as thorite contain more thorium and could easily be used for production if demand rose.<ref>{{cite web |url=https://minerals.usgs.gov/minerals/pubs/commodity/thorium/mcs-2012-thori.pdf |title=Thorium |author=United States Geological Survey |date=2012 |access-date=12 May 2017 |archive-date=29 April 2017 |archive-url=https://web.archive.org/web/20170429174611/https://minerals.usgs.gov/minerals/pubs/commodity/thorium/mcs-2012-thori.pdf |url-status=live }}</ref> Present knowledge of the distribution of thorium resources is poor, as low demand has led to exploration efforts being relatively minor.<ref>{{cite report|url=http://www.iaea.org/inisnkm/nkm/aws/fnss/fulltext/0412_1.pdf |title=An Overview of World Thorium Resources, Incentives for Further Exploration and Forecast for Thorium Requirements in the Near Future |last=Jayaram |first=K. M. V. |url-status=dead |year=1987 |archive-url=https://web.archive.org/web/20110628234922/http://www.iaea.org/inisnkm/nkm/aws/fnss/fulltext/0412_1.pdf |publisher=[[Department of Atomic Energy]]|archive-date=28 June 2011}}</ref> In 2014, world production of the monazite concentrate, from which thorium would be extracted, was 2,700 tonnes.<ref name="USGS">{{Cite report|url=https://minerals.usgs.gov/minerals/pubs/commodity/thorium/index.html#mcs|title=Thorium. Statistics and Information|year=2017|publisher=[[United States Geological Survey]]|language=en|access-date=6 January 2018|archive-date=10 January 2019|archive-url=https://web.archive.org/web/20190110140206/https://minerals.usgs.gov/minerals/pubs/commodity/thorium/index.html#mcs|url-status=live}}</ref>

The common production route of thorium constitutes concentration of thorium minerals; extraction of thorium from the concentrate; purification of thorium; and (optionally) conversion to compounds, such as thorium dioxide.{{sfn|Stoll|2005|p=8}}

===Concentration===
There are two categories of thorium minerals for thorium extraction: primary and secondary. Primary deposits occur in acidic granitic magmas and pegmatites. They are concentrated, but of small size. Secondary deposits occur at the mouths of rivers in granitic mountain regions. In these deposits, thorium is enriched along with other heavy minerals.{{sfn|Stoll|2005|p=6}} Initial concentration varies with the type of deposit.{{sfn|Stoll|2005|p=8}}

For the primary deposits, the source pegmatites, which are usually obtained by mining, are divided into small parts and then undergo [[froth flotation|flotation]]. Alkaline earth metal carbonates may be removed after reaction with [[hydrogen chloride]]; then follow [[thickening]], filtration, and calcination. The result is a concentrate with rare-earth content of up to 90%.{{sfn|Stoll|2005|p=8}} Secondary materials (such as coastal sands) undergo gravity separation. Magnetic separation follows, with a series of magnets of increasing strength. Monazite obtained by this method can be as pure as 98%.{{sfn|Stoll|2005|p=8}}

Industrial production in the 20th century relied on treatment with hot, concentrated sulfuric acid in cast iron vessels, followed by selective precipitation by dilution with water, as on the subsequent steps. This method relied on the specifics of the technique and the concentrate grain size; many alternatives have been proposed, but only one has proven effective economically: alkaline digestion with hot sodium hydroxide solution. This is more expensive than the original method but yields a higher purity of thorium; in particular, it removes phosphates from the concentrate.{{sfn|Stoll|2005|p=8}}

====Acid digestion====
Acid digestion is a two-stage process, involving the use of up to 93% [[sulfuric acid]] at 210–230&nbsp;°C. First, sulfuric acid in excess of 60% of the sand mass is added, thickening the reaction mixture as products are formed. Then, fuming sulfuric acid is added and the mixture is kept at the same temperature for another five hours to reduce the volume of solution remaining after dilution. The concentration of the sulfuric acid is selected based on reaction rate and viscosity, which both increase with concentration, albeit with viscosity retarding the reaction. Increasing the temperature also speeds up the reaction, but temperatures of 300&nbsp;°C and above must be avoided, because they cause insoluble thorium pyrophosphate to form. Since dissolution is very exothermic, the monazite sand cannot be added to the acid too quickly. Conversely, at temperatures below 200&nbsp;°C the reaction does not go fast enough for the process to be practical. To ensure that no precipitates form to block the reactive monazite surface, the mass of acid used must be twice that of the sand, instead of the 60% that would be expected from stoichiometry. The mixture is then cooled to 70&nbsp;°C and diluted with ten times its volume of cold water, so that any remaining monazite sinks to the bottom while the rare earths and thorium remain in solution. Thorium may then be separated by precipitating it as the phosphate at pH&nbsp;1.3, since the rare earths do not precipitate until pH&nbsp;2.{{sfn|Stoll|2005|p=8}}

====Alkaline digestion====
Alkaline digestion is carried out in 30–45% [[sodium hydroxide]] solution at about 140&nbsp;°C for about three hours. Too high a temperature leads to the formation of poorly soluble thorium oxide and an excess of uranium in the filtrate, and too low a concentration of alkali leads to a very slow reaction. These reaction conditions are rather mild and require monazite sand with a particle size under 45&nbsp;μm. Following filtration, the filter cake includes thorium and the rare earths as their hydroxides, uranium as [[sodium diuranate]], and phosphate as [[trisodium phosphate]]. This crystallises trisodium phosphate decahydrate when cooled below 60&nbsp;°C; uranium impurities in this product increase with the amount of [[silicon dioxide]] in the reaction mixture, necessitating recrystallisation before commercial use. The hydroxides are dissolved at 80&nbsp;°C in 37% hydrochloric acid. Filtration of the remaining precipitates followed by addition of 47% sodium hydroxide results in the precipitation of thorium and uranium at about pH&nbsp;5.8. Complete drying of the precipitate must be avoided, as air may oxidise cerium from the +3 to the +4 oxidation state, and the cerium(IV) formed can liberate free [[chlorine]] from the hydrochloric acid. The rare earths again precipitate out at higher pH. The precipitates are neutralised by the original sodium hydroxide solution, although most of the phosphate must first be removed to avoid precipitating rare-earth phosphates. [[Solvent extraction]] may also be used to separate out the thorium and uranium, by dissolving the resultant filter cake in nitric acid. The presence of [[titanium hydroxide]] is deleterious as it binds thorium and prevents it from dissolving fully.{{sfn|Stoll|2005|p=8}}

===Purification===
High thorium concentrations are needed in nuclear applications. In particular, concentrations of atoms with high neutron capture [[cross-section (physics)|cross-sections]] must be very low (for example, [[gadolinium]] concentrations must be lower than one part per million by weight). Previously, repeated dissolution and recrystallisation was used to achieve high purity. Today, liquid solvent extraction procedures involving selective [[complexation]] of {{chem2|Th(4+)}} are used. For example, following alkaline digestion and the removal of phosphate, the resulting nitrato complexes of thorium, uranium, and the rare earths can be separated by extraction with [[tributyl phosphate]] in [[kerosene]].{{sfn|Stoll|2005|p=8}}

==Modern applications==
Non-radioactivity-related uses of thorium have been in decline since the 1950s{{sfn|Stoll|2005|p=32}} due to environmental concerns largely stemming from the radioactivity of thorium and its decay products.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=52–53}}<ref name="Furuta" />

Most thorium applications use its dioxide (sometimes called "thoria" in the industry), rather than the metal. This compound has a melting point of 3300&nbsp;°C (6000&nbsp;°F), the highest of all known oxides; only a few substances have higher melting points.<ref name="Emsley2011" /> This helps the compound remain solid in a flame, and it considerably increases the brightness of the flame; this is the main reason thorium is used in [[gas mantle|gas lamp mantles]].{{sfn|Stoll|2005|p=31}} All substances emit energy (glow) at high temperatures, but the light emitted by thorium is nearly all in the [[visible spectrum]], hence the brightness of thorium mantles.<ref name="Ivey">{{cite journal |first=H.F. |last=Ivey |year=1974 |title=Candoluminescence and radical-excited luminescence |journal=Journal of Luminescence |volume=8 |issue=4 |pages=271–307 |doi=10.1016/0022-2313(74)90001-5 |bibcode=1974JLum....8..271I}}</ref>

Energy, some of it in the form of visible light, is emitted when thorium is exposed to a source of energy itself, such as a cathode ray, heat, or [[ultraviolet light]]. This effect is shared by cerium dioxide, which converts ultraviolet light into visible light more efficiently, but thorium dioxide gives a higher flame temperature, emitting less [[infrared light]].{{sfn|Stoll|2005|p=31}} Thorium in mantles, though still common, has been progressively replaced with yttrium since the late 1990s.<ref name="Matson2011">{{cite book |last=Matson |first=Tim |year=2011 |title=The Book of Non-electric Lighting: The classic guide to the safe use of candles, fuel lamps, lanterns, gaslights, & fire-view stoves |page=60 |publisher=[[Countryman Press]] |isbn=978-1-58157-829-4}}</ref> According to the 2005 review by the United Kingdom's [[National Radiological Protection Board]], "although [thoriated gas mantles] were widely available a few years ago, they are not any more."<ref>{{cite web |last1=Shaw |first1=J. |last2=Dunderdale |first2=J. |last3=Paynter |first3=R.A. |date=9 June 2005 |title=A review of consumer products containing radioactive substances in the European Union |website=NRPB Occupational Services Department |url=https://ec.europa.eu/energy/sites/ener/files/documents/139.pdf |access-date=21 July 2017 |archive-date=10 March 2021 |archive-url=https://web.archive.org/web/20210310074604/https://ec.europa.eu/energy/sites/ener/files/documents/139.pdf |url-status=live }}</ref> Thorium is also used to make cheap permanent [[negative ion generator]]s, such as in [[pseudoscientific]] health bracelets.<ref>{{cite web | title="Negative Ion" Technology—What You Should Know | website=U.S. Nuclear Regulatory Commission | date=28 July 2014 | url=https://public-blog.nrc-gateway.gov/2014/07/28/negative-ion-technology-what-you-should-know/ | access-date=16 June 2021 | archive-date=8 December 2014 | archive-url=https://web.archive.org/web/20141208010325/https://public-blog.nrc-gateway.gov/2014/07/28/negative-ion-technology-what-you-should-know/ | url-status=dead }}</ref>

During the production of [[incandescent]] filaments, [[Recrystallization (chemistry)|recrystallisation]] of tungsten is significantly lowered by adding small amounts of thorium dioxide to the tungsten [[sintering]] powder before drawing the filaments.{{sfn|Stoll|2005|p=32}} A small addition of thorium to tungsten [[hot cathode|thermocathodes]] considerably reduces the [[work function]] of electrons; as a result, electrons are emitted at considerably lower temperatures.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=52–53}} Thorium forms a one-atom-thick layer on the surface of tungsten. The work function from a thorium surface is lowered possibly because of the electric field on the interface between thorium and tungsten formed due to thorium's greater electropositivity.<ref>{{cite book |last=Pridham |first=G.J. |year=2016 |title=Electronic Devices and Circuits |series=The Commonwealth and International Library: Electrical Engineering Division |page=105 |publisher=Elsevier |isbn=978-1-4831-3979-1 |language=en}}</ref> Since the 1920s, thoriated tungsten wires have been used in electronic tubes and in the cathodes and anticathodes of X-ray tubes and rectifiers.The reactivity of thorium with atmospheric oxygen required the introduction of an evaporated [[magnesium]] layer as a [[getter]] for impurities in the evacuated tubes, giving them their characteristic metallic inner coating.<ref>{{Cite book |last=Stokes |first=John W. |title=70 years of radio tubes and valves: a guide for electronic engineers, historians, and collectors |date=1982 |publisher=Vestal Press |isbn=978-0-911572-27-8 |location=Vestal, N.Y}}</ref>{{rp|16}} The introduction of transistors in the 1950s significantly diminished this use, but not entirely.{{sfn|Stoll|2005|p=32}} Thorium dioxide is used in [[gas tungsten arc welding]] (GTAW) to increase the high-temperature strength of tungsten electrodes and improve arc stability.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=52–53}} Thorium oxide is being replaced in this use with other oxides, such as those of zirconium, cerium, and [[lanthanum]].<ref>{{cite book |last=Uttrachi |first=J. |year=2015 |title=Weld Like a Pro: Beginning to advanced techniques |publisher=CarTech Inc. |page=42 |isbn=978-1-61325-221-5}}</ref><ref>{{cite book |last=Jeffus |first=L. |year=2016 |title=Welding: Principles and Applications |publisher=Cengage Learning |page=393 |isbn=978-1-305-49469-5}}</ref>

Thorium dioxide is found in [[refractory]] ceramics, such as high-temperature laboratory [[crucible]]s,{{sfn|Wickleder|Fourest|Dorhout|2006|pp=52–53}} either as the primary ingredient or as an addition to [[zirconium dioxide]]. An alloy of 90% [[platinum]] and 10% thorium is an effective catalyst for oxidising [[ammonia]] to nitrogen oxides, but this has been replaced by an alloy of 95% platinum and 5% [[rhodium]] because of its better mechanical properties and greater durability.{{sfn|Stoll|2005|p=32}}

[[File:Yellowing of thorium lenses.jpg|left|thumb|alt=Three lenses from yellowed to transparent left-to-right|Yellowed thorium dioxide lens (left), a similar lens partially de-yellowed with ultraviolet radiation (centre), and lens without yellowing (right)]]
When added to [[glass]], thorium dioxide helps increase its [[refractive index]] and decrease [[dispersion (optics)|dispersion]]. Such glass finds application in high-quality [[lens (optics)|lenses]] for cameras and scientific instruments.<ref name="CRC" /> The radiation from these lenses can darken them and turn them yellow over a period of years and it degrades film, but the health risks are minimal.<ref>{{cite web |publisher=Oak Ridge Associated Universities |year=2021 |title=Thoriated Camera Lens (ca. 1970s) |url=https://www.orau.org/health-physics-museum/collection/consumer/products-containing-thorium/camera-lens.html |access-date=11 October 2021 |archive-date=24 June 2021 |archive-url=https://web.archive.org/web/20210624165739/https://www.orau.org/health-physics-museum/collection/consumer/products-containing-thorium/camera-lens.html |url-status=live }}</ref> Yellowed lenses may be restored to their original colourless state by lengthy exposure to intense ultraviolet radiation. Thorium dioxide has since been replaced in this application by rare-earth oxides, such as [[Lanthanum oxide#lanthanum glass anchor|lanthanum]], as they provide similar effects and are not radioactive.{{sfn|Stoll|2005|p=32}}

Thorium tetrafluoride is used as an anti-reflection material in multilayered optical coatings. It is transparent to electromagnetic waves having wavelengths in the range of 0.350–12&nbsp;μm, a range that includes near ultraviolet, visible and [[Infrared|mid infrared]] light. Its radiation is primarily due to alpha particles, which can be easily stopped by a thin cover layer of another material.<ref>{{cite book |last=Rancourt |first=J.D. |year=1996 |title=Optical Thin Films |page=196 |series=User Handbook |publisher=[[SPIE Press]] |isbn=978-0-8194-2285-9}}</ref> Replacements for thorium tetrafluoride are being developed as of the 2010s,<ref>{{cite book |last1=Kaiser |first1=N. |last2=Pulker |first2=H. K. |year=2013 |title=Optical Interference Coatings |page=111 |publisher=Springer |isbn=978-3-540-36386-6 |language=en}}</ref> which include [[Lanthanum trifluoride#la fl coating anchor|Lanthanum trifluoride]].

[[Mag-Thor]] alloys (also called thoriated magnesium) found use in some aerospace applications, though such uses have been phased out due to concerns over radioactivity.

==Potential use for nuclear energy==
{{Main|Thorium-based nuclear power|Thorium fuel cycle}}
The main nuclear power source in a reactor is the neutron-induced fission of a nuclide; the synthetic fissile{{efn|name="fissionable"}} nuclei <sup>233</sup>U and <sup>239</sup>Pu can be [[breeder reactor|bred]] from neutron capture by the naturally occurring quantity nuclides <sup>232</sup>Th and <sup>238</sup>U. <sup>235</sup>U occurs naturally in significant amounts and is also fissile.<ref>{{cite journal |last1=Ronen |first1=Yigal |title=A Rule for Determining Fissile Isotopes |journal=Nuclear Science and Engineering |date=March 2006 |volume=152 |issue=3 |pages=334–335 |doi=10.13182/nse06-a2588 |bibcode=2006NSE...152..334R |s2cid=116039197 }}</ref><ref name="anucene">{{Cite journal |last1= Ronen |first1= Y. |title= Some remarks on the fissile isotopes |doi= 10.1016/j.anucene.2010.07.006 |journal= Annals of Nuclear Energy |volume= 37 |issue= 12 |pages= 1783–1784 |year= 2010 |bibcode= 2010AnNuE..37.1783R }}</ref>{{efn|The thirteen fissile actinide isotopes with half-lives over a year are <sup>229</sup>Th, <sup>233</sup>U, <sup>235</sup>U, [[neptunium-236|<sup>236</sup>Np]], <sup>239</sup>Pu, [[plutonium-241|<sup>241</sup>Pu]], [[americium-242m|<sup>242m</sup>Am]], [[curium-243|<sup>243</sup>Cm]], [[curium-245|<sup>245</sup>Cm]], [[curium-247|<sup>247</sup>Cm]], [[californium-249|<sup>249</sup>Cf]], [[californium-251|<sup>251</sup>Cf]], and [[einsteinium-252|<sup>252</sup>Es]]. Of these, only <sup>235</sup>U have significant amounts in nature, and only <sup>233</sup>U and <sup>239</sup>Pu can be bred from naturally occurring nuclei with a single neutron capture.<ref name="anucene" />}} In the thorium fuel cycle, the fertile isotope <sup>232</sup>Th is bombarded by [[slow neutron]]s, undergoing neutron capture to become <sup>233</sup>Th, which undergoes two consecutive beta decays to become first [[protactinium-233|<sup>233</sup>Pa]] and then the fissile <sup>233</sup>U:{{sfn|Wickleder|Fourest|Dorhout|2006|pp=52–53}}

:<chem>{^{232}_{90}Th} ->[\text{(n,}\gamma\text{)}] {^{233}_{90}Th}->[\beta^-][\text{21.8 min}] {^{233}_{91}Pa} ->[\beta^-][\text{27 days}] {^{233}_{92}U} \ (->[\alpha][1.60 \times 10^5\text{years}]) </chem>

{{Thorium Cycle Transmutation}}
<sup>233</sup>U is fissile and can be used as a nuclear fuel in the same way as <sup>235</sup>U or [[plutonium-239|<sup>239</sup>Pu]]. When <sup>233</sup>U undergoes nuclear fission, the neutrons emitted can strike further <sup>232</sup>Th nuclei, continuing the cycle.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=52–53}} This parallels the uranium fuel cycle in [[fast breeder reactor]]s where <sup>238</sup>U undergoes neutron capture to become <sup>239</sup>U, beta decaying to first <sup>239</sup>Np and then fissile <sup>239</sup>Pu.<ref>{{cite web|url=http://www.world-nuclear.org/information-library/nuclear-fuel-cycle/fuel-recycling/plutonium.aspx|title=Plutonium|year=2017|publisher=World Nuclear Association|access-date=29 September 2017|archive-date=5 October 2017|archive-url=https://web.archive.org/web/20171005061205/http://www.world-nuclear.org/information-library/nuclear-fuel-cycle/fuel-recycling/plutonium.aspx|url-status=live}}</ref>

The fission of {{Nuclide|Uranium|233}} produces 2.48 neutrons on average.<ref>{{cite web|url=https://www.nuclear-power.com/nuclear-power-plant/nuclear-fuel/uranium/uranium-233/uranium-233-fission/|title=Uranium 233 Fission|year=2023|publisher=Nuclear Power|access-date=28 April 2023}}</ref>
One neutron is needed to keep the fission reaction going. For a self-contained continuous breeding cycle, one more neutron is needed to breed a new {{Nuclide|Uranium|233}} atom from the fertile  {{Nuclide|Thorium|232}}. This leaves a margin of 0.45 neutrons (or 18% of the neutron flux) for losses.

===Advantages===
Thorium is more abundant than uranium, and can satisfy world energy demands for longer.{{sfn|Greenwood|Earnshaw|1997|p=1259}} It is particularly suitable for being used as fertile material in [[molten salt reactor]]s.

<sup>232</sup>Th absorbs neutrons more readily than <sup>238</sup>U, and <sup>233</sup>U has a higher probability of fission upon neutron capture (92.0%) than <sup>235</sup>U (85.5%) or <sup>239</sup>Pu (73.5%).<ref>{{cite web |url=http://www.nndc.bnl.gov/chart/reColor.jsp?newColor=sigf |title=Interactive Chart of Nuclides |publisher=[[Brookhaven National Laboratory]] |access-date=12 August 2013 |archive-date=24 January 2017 |archive-url=https://web.archive.org/web/20170124175936/http://www.nndc.bnl.gov/chart/reColor.jsp?newColor=sigf |url-status=dead }}</ref> It also releases more neutrons upon fission on average.{{sfn|Greenwood|Earnshaw|1997|p=1259}} A single neutron capture by <sup>238</sup>U produces transuranic waste along with the fissile <sup>239</sup>Pu, but <sup>232</sup>Th only produces this waste after five captures, forming <sup>237</sup>Np. This number of captures does not happen for 98–99% of the <sup>232</sup>Th nuclei because the intermediate products <sup>233</sup>U or <sup>235</sup>U undergo fission, and fewer long-lived transuranics are produced. Because of this, thorium is a potentially attractive alternative to uranium in [[MOX fuel|mixed oxide fuels]] to minimise the generation of transuranics and maximise the destruction of [[plutonium]].<ref name="wnn-20130621">{{cite news |url=http://www.world-nuclear-news.org/ENF_Thorium_test_begins_2106131.html |title=Thorium test begins |publisher=World Nuclear News |year=2013 |access-date=21 July 2013 |archive-date=19 July 2013 |archive-url=https://web.archive.org/web/20130719084439/http://www.world-nuclear-news.org/ENF_Thorium_test_begins_2106131.html |url-status=live }}</ref>

Thorium fuels result in a safer and better-performing [[reactor core]]{{sfn|Wickleder|Fourest|Dorhout|2006|pp=52–53}} because thorium dioxide has a higher melting point, higher [[thermal conductivity]], and a lower [[coefficient of thermal expansion]]. It is more stable chemically than the now-common fuel uranium dioxide, because the latter oxidises to [[triuranium octoxide]] ({{chem2|U3O8}}), becoming substantially less dense.<ref name="Thorium Fuel Cycle – Potential Benefits and Challenges">{{cite web|url=http://www-pub.iaea.org/MTCD/publications/PDF/TE_1450_web.pdf|publisher=International Atomic Energy Agency|title=IAEA-TECDOC-1450 Thorium Fuel Cycle – Potential Benefits and Challenges|date=2005|access-date=23 March 2009|archive-date=4 August 2016|archive-url=https://web.archive.org/web/20160804054758/http://www-pub.iaea.org/MTCD/publications/PDF/TE_1450_web.pdf|url-status=live}}</ref>

===Disadvantages===
The used fuel is difficult and dangerous to reprocess because many of the daughters of <sup>232</sup>Th and <sup>233</sup>U are strong gamma emitters.{{sfn|Greenwood|Earnshaw|1997|p=1259}} All <sup>233</sup>U production methods result in impurities of [[uranium-232|<sup>232</sup>U]], either from parasitic knock-out (n,2n) reactions on <sup>232</sup>Th, <sup>233</sup>Pa, or <sup>233</sup>U that result in the loss of a neutron, or from double neutron capture of <sup>230</sup>Th, an impurity in natural <sup>232</sup>Th:<ref name="Intro2WMD" />
:{{nuclide|Th|230}} + n → {{nuclide|Th|231}} + {{math|γ}} {{overunderset|→|''β''<sup>−</sup>|25.5 h}} {{nuclide|Pa|231}} ( {{overunderset|→|''α''|3.28 × 10{{su|p=4}} y}} {{nuclide|Ac|227}} )
:{{nuclide|Pa|231}} + n → {{nuclide|Pa|232}} + {{math|γ}} {{overunderset|→|''β''<sup>−</sup>|1.3 d}} {{nuclide|U|232}} {{overunderset|→|''α''|69 y}}

<sup>232</sup>U by itself is not particularly harmful, but quickly decays to produce the strong gamma emitter [[Isotopes of thallium|<sup>208</sup>Tl]]. (<sup>232</sup>Th follows the same decay chain, but its much longer half-life means that the quantities of <sup>208</sup>Tl produced are negligible.){{sfn|Stoll|2005|p=30}} These impurities of <sup>232</sup>U make <sup>233</sup>U easy to detect and dangerous to work on, and the impracticality of their separation limits the possibilities of [[nuclear proliferation]] using <sup>233</sup>U as the fissile material.<ref name="Intro2WMD">{{cite book |title= Introduction to Weapons of Mass Destruction: Radiological, Chemical, and Biological |last= Langford |first= R. E. |year= 2004 |publisher= John Wiley & Sons |isbn=978-0-471-46560-7 |page=85 }}</ref> <sup>233</sup>Pa has a relatively long half-life of 27&nbsp;days and a high [[cross section (physics)|cross section]] for neutron capture. Thus it is a [[neutron poison]]: instead of rapidly decaying to the useful <sup>233</sup>U, a significant amount of <sup>233</sup>Pa converts to <sup>234</sup>U and consumes neutrons, degrading [[neutron economy|the reactor efficiency]]. To avoid this, <sup>233</sup>Pa is extracted from the active zone of thorium [[molten salt reactor]]s during their operation, so that it does not have a chance to capture a neutron and will only decay to <sup>233</sup>U.<ref name="NakajimaGroult2005">{{cite book|last1=Nakajima|first1=Ts.|last2=Groult|first2=H.|title=Fluorinated Materials for Energy Conversion|year=2005|publisher=Elsevier|isbn=978-0-08-044472-7|pages=562–565}}</ref>

The irradiation of <sup>232</sup>Th with neutrons, followed by its processing, need to be mastered before these advantages can be realised, and this requires more advanced technology than the uranium and plutonium fuel cycle;{{sfn|Wickleder|Fourest|Dorhout|2006|pp=52–53}} research continues in this area. Others cite the low commercial viability of the thorium fuel cycle:<ref>{{cite news|url=https://www.theguardian.com/environment/2011/jun/23/thorium-nuclear-uranium|title=Don't believe the spin on thorium being a greener nuclear option|last=Rees|first=E.|year=2011|newspaper=[[The Guardian]]|access-date=29 September 2017|archive-date=27 September 2017|archive-url=https://web.archive.org/web/20170927111531/https://www.theguardian.com/environment/2011/jun/23/thorium-nuclear-uranium|url-status=live}}</ref><ref name="SovacoolValentine2012">{{cite book|last1=Sovacool |first1=B. K. |last2=Valentine |first2=S. V. |title=The National Politics of Nuclear Power: Economics, Security, and Governance|date=2012|publisher=[[Routledge]]|isbn=978-1-136-29437-2|page=226}}</ref><ref>{{cite web |url=http://www.ne.anl.gov/pdfs/NuclearEnergyFAQ.pdf |title=Nuclear Energy FAQs |publisher=[[Argonne National Laboratory]] |year=2014 |access-date=13 January 2018 |archive-date=7 October 2014 |archive-url=https://web.archive.org/web/20141007005609/http://www.ne.anl.gov/pdfs/NuclearEnergyFAQ.pdf |url-status=live }}</ref> the international [[Nuclear Energy Agency]] predicts that the thorium cycle will never be commercially viable while uranium is available in abundance—a situation which may persist "in the coming decades".<ref name="FindlayFindlay2011">{{cite book|last=Findlay |first=T. |author-link=Trevor Findlay |title=Nuclear Energy and Global Governance: Ensuring Safety, Security and Non-proliferation|date=2011|publisher=Routledge|isbn=978-1-136-84993-0|page=9}}</ref> The isotopes produced in the thorium fuel cycle are mostly not transuranic, but some of them are still very dangerous, such as <sup>231</sup>Pa, which has a half-life of 32,760&nbsp;years and is a major contributor to the long-term [[radiotoxic]]ity of spent nuclear fuel.<ref name="NakajimaGroult2005" />

==Hazards and health effects==
[[File:PSM V74 D233 Thorium radioactive incandescent gas mantle placed above plant seeds.png|thumb|alt=Thorium mantle installed over a small sprout of grass|Experiment on the effect of radiation (from an unburned thorium gas mantle) on the germination and growth of [[timothy-grass]] seed]]

===Radiological===

Natural thorium decays very slowly compared to many other radioactive materials, and the emitted [[alpha radiation]] cannot penetrate human skin. As a result, handling small amounts of thorium, such as those in gas mantles, is considered safe, although the use of such items may pose some risks.<ref name="epa">{{cite web|title=Thorium: Radiation Protection|url=http://www.epa.gov/radiation/radionuclides/thorium.html|archive-url=https://web.archive.org/web/20061001225000/http://www.epa.gov/radiation/radionuclides/thorium.htm|url-status=dead|publisher=United States Environmental Protection Agency|access-date=27 February 2016|archive-date=1 October 2006}}</ref> Exposure to an aerosol of thorium, such as contaminated dust, can lead to increased risk of [[cancer]]s of the [[lung]], [[pancreas]], and [[blood]], as lungs and other internal organs can be penetrated by alpha radiation.<ref name="epa" /> Internal exposure to thorium leads to increased risk of [[liver]] diseases.<ref name="arpansa" />

The decay products of <sup>232</sup>Th include more dangerous radionuclides such as radium and radon. Although relatively little of those products are created as the result of the slow decay of thorium, a proper assessment of the radiological toxicity of <sup>232</sup>Th must include the contribution of its daughters, some of which are dangerous [[gamma radiation|gamma]] emitters,<ref>{{cite web|url=http://gonuke.org/ComprehensiveTeachingToolkits/Radiation%20Protection/ChSCC_RP/Columbia%20Basin%20RPT-111/Supplementary%20materials/natural-decay-series.pdf |title=Natural Decay Series: Uranium, Radium, and Thorium |publisher=Argonne National Laboratory |year=2005 |archive-url=https://web.archive.org/web/20160817010031/http://gonuke.org/ComprehensiveTeachingToolkits/Radiation%20Protection/ChSCC_RP/Columbia%20Basin%20RPT-111/Supplementary%20materials/natural-decay-series.pdf |archive-date=17 August 2016 |access-date=30 September 2017}}</ref> and which are built up quickly following the initial decay of <sup>232</sup>Th due to the absence of long-lived nuclides along the decay chain.{{sfn|Stoll|2005|p=35}} As the dangerous daughters of thorium have much lower melting points than thorium dioxide, they are volatilised every time the mantle is heated for use. In the first hour of use large fractions of the thorium daughters <sup>224</sup>Ra, <sup>228</sup>Ra, <sup>212</sup>Pb, and <sup>212</sup>Bi are released.<ref>{{cite journal |title=Radioactivity released from burning gas lantern mantles |first1=J. W. |last1=Luetzelschwab |first2=S. W. |last2=Googins |date=1984 |journal=Health Phys. |volume=46 |issue=4 |pages=873–881 |pmid=6706595 |doi=10.1097/00004032-198404000-00013|bibcode=1984HeaPh..46..873L }}</ref> Most of the radiation dose by a normal user arises from inhaling the radium, resulting in a radiation dose of up to 0.2&nbsp;[[sievert|millisieverts]] per use, about a third of the dose sustained during a [[mammogram]].<ref>{{cite journal |last1=Huyskens |first1=C. J. |last2=Hemelaar |first2=J. T. |last3=Kicken |first3=P. J. |date=1985 |title=Dose estimates for exposure to radioactivity in gas mantles |journal=Sci. Total Environ. |volume=45 |pages=157–164 |pmid=4081711|bibcode=1985ScTEn..45..157H |doi=10.1016/0048-9697(85)90216-5 |s2cid=39901914 |url=https://research.tue.nl/nl/publications/dose-estimates-for-exposure-to-radioactivity-in-gas-mantles(0c586a27-db9f-44ac-b8a8-ef94155f9c6a).html }}</ref>

Some [[nuclear safety]] agencies make recommendations about the use of thorium mantles and have raised safety concerns regarding their [[Gas mantle#Safety concerns|manufacture]] and disposal; the radiation dose from one mantle is not a serious problem, but that from many mantles gathered together in factories or landfills is.<ref name="arpansa">{{cite web|archive-url=https://web.archive.org/web/20071014211034/http://arpansa.gov.au/RadiationProtection/Factsheets/is_lantern.cfm |url=http://arpansa.gov.au/RadiationProtection/Factsheets/is_lantern.cfm |title=Radioactivity in Lantern Mantles |publisher=[[Australian Radiation Protection and Nuclear Safety Agency]] |archive-date=14 October 2007 |access-date=29 September 2017}}</ref>

===Biological===
Thorium is odourless and tasteless.<ref>{{cite web|url=https://www.atsdr.cdc.gov/toxprofiles/tp147.pdf|title=Toxicological Profile for Thorium|publisher=Agency for Toxic Substances and Disease Registry U.S. Public Health Service|date=1990|page=4|access-date=5 September 2016|archive-date=21 October 2004|archive-url=https://web.archive.org/web/20041021114032/https://www.atsdr.cdc.gov/toxprofiles/tp147.pdf|url-status=live}}</ref> The chemical toxicity of thorium is low because thorium and its most common compounds (mostly the dioxide) are poorly soluble in water,<ref name="Schneckenstein">{{cite report|last1=Merkel |first1=B. |last2=Dudel |first2=G. |display-authors=etal |year=1988 |archive-url=https://web.archive.org/web/20130108094057/http://www.geo.tu-freiberg.de/~merkel/schneckenstein.PDF |archive-date=8 January 2013 |url=http://www.geo.tu-freiberg.de/~merkel/schneckenstein.PDF |title=Untersuchungen zur radiologischen Emission des Uran-Tailings Schneckenstein |publisher=Sächsisches Staatsministerium für Umwelt und Landesentwicklung |language=de}}</ref> precipitating out before entering the body as the hydroxide.{{sfn|Stoll|2005|p=34}} Some thorium compounds are chemically moderately [[toxic]], especially in the presence of strong complex-forming ions such as citrate that carry the thorium into the body in soluble form.{{sfn|Stoll|2005|p=35}} If a thorium-containing object has been chewed or sucked, it loses 0.4% of thorium and 90% of its dangerous daughters to the body.<ref name="Poljanc" /> Three-quarters of the thorium that has penetrated the body accumulates in the [[skeleton]]. Absorption through the skin is possible, but is not a likely means of exposure.<ref name="epa" /> Thorium's low solubility in water also means that excretion of thorium by the kidneys and faeces is rather slow.{{sfn|Stoll|2005|p=35}}

Tests on the thorium uptake of workers involved in monazite processing showed thorium levels above recommended limits in their bodies, but no adverse effects on health were found at those moderately low concentrations. No chemical toxicity has yet been observed in the [[tracheobronchial tract]] and the lungs from exposure to thorium.{{sfn|Stoll|2005|p=34}} People who work with thorium compounds are at a risk of [[dermatitis]]. It can take as much as thirty years after the ingestion of thorium for symptoms to manifest themselves.<ref name="Emsley2011" /> Thorium has no known biological role.<ref name="Emsley2011" />

===Chemical===
Powdered thorium metal is pyrophoric: it ignites spontaneously in air.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=61–63}} In 1964, the [[United States Department of the Interior]] listed thorium as "severe" on a table entitled "Ignition and explosibility of metal powders". Its ignition temperature was given as 270&nbsp;°C (520&nbsp;°F) for dust clouds and 280&nbsp;°C (535&nbsp;°F) for layers. Its minimum explosive concentration was listed as 0.075&nbsp;oz/cu ft (0.075&nbsp;kg/m<sup>3</sup>); the minimum igniting energy for (non-submicron) dust was listed as 5&nbsp;[[Joule|mJ]].<ref>{{cite report|url=http://apps.dtic.mil/dtic/tr/fulltext/u2/b270510.pdf|archive-url=https://web.archive.org/web/20160803175920/http://www.dtic.mil/dtic/tr/fulltext/u2/b270510.pdf|url-status=live|archive-date=3 August 2016|title=Explosibility of metal powders|first1=M. |last1=Jacobson |first2=A. R. |last2=Cooper |first3=J. |last3=Nagy |publisher=[[United States Department of the Interior]] |year=1964 |access-date=29 September 2017}}</ref>

In 1956, the [[Sylvania Electric Products explosion]] occurred during reprocessing and burning of thorium sludge in [[New York City]], United States. Nine people were injured; one died of complications caused by [[third-degree burns]].<ref name="AP">{{cite news |agency=Associated Press |url=https://news.google.com/newspapers?nid=1129&dat=19560703&id=eSgNAAAAIBAJ&sjid=BmwDAAAAIBAJ&pg=4985,347025 |title=Nine Injured in Atomic Lab Blasts |newspaper=[[Pittsburgh Post-Gazette]] |year=1956 |page=2 |access-date=29 September 2017 |archive-date=16 March 2020 |archive-url=https://web.archive.org/web/20200316201104/https://news.google.com/newspapers?nid=1129&dat=19560703&id=eSgNAAAAIBAJ&sjid=BmwDAAAAIBAJ&pg=4985%2C347025 |url-status=live }}</ref><ref>{{cite news |agency=Associated Press |url=https://news.google.com/newspapers?nid=888&dat=19560703&id=n6RSAAAAIBAJ&sjid=M3YDAAAAIBAJ&pg=7412,1311234 |title=No Radiation Threat Seen in A-laboratory Blast |newspaper=[[The St. Petersburg Times (Russia)|The St. Petersburg Times]] |date=1956 |page=2 |access-date=29 September 2017 |archive-date=16 March 2020 |archive-url=https://web.archive.org/web/20200316213830/https://news.google.com/newspapers?nid=888&dat=19560703&id=n6RSAAAAIBAJ&sjid=M3YDAAAAIBAJ&pg=7412%2C1311234 |url-status=live }}</ref><ref name="Newsday">{{cite news|last=Harrington|first=M.|url=http://www.newsday.com/columnists/glenn-gamboa/2.1091/sad-memories-of-56-sylvania-explosion-1.451270|title=Sad Memories of '56 Sylvania Explosion|work=[[New York Newsday]]|date=2003|archive-url=https://web.archive.org/web/20120204183944/http://www.newsday.com/columnists/glenn-gamboa/2.1091/sad-memories-of-56-sylvania-explosion-1.451270|archive-date=4 February 2012|access-date=29 September 2017}}</ref>

===Exposure routes===
Thorium exists in very small quantities everywhere on Earth although larger amounts exist in certain parts: the average human contains about 40&nbsp;[[microgram]]s of thorium and typically consumes three micrograms per day.<ref name="Emsley2011" /> Most thorium exposure occurs through dust inhalation; some thorium comes with food and water, but because of its low solubility, this exposure is negligible.{{sfn|Stoll|2005|p=35}}

Exposure is raised for people who live near thorium deposits or radioactive waste disposal sites, those who live near or work in uranium, phosphate, or tin processing factories, and for those who work in gas mantle production.<ref name="ATSDR">{{cite web |url=http://www.atsdr.cdc.gov/toxfaqs/tfacts147.pdf |title=Thorium ToxFAQs |publisher=[[Agency for Toxic Substances and Disease Registry]] |access-date=29 September 2017 |archive-date=5 June 2012 |archive-url=https://web.archive.org/web/20120605122824/http://www.atsdr.cdc.gov/toxfaqs/tfacts147.pdf |url-status=live }}</ref> Thorium is especially common in the [[Tamil Nadu]] coastal areas of India, where residents may be exposed to a naturally occurring radiation dose ten times higher than the worldwide average.<ref>{{cite web |url=http://www.dae.gov.in/iandm/minesback.htm |title=Compendium of Policy And Statutory Provisions Relating To Exploitation of Beach Sand Minerals |publisher=Department of Atomic Energy |access-date=19 December 2008 |archive-url=https://web.archive.org/web/20081204114125/http://www.dae.gov.in/iandm/minesback.htm |archive-date=4 December 2008 |url-status=dead}}</ref> It is also common in northern [[Brazil]]ian coastal areas, from south [[Bahia]] to [[Guarapari]], a city with radioactive monazite sand beaches, with radiation levels up to 50 times higher than world average background radiation.<ref>{{cite journal|journal=An. Acad. Bras. Ciênc.|year=1981|volume=53|issue=4|pages=683–691|pmid=7345962 |title=Measurements of environmental radiation exposure dose rates at selected sites in Brazil |author1=Pfeiffer, W. C.|author2=Penna-Franca, E.|author3=Ribeiro, C. C.|author4=Nogueira, A. R.|author5=Londres, H.|author6=Oliveira, A. E.}}</ref>

Another possible source of exposure is thorium dust produced at weapons testing ranges, as thorium is used in the guidance systems of some missiles. This has been blamed for a high incidence of birth defects and cancer at [[Salto di Quirra]] on the Italian island of [[Sardinia]].<ref>{{cite news |url=https://www.abc.net.au/news/2019-01-29/sardinia-military-weapons-testing-birth-defects/10759614 |title=Italian military officials' trial ignites suspicions of links between weapon testing and birth defects in Sardinia |last=Alberici |first=Emma |date=29 January 2019 |website=ABC News |publisher=Australian Broadcasting Corporation |access-date=29 January 2019 |archive-date=29 January 2019 |archive-url=https://web.archive.org/web/20190129165810/https://www.abc.net.au/news/2019-01-29/sardinia-military-weapons-testing-birth-defects/10759614 |url-status=live }}</ref>

==See also==
*[[Thorium Energy Alliance]]

== Explanatory notes ==
{{Notelist|30em}}

== Citations ==
{{Reflist|30em}}

== General and cited references ==
* {{cite book |last1=Greenwood |first1=N. N. |author-link1=Norman Greenwood |last2=Earnshaw |first2=A. |year=1997 |title=Chemistry of the Elements |edition=2nd |publisher=Butterworth-Heinemann |isbn=978-0-08-037941-8}}
* {{cite book |first=W. |last=Stoll |chapter=Thorium and Thorium Compounds |doi=10.1002/14356007.a27_001 |title=Ullmann's Encyclopedia of Industrial Chemistry |publisher=Wiley-VCH |year=2005 |isbn=978-3-527-31097-5}}
* {{cite book |doi=10.1007/1-4020-3598-5_3 |chapter=Thorium |title=The Chemistry of the Actinide and Transactinide Elements |year=2006 |last1=Wickleder |first1=Mathias S. |last2=Fourest |first2=Blandine |last3=Dorhout |first3=Peter K. |pages=52–160 |isbn=978-1-4020-3555-5 }}

==Further reading==
* {{cite web |first1=B. W. |last1=Jordan |first2=R. |last2=Eggert |first3=B. |last3=Dixon|first4=B. |last4=Carlsen |display-authors=3 |year=2014 |url=http://econbus.mines.edu/working-papers/wp201407.pdf |archive-url=https://web.archive.org/web/20170630194355/http://econbus.mines.edu/working-papers/wp201407.pdf |archive-date=30 June 2017 |title=Thorium: Does Crustal Abundance Lead to Economic Availability?|publisher=[[Colorado School of Mines]] |access-date=29 September 2017}}
* International Atomic Energy Agency (2005). [http://www-pub.iaea.org/mtcd/publications/pdf/te_1450_web.pdf Thorium fuel cycle – Potential benefits and challenges]

== External links ==
* {{Wiktionary inline}}
* {{Commons-inline}}

{{Periodic table (navbox)}}
{{Thorium compounds}}
{{Authority control}}

{{DEFAULTSORT:Thorium}}
[[Category:Thorium| ]]
[[Category:Actinides]]
[[Category:Carcinogens]]
[[Category:Chemical elements]]
[[Category:Chemical elements with face-centered cubic structure]]
[[Category:Nuclear fuels]]
[[Category:Nuclear materials]]
[[Category:Thor]]