View source for Fermium

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{{use dmy dates|date=March 2018}}
{{infobox fermium}}
'''Fermium''' is a [[synthetic element|synthetic chemical element]]; it has [[Chemical symbol|symbol]] '''Fm''' and [[atomic number]] 100. It is an [[actinide]] and the heaviest element that can be formed by [[neutron]] bombardment of lighter elements, and hence the last element that can be prepared in macroscopic quantities, although pure fermium metal has not been  prepared yet.<ref name="Silva" /> A total of 20 isotopes are known, with <sup>257</sup>Fm being the longest-lived with a half-life of 100.5 days.

Fermium was discovered in the debris of the [[Ivy Mike|first]] [[hydrogen bomb]] explosion in 1952, and named after [[Enrico Fermi]], one of the pioneers of [[nuclear physics]]. Its chemistry is typical for the late actinides, with a preponderance of the +3 [[oxidation state]] but also an accessible +2 oxidation state. Owing to the small amounts of produced fermium and all of its isotopes having relatively short half-lives, there are currently no uses for it outside basic scientific research.

==Discovery==
[[File:Ivy Mike - mushroom cloud.jpg|thumb|left|Fermium was first observed in the fallout from the ''Ivy Mike'' nuclear test.]]
[[File:Enrico Fermi 1943-49.jpg|thumb|left|The element was named after [[Enrico Fermi]].]]
[[File:Albert Ghiorso ca 1970.jpg|thumb|right|The element was discovered by a team headed by [[Albert Ghiorso]].]]
Fermium was first discovered in the fallout from the '[[Ivy Mike]]' nuclear test (1&nbsp;November 1952), the first successful test of a hydrogen bomb.<ref name="lanl">{{cite web| url=http://periodic.lanl.gov/elements/99.html|title=Einsteinium|access-date=2007-12-07|archive-url=https://web.archive.org/web/20071026052909/http://periodic.lanl.gov/elements/99.html <!--Added by H3llBot-->|archive-date=2007-10-26}}</ref><ref name="nrc">[http://www.nrc-cnrc.gc.ca/eng/education/elements/el/fm.html Fermium – National Research Council Canada] {{webarchive|url=https://web.archive.org/web/20101225025414/http://www.nrc-cnrc.gc.ca/eng/education/elements/el/fm.html |date=2010-12-25 }}. Retrieved 2 December 2007</ref><ref name="Ghiorso" /> Initial examination of the debris from the explosion had shown the production of a new isotope of [[plutonium]], [[plutonium-244|{{nuclide|Pu|Z=94|A=244}}]]: this could only have formed by the absorption of six [[neutron]]s by a [[uranium-238]] nucleus followed by two [[Beta decay|β<sup>−</sup>&nbsp;decays]]. At the time, the absorption of neutrons by a heavy nucleus was thought to be a rare process, but the identification of {{nuclide|Pu|Z=94|A=244}} raised the possibility that still more neutrons could have been absorbed by the uranium nuclei, leading to new elements.<ref name="Ghiorso">{{cite journal|first = Albert|last = Ghiorso|author-link = Albert Ghiorso|date = 2003 |title = Einsteinium and Fermium|journal = Chemical and Engineering News|url = http://pubs.acs.org/cen/80th/einsteiniumfermium.html|volume = 81|issue = 36|doi = 10.1021/cen-v081n036.p174|pages = 174–175|url-access = subscription}}</ref>

Element&nbsp;99 ([[einsteinium]]) was quickly discovered on filter papers which had been flown through clouds from the explosion (the same sampling technique that had been used to discover {{nuclide|Pu|Z=94|A=244}}).<ref name="Ghiorso" /> It was then identified in December 1952 by [[Albert Ghiorso]] and co-workers at the [[University of California at Berkeley]].<ref name="lanl" /><ref name="nrc" /><ref name="Ghiorso" /> They discovered the isotope <sup>253</sup>Es ([[half-life]] {{val|20.5|u=days}}) that was made by the [[neutron capture|capture]] of 15 [[neutron]]s by [[uranium-238]] nuclei – which then underwent seven successive [[beta decay]]s:
<blockquote>
{{NumBlk|:|<math chem>\ce{^{238}_{92}U ->[\begin{matrix} \\ +15\ce{n}\end{matrix}][\begin{matrix} 7\beta^- \\ \end{matrix}] {}^{253}_{99}Es}</math>|{{EquationRef|1}}}}
</blockquote>
Some <sup>238</sup>U atoms, however, could capture another amount of neutrons (most likely, 16 or 17).

The discovery of fermium ({{nowrap|''Z'' {{=}} 100}}) required more material, as the yield was expected to be at least an order of magnitude lower than that of element&nbsp;99, and so contaminated coral from the [[Enewetak atoll]] (where the test had taken place) was shipped to the [[University of California Radiation Laboratory]] in [[Berkeley, California]], for processing and analysis. About two months after the test, a new component was isolated emitting high-energy [[α-particle]]s ({{val|7.1|ul=MeV}}) with a [[half-life]] of about a day. With such a short half-life, it could only arise from the β<sup>−</sup>&nbsp;decay of an isotope of einsteinium, and so had to be an isotope of the new element&nbsp;100: it was quickly identified as <sup>255</sup>Fm ({{nowrap|''t'' {{=}} {{val|20.07|(7)|ul=hours}}}}).<ref name="Ghiorso" />

The discovery of the new elements, and the new data on neutron capture, was initially kept secret on the orders of the U.S.&nbsp;military until 1955 due to [[Cold War]] tensions.<ref name="Ghiorso" /><ref name = "PhysRev.99.1048" >{{cite journal
| last1 = Ghiorso
| first1 = A.
| last2 = Thompson
| first2 = S.
| last3 = Higgins
| first3 = G.
| last4 = Seaborg
| first4 = Glenn T.
| last5 = Studier
| first5 = M.
| last6 = Fields
| first6 = P.
| last7 = Fried
| first7 = S.
| last8 = Diamond
| first8 = H.
| last9 = Mech
| first9 = J.
| last10 = Pyle
| first10 = G.
| last11 = Huizenga
| first11 = J.
| last12 = Hirsch
| first12 = A.
| last13 = Manning
| first13 = W.
| last14 = Browne
| first14 = C.
| last15 = Smith
| first15 = H.
| last16 = Spence
| first16 = R.
| title = New Elements Einsteinium and Fermium, Atomic Numbers 99 and 100
| journal = Phys. Rev.
| volume = 99
| issue = 3
| doi = 10.1103/PhysRev.99.1048
| pages = 1048–1049
| date = 1955|bibcode = 1955PhRv...99.1048G | display-authors = 8
| url = https://cloudfront.escholarship.org/dist/prd/content/qt70q401ct/qt70q401ct.pdf
| doi-access = free
}}</ref><ref>Fields, P. R.; Studier, M. H.; Diamond, H.; Mech, J. F.; Inghram, M. G. Pyle, G. L.; Stevens, C. M.; Fried, S.; Manning, W. M. (Argonne National Laboratory, Lemont, Illinois); Ghiorso, A.; Thompson, S. G.; Higgins, G. H.; Seaborg, G. T. (University of California, Berkeley, California): "Transplutonium Elements in Thermonuclear Test Debris", in: {{cite journal|last1=Fields|first1=P.|last2=Studier|first2=M.|last3=Diamond|first3=H.|last4=Mech|first4=J.|last5=Inghram|first5=M.|last6=Pyle|first6=G.|last7=Stevens|first7=C.|last8=Fried|first8=S.|last9=Manning|first9=W. |last10=Ghiorso|first10=A.|last11=Thompson|first11=S.|last12=Higgins|first12=G.|last13=Seaborg|first13=G.|title=Transplutonium Elements in Thermonuclear Test Debris|journal=Physical Review|volume=102|issue=1|pages=180|date=1956|doi=10.1103/PhysRev.102.180|bibcode = 1956PhRv..102..180F }}</ref> Nevertheless, the Berkeley team was able to prepare elements 99 and 100 by civilian means, through the neutron bombardment of [[plutonium-239]], and published this work in 1954 with the disclaimer that it was not the first studies that had been carried out on the elements.<ref name = "PhysRev.93.908">{{cite journal|first1 = S. G.|last1 = Thompson |first2 = A.|last2 = Ghiorso|author-link2 = Albert Ghiorso|first3 = B. G.|last3 = Harvey|first4 = G. R.|last4 = Choppin|title = Transcurium Isotopes Produced in the Neutron Irradiation of Plutonium|journal = Physical Review|volume = 93|issue = 4|page = 908|date = 1954|doi = 10.1103/PhysRev.93.908|bibcode = 1954PhRv...93..908T |url = https://escholarship.org/content/qt2wj6c5kh/qt2wj6c5kh.pdf?t=p0wtgb |doi-access = free}}</ref><ref>{{cite journal|first1 = G. R.|last1 = Choppin|first2 = S. G.|last2 = Thompson|first3 = A.|last3 = Ghiorso|author-link3 = Albert Ghiorso|first4 = B. G.|last4 = Harvey|title = Nuclear Properties of Some Isotopes of Californium, Elements 99 and 100|journal = Physical Review|volume = 94|issue = 4|pages = 1080–1081|date = 1954|doi = 10.1103/PhysRev.94.1080|bibcode = 1954PhRv...94.1080C |doi-access = free}}</ref> The "Ivy Mike" studies were declassified and published in 1955.<ref name = "PhysRev.99.1048" />

The Berkeley team had been worried that another group might discover lighter isotopes of element&nbsp;100 through ion-bombardment techniques before they could publish their classified research,<ref name="Ghiorso" /> and this proved to be the case. A group at the Nobel Institute for Physics in Stockholm independently discovered the element, producing an [[isotope]] later confirmed to be <sup>250</sup>Fm ({{nowrap|''t''<sub>1/2</sub> {{=}} {{val|30|ul=minutes}}}}) by bombarding a {{nuclide|U|Z=92|A=238}} target with [[oxygen-16]] ions, and published their work in May 1954.<ref>{{cite journal|last1 = Atterling|first1 = Hugo|last2 = Forsling|first2 = Wilhelm|last3 = Holm |first3 = Lennart W.|last4 = Melander|first4 = Lars|last5 = Åström|first5 = Björn|date = 1954|title = Element 100 Produced by Means of Cyclotron-Accelerated Oxygen Ions|journal = Physical Review|volume = 95|issue = 2|pages = 585–586|doi = 10.1103/PhysRev.95.585.2|bibcode = 1954PhRv...95..585A }}</ref> Nevertheless, the priority of the Berkeley team was generally recognized, and with it the prerogative to name the new element in honour of [[Enrico Fermi]], the developer of the first artificial self-sustained nuclear reactor. Fermi was still alive when the name was proposed, but had died by the time it became official.<ref>{{cite book |last1=Hoffman |first1=D. C. |author-link=Darleane C. Hoffman |last2=Ghiorso |first2=A. |author-link2=Albert Ghiorso |last3=Seaborg |first3=G. T. |title=The Transuranium People: The Inside Story |year=2000 |publisher=[[World Scientific]] |isbn=978-1-78-326244-1 |pages=187–189}}</ref>

==Isotopes==
{{Main|Isotopes of fermium}}
[[File:Decay of Fermium-257.PNG|thumb|center|upright=3.5|Decay pathway of fermium-257]]
{{clear}}
There are 20 isotopes of fermium listed in N<small>UBASE</small>&nbsp;2016,{{NUBASE2016|ref}} with atomic weights of 241 to 260,{{efn|The discovery of {{sup|260}}Fm is considered "unproven" in N<small>UBASE</small>&nbsp;2003.<ref name="NUBASE2003">{{NUBASE 2003}}</ref>|name=Fm-260}} of which {{sup|257}}Fm is the longest-lived with a [[half-life]] of 100.5&nbsp;days. {{sup|253}}Fm has a half-life of 3 days, while {{sup|251}}Fm of 5.3&nbsp;h, {{sup|252}}Fm of 25.4&nbsp;h, {{sup|254}}Fm of 3.2&nbsp;h, {{sup|255}}Fm of 20.1&nbsp;h, and {{sup|256}}Fm of 2.6&nbsp;hours. All the remaining ones have half-lives ranging from 30 minutes to less than a millisecond.<ref name="NUBASE2003" />
The neutron capture product of fermium-257, {{sup|258}}Fm, undergoes [[spontaneous fission]] with a half-life of just 370(14)&nbsp;microseconds; {{sup|259}}Fm and {{sup|260}}Fm also undergo spontaneous fission (''t''<sub>1/2</sub>&nbsp;= 1.5(3)&nbsp;s and 4&nbsp;ms respectively).<ref name="NUBASE2003" /> This means that neutron capture cannot be used to create [[nuclide]]s with a [[mass number]] greater than 257, unless carried out in a nuclear explosion. As {{sup|257}}Fm [[alpha decay]]s to {{sup|253}}Cf, and no known fermium isotopes undergo [[beta minus decay]] to the next element, [[mendelevium]], fermium is also the last element that can be synthesized by neutron-capture.<ref name="Silva">{{cite book |last=Silva |first=Robert J. |chapter=Fermium, Mendelevium, Nobelium, and Lawrencium |title=The Chemistry of the Actinide and Transactinide Elements |editor1-first=Lester R. |editor1-last=Morss |editor2-first=Norman M. |editor2-last=Edelstein |editor3-first=Jean |editor3-last=Fuger |edition=3rd |date=2006 |volume=3 |publisher=Springer |location=Dordrecht |pages=1621–1651 |chapter-url=http://radchem.nevada.edu/classes/rdch710/files/Fm%20to%20Lr.pdf |doi=10.1007/1-4020-3598-5_13 |isbn=978-1-4020-3555-5 |archive-url=https://web.archive.org/web/20100717155410/http://radchem.nevada.edu/classes/rdch710/files/Fm%20to%20Lr.pdf |archive-date=2010-07-17}}</ref><ref name="G&E">{{Greenwood&Earnshaw1st|page=1262}}</ref><ref name="nuclidetable">{{cite web |url=http://www.nndc.bnl.gov/chart/reCenter.jsp?z=100&n=147 |title=Interactive Chart of Nuclides |publisher=Brookhaven National Laboratory |author=Sonzogni, Alejandro |location=National Nuclear Data Center |access-date=2008-06-06 |archive-date=21 June 2018 |archive-url=https://web.archive.org/web/20180621170934/http://www.nndc.bnl.gov/chart/reCenter.jsp?z=100&n=147 |url-status=dead }}</ref> Because of this impediment in forming heavier isotopes, these short-lived isotopes {{sup|258–260}}Fm constitute the "fermium gap."<ref name="Zagrebaev">{{cite journal|title=Future of superheavy element research: Which nuclei could be synthesized within the next few years?|url=http://nrv.jinr.ru/pdf_file/J_phys_2013.pdf|first1=Valeriy|last1=Zagrebaev|first2=Alexander|last2=Karpov|first3=Walter|last3=Greiner|date=2013|journal=Journal of Physics|volume=420|number=12001|page=11|doi=10.1088/1742-6596/420/1/012001|arxiv=1207.5700|bibcode=2013JPhCS.420a2001Z|s2cid=55434734}}</ref> 
{{clear}}

==Occurrence==
===Production===
[[File:Elutionskurven Fm Es Cf Bk Cm Am.png|thumb|[[Elution]]: chromatographic separation of Fm(100), Es(99), Cf, Bk, Cm and Am]]
Fermium is produced by the bombardment of lighter [[actinide]]s with [[neutron]]s in a nuclear reactor. Fermium-257 is the heaviest isotope that is obtained via neutron capture, and can only be produced in picogram quantities.{{efn|All isotopes of elements Z &gt; 100 can only be produced by accelerator-based nuclear reactions with charged particles and can be obtained only in tracer quantities (e.g., 1 million atoms for Md (''Z''&nbsp;{{=}}&nbsp;101) per hour of irradiation (see Silva 2006).}}<ref>{{cite book |last1=Luig |first1=Heribert |last2=Keller |first2=Cornelius |last3=Wolf |first3=Walter |last4=Shani |first4=Jashovam |last5=Miska |first5=Horst |last6=Zyball |first6=Alfred |last7=Gervé |first7=Andreas |last8=Balaban |first8=Alexandru T. |last9=Kellerer |first9=Albrecht M. |last10=Griebel |first10=Jürgen |title=Ullmann's Encyclopedia of Industrial Chemistry |date=2000 |doi=10.1002/14356007.a22_499 |chapter=Radionuclides |isbn=978-3527306732}}</ref> The major source is the 85&nbsp;MW [[High Flux Isotope Reactor]] (HFIR) at the [[Oak Ridge National Laboratory]] in [[Tennessee]], USA, which is dedicated to the production of transcurium (''Z''&nbsp;>&nbsp;96) elements.<ref>{{cite web|title = High Flux Isotope Reactor|url = http://neutrons.ornl.gov/facilities/HFIR/|publisher = Oak Ridge National Laboratory|access-date = 2010-09-23}}</ref> Lower mass fermium isotopes are available in greater quantities, though these isotopes (<sup>254</sup>Fm and <sup>255</sup>Fm) are comparatively short-lived. In a "typical processing campaign" at Oak Ridge, tens of grams of [[curium]] are irradiated to produce decigram quantities of [[californium]], milligram quantities of [[berkelium]] and [[einsteinium]], and picogram quantities of fermium.<ref>{{cite journal|first1 = C. E.|last1 = Porter| first2 = F. D. Jr. |last2 = Riley|first3 = R. D.|last3 = Vandergrift|first4 = L. K.|last4 = Felker|title = Fermium Purification Using Teva Resin Extraction Chromatography|journal = Sep. Sci. Technol.|volume = 32|issue = 1–4|date = 1997|pages = 83–92|doi = 10.1080/01496399708003188|url = https://zenodo.org/record/1234415}}</ref> However, nanogram<ref>{{cite journal|first1 = M.|last1 = Sewtz|first2 = H.|last2 = Backe|first3 = A.|last3 = Dretzke|first4 = G.|last4 = Kube|first5 = W.|last5 = Lauth|first6 = P.|last6 = Schwamb|first7 = K.|last7 = Eberhardt|first8 = C.|last8 = Grüning|first9 = P.|last10 = Trautmann|first10 = N.|last11 = Kunz|first11 = P.|last12 = Lassen|first12 = J.|last13 = Passler|first13 = G.|last14 = Dong|first14 = C.|last15 = Fritzsche|first15 = S.|last16 = Haire|first16 = R.|last9 = Thörle
|s2cid = 16234935|title = First Observation of Atomic Levels for the Element Fermium (''Z''=100)|journal = Phys. Rev. Lett.|volume = 90|issue = 16|page = 163002|date = 2003|doi = 10.1103/PhysRevLett.90.163002|bibcode=2003PhRvL..90p3002S|pmid=12731975}}</ref> quantities of fermium can be prepared for specific experiments. The quantities of fermium produced in 20–200&nbsp;kiloton thermonuclear explosions is believed to be of the order of milligrams, although it is mixed in with a huge quantity of debris; 4.0&nbsp;picograms of <sup>257</sup>Fm was recovered from 10&nbsp;kilograms of debris from the "[[Operation Mandrel|Hutch]]" test (16&nbsp;July 1969).<ref>{{cite journal|last1 = Hoff|first1 = R. W.|last2 = Hulet|first2 = E. K.|date = 1970|title = Engineering with Nuclear Explosives|volume = 2|pages = 1283–1294}}</ref> The Hutch experiment produced an estimated total of 250 micrograms of <sup>257</sup>Fm.

After production, the fermium must be separated from other actinides and from [[lanthanide]] fission products. This is usually achieved by [[ion-exchange chromatography]], with the standard process using a cation exchanger such as Dowex&nbsp;50 or T<small>EVA</small> eluted with a solution of ammonium α-hydroxyisobutyrate.<ref name="Silva" /><ref>{{cite journal|last1 = Choppin|first1 = G. R.|last2 = Harvey|first2 = B. G.|last3 = Thompson|first3 = S. G.|date = 1956|title = A new eluant for the separation of the actinide elements|journal = J. Inorg. Nucl. Chem.|volume = 2|issue = 1|pages = 66–68|doi = 10.1016/0022-1902(56)80105-X|url = https://escholarship.org/content/qt73d377r3/qt73d377r3.pdf?t=p0fvlf}}</ref> Smaller cations form more stable complexes with the α-hydroxyisobutyrate anion, and so are preferentially eluted from the column.<ref name="Silva" /> A rapid [[fractional crystallization (chemistry)|fractional crystallization]] method has also been described.<ref name="Silva" /><ref>{{cite journal|last1 = Mikheev|first1 = N. B.|last2 = Kamenskaya|first2 = A. N.|last3 = Konovalova|first3 = N. A.|last4 = Rumer|first4 = I. A.|last5 = Kulyukhin|first5 = S. A.|title = High-speed method for the separation of fermium from actinides and lanthanides|date =1983|journal = Radiokhimiya|volume = 25|issue = 2|pages = 158–161}}</ref>

Although the most stable isotope of fermium is <sup>257</sup>Fm, with a [[half-life]] of 100.5&nbsp;days, most studies are conducted on <sup>255</sup>Fm (''t''<sub>1/2</sub>&nbsp;= 20.07(7)&nbsp;hours), since this isotope can be easily isolated as required as the decay product of <sup>255</sup>Es (''t''<sub>1/2</sub>&nbsp;= 39.8(12)&nbsp;days).<ref name="Silva" />

===Synthesis in nuclear explosions===
The analysis of the debris at the 10-[[TNT equivalent|megaton]] ''Ivy Mike'' nuclear test was a part of long-term project, one of the goals of which was studying the efficiency of production of transuranium elements in high-power nuclear explosions. The motivation for these experiments was as follows: synthesis of such elements from uranium requires multiple neutron capture. The probability of such events increases with the neutron flux, and nuclear explosions are the most powerful neutron sources, providing densities on the order 10{{sup|23}} neutrons/cm{{sup|2}} within a microsecond, i.e. about 10{{sup|29}} neutrons/(cm{{sup|2}}·s). For comparison, the flux of the HFIR reactor is 5{{e|15}} neutrons/(cm{{sup|2}}·s). A dedicated laboratory was set up right at [[Enewetak Atoll]] for preliminary analysis of debris, as some isotopes could have decayed by the time the debris samples reached the U.S. The laboratory was receiving samples for analysis, as soon as possible, from airplanes equipped with paper filters which flew over the atoll after the tests. Whereas it was hoped to discover new chemical elements heavier than fermium, those were not found after a series of megaton explosions conducted between 1954 and 1956 at the atoll.<ref name="s39">Seaborg, p. 39</ref>

[[File:ActinideExplosionSynthesis.png|thumb|upright=1.4|left|Estimated yield of transuranium elements in the U.S. nuclear tests Hutch and Cyclamen.<ref name="s40" />]]
The atmospheric results were supplemented by the underground test data accumulated in the 1960s at the [[Nevada National Security Site|Nevada Test Site]], as it was hoped that powerful explosions conducted in confined space might result in improved yields and heavier isotopes. Apart from traditional uranium charges, combinations of uranium with americium and thorium have been tried, as well as a mixed plutonium-neptunium charge. They were less successful in terms of yield, which was attributed to stronger losses of heavy isotopes due to enhanced fission rates in heavy-element charges. Isolation of the products was found to be rather problematic, as the explosions were spreading debris through melting and vaporizing rocks under the great depth of 300–600 meters, and drilling to such depth in order to extract the products was both slow and inefficient in terms of collected volumes.<ref name="s39" /><ref name="s40">Seaborg, p. 40</ref>

Among the nine underground tests, which were carried between 1962 and 1969 and codenamed Anacostia (5.2 [[TNT equivalent|kilotons]], 1962), Kennebec (<5 kilotons, 1963), Par (38 kilotons, 1964), Barbel (<20 kilotons, 1964), Tweed (<20 kilotons, 1965), Cyclamen (13 kilotons, 1966), Kankakee (20-200 kilotons, 1966), Vulcan (25 kilotons, 1966) and Hutch (20-200 kilotons, 1969),<ref>[http://www.nv.doe.gov/library/publications/historical/DOENV_209_REV15.pdf United States Nuclear Tests July 1945 through September 1992] {{webarchive |url=https://web.archive.org/web/20100615231826/http://www.nv.doe.gov/library/publications/historical/DOENV_209_REV15.pdf |date=June 15, 2010 }}, DOE/NV--209-REV 15, December 2000</ref> the last one was most powerful and had the highest yield of transuranium elements. In the dependence on the atomic mass number, the yield showed a saw-tooth behavior with the lower values for odd isotopes, due to their higher fission rates.<ref name="s40" /> The major practical problem of the entire proposal, however, was collecting the radioactive debris dispersed by the powerful blast. Aircraft filters adsorbed only about 4{{e|-14}} of the total amount and collection of tons of corals at Enewetak Atoll increased this fraction by only two orders of magnitude. Extraction of about 500 kilograms of underground rocks 60 days after the Hutch explosion recovered only about 10{{sup|−7}} of the total charge. The amount of transuranium elements in this 500-kg batch was only 30 times higher than in a 0.4&nbsp;kg rock picked up 7 days after the test. This observation demonstrated the highly nonlinear dependence of the transuranium elements yield on the amount of retrieved radioactive rock.<ref name="s43">Seaborg, p. 43</ref> In order to accelerate sample collection after the explosion, shafts were drilled at the site not after but before the test, so that the explosion would expel radioactive material from the epicenter, through the shafts, to collecting volumes near the surface. This method was tried in the Anacostia and Kennebec tests and instantly provided hundreds of kilograms of material, but with actinide concentrations 3 times lower than in samples obtained after drilling; whereas such a method could have been efficient in scientific studies of short-lived isotopes, it could not improve the overall collection efficiency of the produced actinides.<ref name="s44">Seaborg, p. 44</ref>

Though no new elements (apart from einsteinium and fermium) could be detected in the nuclear test debris, and the total yields of transuranium elements were disappointingly low, these tests did provide significantly higher amounts of rare heavy isotopes than previously available in laboratories. For example, 6{{e|9}} atoms of {{sup|257}}Fm could be recovered after the Hutch detonation. They were then used in the studies of thermal-neutron induced fission of {{sup|257}}Fm and in discovery of a new fermium isotope {{sup|258}}Fm. Also, the rare isotope {{sup|250}}Cm was synthesized in large quantities, which is very difficult to produce in nuclear reactors from its progenitor {{sup|249}}Cm; the half-life of {{sup|249}}Cm (64 minutes) is much too short for months-long reactor irradiations, but is very "long" on the explosion timescale.<ref name="s47">Seaborg, p. 47</ref>

===Natural occurrence===
Because of the short half-life of all known isotopes of fermium, any [[Primordial nuclide|primordial]] fermium, that is fermium present on Earth during its formation, has decayed by now. Synthesis of fermium from naturally occurring uranium and thorium in the Earth's crust requires multiple neutron captures, which is extremely unlikely. Therefore, most fermium is produced on Earth in laboratories, high-power nuclear reactors, or in [[nuclear test]]s, and is present for only a few months afterward. The [[transuranic element]]s [[americium]] to fermium did occur naturally in the [[natural nuclear fission reactor]] at [[Oklo]], but no longer do so.<ref name="emsley">{{cite book|last=Emsley|first=John|title=Nature's Building Blocks: An A-Z Guide to the Elements|edition=New|date=2011|publisher=Oxford University Press|location=New York, NY|isbn=978-0-19-960563-7}}</ref>

==Chemistry==
[[File:Fermium-Ytterbium Alloy.jpg|thumb|A fermium-[[ytterbium]] alloy used for measuring the [[enthalpy of sublimation]] of fermium metal<ref>{{Cite journal|last1=Haire|first1=Richard G.|last2=Gibson|first2=John K.|title=The enthalpy of sublimation and thermodynamic functions of fermium|url=https://pubs.aip.org/aip/jcp/article-abstract/91/11/7085/95919/The-enthalpy-of-sublimation-and-thermodynamic|journal=The Journal of Chemical Physics|date=1989 |language=en|volume=91|issue=11 |pages=7085–7096|doi=10.1063/1.457326|bibcode=1989JChPh..91.7085H }}</ref>]]

The chemistry of fermium has only been studied in solution using tracer techniques, and no solid compounds have been prepared. Under normal conditions, fermium exists in solution as the Fm<sup>3+</sup> ion, which has a [[hydration number]] of 16.9 and an [[acid dissociation constant]] of 1.6{{e|−4}} (p''K''{{sub|a}}&nbsp;= 3.8).<ref>{{cite journal|last1 = Lundqvist|first1 = Robert|last2 = Hulet|first2 = E. K.|last3 = Baisden|first3 = T. A.|date = 1981|last4 = Näsäkkälä|first4 = Elina|last5 = Wahlberg|first5 = Olof|title = Electromigration Method in Tracer Studies of Complex Chemistry. II. Hydrated Radii and Hydration Numbers of Trivalent Actinides|journal = Acta Chemica Scandinavica A|volume = 35|pages = 653–661|doi = 10.3891/acta.chem.scand.35a-0653|doi-access = free}}</ref><ref>{{cite journal|last1 = Hussonnois|first1 = H.|last2 = Hubert|first2 = S.|last3 = Aubin|first3 = L.|last4 = Guillaumont|first4 = R.|author-link4=Robert Guillaumont|last5 = Boussieres|first5 = G.|date = 1972|journal = Radiochem. Radioanal. Lett.|volume = 10|pages = 231–238}}</ref> Fm{{sup|3+}} forms complexes with a wide variety of organic ligands with [[HSAB theory|hard]] donor atoms such as oxygen, and these complexes are usually more stable than those of the preceding actinides.<ref name="Silva" /> It also forms anionic complexes with ligands such as [[chloride]] or [[nitrate]] and, again, these complexes appear to be more stable than those formed by [[einsteinium]] or [[californium]].<ref>{{cite journal|last1 = Thompson|first1 = S. G.|last2 = Harvey|first2 = B. G.|last3 = Choppin|first3 = G. R.|last4 = Seaborg|first4 = G. T.|author-link4 = Glenn T. Seaborg|date = 1954|title = Chemical Properties of Elements 99 and 100|journal = J. Am. Chem. Soc.|volume = 76|issue = 24|pages = 6229–6236|doi = 10.1021/ja01653a004|url = https://digital.library.unt.edu/ark:/67531/metadc1023183/|bibcode = 1954JAChS..76.6229T}}</ref> It is believed that the bonding in the complexes of the later actinides is mostly [[Ionic bond|ionic]] in character: the Fm{{sup|3+}} ion is expected to be smaller than the preceding An{{sup|3+}} ions because of the higher [[effective nuclear charge]] of fermium, and hence fermium would be expected to form shorter and stronger metal–ligand bonds.<ref name="Silva" />

Fermium(III) can be fairly easily reduced to fermium(II),<ref>{{cite journal|last = Malý|first = Jaromír|date = 1967|title = The amalgamation behaviour of heavy elements 1. Observation of anomalous preference in formation of amalgams of californium, einsteinium, and fermium|journal = Inorg. Nucl. Chem. Lett.|volume = 3|issue = 9|pages = 373–381|doi = 10.1016/0020-1650(67)80046-1}}</ref> for example with [[samarium(II) chloride]], with which fermium(II) coprecipitates.<ref>{{cite journal|last1 = Mikheev|first1 = N. B.|last2 = Spitsyn|first2 = V. I.|last3 = Kamenskaya|first3 = A. N.|last4 = Gvozdec|first4 = B. A.|last5 = Druin|first5 = V. A.|last6 = Rumer|first6 = I. A.|last7 = Dyachkova|first7 = R. A.|last8 = Rozenkevitch|first8 = N. A.|last9 = Auerman|first9 = L. N.|date = 1972|title = Reduction of fermium to divalent state in chloride aqueous ethanolic solutions|journal = Inorg. Nucl. Chem. Lett.|volume = 8|issue = 11|pages = 929–936|doi = 10.1016/0020-1650(72)80202-2}}</ref><ref>{{cite journal|last1 = Hulet|first1 = E. K.|last2 = Lougheed|first2 = R. W.|last3 = Baisden|first3 = P. A.|last4 = Landrum|first4 = J. H.|last5 = Wild|first5 = J. F.|last6 = Lundqvist|first6 = R. F.|date = 1979|title = Non-observance of monovalent Md|journal = J. Inorg. Nucl. Chem.|volume = 41|issue = 12|pages = 1743–1747|doi = 10.1016/0022-1902(79)80116-5}}</ref> In the precipitate, the compound fermium(II) chloride (FmCl{{sub|2}}) was produced, though it was not purified or studied in isolation.<ref>{{cite book |title=Dictionary of Inorganic Compounds |page=2873 |date=1992 |volume=3 |edition=1 |publisher=Chapman & Hall |isbn=0412301202}}</ref> The [[electrode potential]] has been estimated to be similar to that of the [[ytterbium]](III)/(II) couple, or about −1.15&nbsp;V with respect to the [[standard hydrogen electrode]],<ref>{{cite journal|last1 = Mikheev|first1 = N. B.|last2 = Spitsyn|first2 = V. I.|last3 = Kamenskaya|first3 = A. N.|last4 = Konovalova|first4 = N. A.|last5 = Rumer|first5 = I. A.|last6 = Auerman|first6 = L. N.|last7 = Podorozhnyi|first7 = A. M.|date = 1977|title = Determination of oxidation potential of the pair Fm{{sup|2+}}/Fm{{sup|3+}}|journal = Inorg. Nucl. Chem. Lett.|volume = 13|issue = 12|pages = 651–656|doi = 10.1016/0020-1650(77)80074-3}}</ref> a value which agrees with theoretical calculations.<ref>{{cite journal|last = Nugent|first = L. J.|date = 1975|journal = MTP Int. Rev. Sci.: Inorg. Chem. |volume = 7|pages = 195–219}}</ref> The Fm{{sup|2+}}/Fm{{sup|0}} couple has an electrode potential of −2.37(10)&nbsp;V based on [[Polarography|polarographic]] measurements.<ref>{{cite journal|last1 = Samhoun|first1 = K.|last2 = David|first2 = F.|last3 = Hahn|first3 = R. L.|last4 = O'Kelley|first4 = G. D.|last5 = Tarrant|first5 = J. R.|last6 = Hobart|first6 = D. E.|date = 1979|title = Electrochemical study of mendelevium in aqueous solution: No evidence for monovalent ions|journal = J. Inorg. Nucl. Chem.| volume = 41|issue = 12|pages = 1749–1754|doi = 10.1016/0022-1902(79)80117-7}}</ref>

==Toxicity==
Though few people come in contact with fermium, the [[International Commission on Radiological Protection]] has set annual exposure limits for the two most stable isotopes. For fermium-253, the ingestion limit was set at 10{{sup|7}} [[becquerel]]s (1 Bq equals one decay per second), and the inhalation limit at 10{{sup|5}} Bq; for fermium-257, at 10{{sup|5}} Bq and 4,000 Bq respectively.<ref>{{cite book|last1=Koch|first1=Lothar|title=Transuranium Elements, in Ullmann's Encyclopedia of Industrial Chemistry|publisher=Wiley|date=2000|doi=10.1002/14356007.a27_167|chapter=Transuranium Elements|isbn=978-3527306732}}</ref>

==Notes and references==

===Notes===
{{notelist}}

===References===
{{Reflist|30em}}

==Further reading==
* Robert J. Silva: [https://web.archive.org/web/20100717155410/http://radchem.nevada.edu/classes/rdch710/files/Fm%20to%20Lr.pdf Fermium, Mendelevium, Nobelium, and Lawrencium], in: Lester R. Morss, Norman M. Edelstein, Jean Fuger (Hrsg.): ''The Chemistry of the Actinide and Transactinide Elements'', Springer, Dordrecht 2006; {{ISBN|1-4020-3555-1}}, p.&nbsp;1621–1651; {{doi|10.1007/1-4020-3598-5_13}}.
* [[Glenn T. Seaborg|Seaborg, Glenn T.]] (ed.) (1978) ''[http://www.escholarship.org/uc/item/92g2p7cd.pdf Proceedings of the Symposium Commemorating the 25th Anniversary of Elements 99 and 100]'', 23 January 1978, Report LBL-7701
* ''[[Gmelins Handbuch der anorganischen Chemie]]'', System Nr. 71, Transurane: Teil A 1 II, p.&nbsp;19–20; Teil A 2, p.&nbsp;47; Teil B 1, p.&nbsp;84.

==External links==
{{Commons}}
{{Wiktionary|fermium}}
* [http://www.periodicvideos.com/videos/100.htm Fermium] at ''[[The Periodic Table of Videos]]'' (University of Nottingham)

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