View source for Hydrogen

{{About|the chemical element}}
{{pp-vandalism|small=yes}}
{{Use dmy dates|date=March 2022}}
{{Infobox hydrogen}}
'''Hydrogen''' is a [[chemical element]]; it has [[chemical symbol|symbol]] '''H''' and [[atomic number]] 1. It is the lightest element and, at [[standard conditions]], is a [[gas]] of [[diatomic molecule]]s with the [[chemical formula|formula]] {{chem2|H2}}, sometimes called '''dihydrogen'''<!--[[dihydrogen]] is a redirect to hydrogen also called '''diprotium'''-->,<ref>{{cite web |title=Dihydrogen |url=http://www.usm.maine.edu/~newton/Chy251_253/Lectures/LewisStructures/Dihydrogen.html |archive-url=https://web.archive.org/web/20090213174645/http://usm.maine.edu/~newton/Chy251_253/Lectures/LewisStructures/Dihydrogen.html |archive-date=13 February 2009 |access-date=6 April 2009 |work=O{{=}}CHem Directory |publisher=[[University of Southern Maine]] |df=dmy-all}}</ref> '''hydrogen gas''', '''molecular hydrogen''', or simply hydrogen. It is colorless, odorless,<ref>{{Cite web|title=Hydrogen|url=https://www.britannica.com/science/hydrogen|url-status=live|access-date=25 December 2021|website=[[Encyclopædia Britannica]]|language=en|archive-date=24 December 2021|archive-url=https://web.archive.org/web/20211224165150/https://www.britannica.com/science/hydrogen}}</ref> non-toxic, and highly [[combustible]]. Constituting about 75% of all [[baryon|normal]] [[matter]], hydrogen is the [[abundance of the chemical elements|most abundant]] chemical element in the [[universe]].<ref>{{cite web
|last=Boyd
|first=Padi
|title=What is the chemical composition of stars?
|url=https://imagine.gsfc.nasa.gov/ask_astro/stars.html#961112a
|publisher=[[NASA]]
|date=19 July 2014
|access-date=5 February 2008
|archive-url=https://web.archive.org/web/20150115074556/http://imagine.gsfc.nasa.gov/ask_astro/stars.html#961112a
|archive-date=15 January 2015
|url-status=live
}}</ref> [[Stars]], including the [[Sun]], mainly consist of hydrogen in a [[plasma state]], while on Earth, hydrogen is found in [[water]], [[organic compounds]], as the gas H<sub>2</sub> (dihydrogen), and in other [[molecular form]]s. The most common [[isotope of hydrogen]] (protium, {{sup|1}}H) consists of one [[proton]], one [[electron]], and no [[neutron]]s.

In the [[early universe]], the formation of hydrogen's protons occurred in the first second after the [[Big Bang]]; neutral hydrogen atoms only formed about 370,000 years later during the [[Recombination (cosmology)|recombination epoch]] as the universe expanded and plasma had cooled enough for electrons to remain bound to protons.<ref>{{cite journal |last=Tanabashi |first=M. |display-authors=etal |year=2018 |journal=[[Physical Review D]] |volume=98 |issue=3 |via=[[Particle Data Group]] at [[Lawrence Berkeley National Laboratory]] |url=http://pdg.lbl.gov/2018/reviews/rpp2018-rev-bbang-cosmology.pdf |page=358 |quote=Chapter 21.4.1 - This occurred when the age of the Universe was about 370,000 years. |title=Big-Bang Cosmology |url-status=live |archive-url=https://web.archive.org/web/20210629034426/https://pdg.lbl.gov/2018/reviews/rpp2018-rev-bbang-cosmology.pdf |archive-date=29 June 2021 |doi=10.1103/PhysRevD.98.030001 |doi-access=free }} (Revised September 2017) by [[Keith Olive|Keith A. Olive]] and [[John A. Peacock]].</ref> Hydrogen gas was first produced artificially in the early 16th century by the reaction of acids with metals. [[Henry Cavendish]], in 1766–81, identified hydrogen gas as a distinct substance<ref>{{Cite episode
|title = Discovering the Elements
|url = http://www.bbc.co.uk/programmes/b00q2mk5
|series = Chemistry: A Volatile History
|credits = Presenter: Professor Jim Al-Khalili
|network = [[BBC]]
|station = [[BBC Four]]
|air-date = 21 January 2010
|minutes = 25:40
|access-date = 9 February 2010
|archive-url = https://web.archive.org/web/20100125010949/http://www.bbc.co.uk/programmes/b00q2mk5
|archive-date = 25 January 2010
|url-status = live
}}</ref> and discovered its property of producing water when burned; hence its name means "water-former" in Greek. Understanding the colors of light absorbed and emitted by hydrogen was a crucial part of developing [[quantum mechanics]].

Hydrogen, typically [[nonmetallic]] except under [[High pressure|extreme pressure]], readily forms [[covalent bonds]] with most nonmetals, contributing to the formation of compounds like water and various organic substances. Its role is crucial in [[acid-base reactions]], which mainly involve proton exchange among soluble molecules. In [[ionic compounds]], hydrogen can take the form of either a negatively charged [[anion]], where it is known as [[hydride]], or as a positively charged [[cation]], H{{sup|+}}, called a proton. Although tightly bonded to water molecules, protons strongly affect the behavior of [[aqueous solution]]s, as reflected in the importance of pH.  Hydride on the other hand, is rarely observed because it tends to deprotonate solvents, yielding  H<sub>2</sub>.

Nearly all [[hydrogen production]] is done by transforming fossil fuels, particularly [[steam reforming]] of [[natural gas]]. It can also be produced from electricity by [[electrolysis]], however this process is more expensive.<ref name="Evans-2020" /> Its main industrial uses include [[fossil fuel]] processing and [[ammonia production]]. Emerging uses for hydrogen include the use of [[fuel cell]]s to generate electricity.<ref>{{cite journal |doi=10.1155/2024/7271748 |title=A Recent Comprehensive Review of Fuel Cells: History, Types, and Applications |date=2024 |last1=Qasem |first1=Naef A. A. |last2=Abdulrahman |first2=Gubran A. Q. |journal=International Journal of Energy Research |issue=1 |doi-access=free |bibcode=2024IJER.202471748Q }}</ref>
{{Toclimit}}

== Properties ==
=== Combustion ===
[[File:19. Експлозија на смеса од водород и воздух.webm|thumb|Combustion of hydrogen with the oxygen in the air. When the bottom cap is removed, allowing air to enter at the bottom, the hydrogen in the container rises out of top and burns as it mixes with the air.]]
[[File:Shuttle Main Engine Test Firing cropped edited and reduced.jpg|thumb|[[Space Shuttle Main Engine]] burning hydrogen with oxygen, produces a nearly invisible flame at full thrust.|alt=A black inverted funnel with blue glow emerging from its opening.]]

Hydrogen gas is highly flammable:
:{{chem2|2 H2(g) + O2(g) → 2 H2O(l)}} (572&nbsp;kJ/2 mol = 286&nbsp;kJ/mol = 141.865 MJ/kg)<ref group="note">286&nbsp;kJ/mol: energy per mole of the combustible material (molecular hydrogen).</ref>
[[Enthalpy of combustion]]: −286&nbsp;kJ/mol.<ref>{{cite book
|author=Committee on Alternatives and Strategies for Future Hydrogen Production and Use
|date=2004
|title=The Hydrogen Economy: Opportunities, Costs, Barriers, and R&D Needs
|page=240
|publisher=[[National Academies Press]]
|isbn=978-0-309-09163-3
|url=https://books.google.com/books?id=ugniowznToAC&pg=PA240
|access-date=3 September 2020
|archive-date=29 January 2021
|archive-url=https://web.archive.org/web/20210129015745/https://books.google.com/books?id=ugniowznToAC&pg=PA240
|url-status=live
}}</ref>

Hydrogen gas forms explosive mixtures with air in concentrations from 4–74%<ref>{{cite journal
|last1=Carcassi|first1=M. N.
|last2=Fineschi|first2=F.
|title=Deflagrations of H<sub>2</sub>–air and CH<sub>4</sub>–air lean mixtures in a vented multi-compartment environment
|journal=Energy
|volume=30|issue=8|pages=1439–1451
|date=2005
|doi=10.1016/j.energy.2004.02.012
|bibcode=2005Ene....30.1439C
}}</ref> and with chlorine at 5–95%. The hydrogen [[autoignition temperature]], the temperature of spontaneous ignition in air, is {{convert|500|C|F}}.<ref>{{cite book
|url=https://books.google.com/books?id=-CRRJBVv5d0C&pg=PA402
|page=402
|title=A Comprehensive Guide to the Hazardous Properties of Chemical Substances
|publisher=Wiley-Interscience
|isbn=978-0-471-71458-3
|date=2007
|last=Patnaik
|first=P.
|access-date=3 September 2020
|archive-date=26 January 2021
|archive-url=https://web.archive.org/web/20210126131413/https://books.google.com/books?id=-CRRJBVv5d0C&pg=PA402
|url-status=live
}}</ref> In a high-pressure hydrogen leak, the shock wave from the leak itself can heat air to the autoignition temperature, leading to flaming and possibly explosion.<ref>{{Cite journal |last=Yamada |first=Eisuke |last2=Kitabayashi |first2=Naoki |last3=Hayashi |first3=A. Koichi |last4=Tsuboi |first4=Nobuyuki |date=2011-02-01 |title=Mechanism of high-pressure hydrogen auto-ignition when spouting into air |url=https://linkinghub.elsevier.com/retrieve/pii/S0360319910009468 |journal=International Journal of Hydrogen Energy |series=The Third Annual International Conference on Hydrogen Safety |volume=36 |issue=3 |pages=2560–2566 |doi=10.1016/j.ijhydene.2010.05.011 |issn=0360-3199}}</ref>

Hydrogen flames emit faint blue and [[ultraviolet]] light.<ref name=":1">{{cite journal |last1=Schefer |first1=E. W. |last2=Kulatilaka |first2=W. D. |last3=Patterson |first3=B. D. |last4=Settersten |first4=T. B. |date=June 2009 |title=Visible emission of hydrogen flames |url=https://zenodo.org/record/1258847 |url-status=live |journal=Combustion and Flame |volume=156 |issue=6 |pages=1234–1241 |bibcode=2009CoFl..156.1234S |doi=10.1016/j.combustflame.2009.01.011 |archive-url=https://web.archive.org/web/20210129015717/https://zenodo.org/record/1258847 |archive-date=29 January 2021 |access-date=30 June 2019}}</ref>  [[Flame detector]]s are used to detect hydrogen fires as they are nearly invisible to the naked eye in daylight.<ref>{{Cite web |title=Making Visible the Invisible {{!}} NASA Spinoff |url=https://spinoff.nasa.gov/spinoff1999/er5.htm |access-date=2025-02-09 |website=spinoff.nasa.gov}}</ref><ref name=":3" />

=== Electron energy levels ===
{{Main|Hydrogen atom}}
[[File:hydrogen atom.svg|thumb|A depiction of a hydrogen atom with size of central proton shown, and the atomic diameter shown as about twice the [[Bohr model]] radius (image not to scale)|alt=Drawing of a light-gray large sphere with a cut off quarter and a black small sphere and numbers 1.7×10{{sup|−5}} illustrating their relative diameters.]]
The [[ground state]] [[energy level]] of the electron in a hydrogen atom is −13.6&nbsp;[[Electronvolt|eV]],<ref>{{cite web|author1=NAAP Labs|title=Energy Levels|url=http://astro.unl.edu/naap/hydrogen/levels.html|publisher=University of Nebraska Lincoln|access-date=20 May 2015|date=2009|archive-url=https://web.archive.org/web/20150511120536/http://astro.unl.edu/naap/hydrogen/levels.html|archive-date=11 May 2015|url-status=live}}</ref> equivalent to an [[ultraviolet]] [[photon]] of roughly 91&nbsp;nm wavelength.<ref>{{cite web|url=http://www.wolframalpha.com/input/?i=photon+wavelength+13.6+ev|title=photon wavelength 13.6 eV|access-date=20 May 2015|date=20 May 2015|work=Wolfram Alpha|archive-url=https://web.archive.org/web/20160512221720/http://www.wolframalpha.com/input/?i=photon+wavelength+13.6+ev|archive-date=12 May 2016|url-status=live}}</ref> The energy levels of hydrogen are referred to by consecutive [[quantum number]]s, with <math>n=1</math> being the ground state. The [[hydrogen spectral series]] corresponds to emission of light due to transitions from higher to lower energy levels.<ref>{{Cite book |last=Levine |first=Ira N. |title=Quantum chemistry |date=1970 |publisher=Pearson |isbn=978-0-321-89060-3 |edition=2 |series=Pearson advanced chemistry series |location=Boston}}</ref>{{rp|105}}

The energy levels of hydrogen can be calculated fairly accurately using the [[Bohr model]] of the atom, in which the electron "orbits" the proton, like how Earth orbits the Sun. However, the electron and proton are held together by electrostatic attraction, while planets and celestial objects are held by [[gravity]]. Due to the discretization of [[angular momentum]] postulated in early [[quantum mechanics]] by Bohr, the electron in the Bohr model can only occupy certain allowed distances from the proton, and therefore only certain allowed energies.<ref>{{cite web
|last=Stern
|first=D. P.
|date=16 May 2005
|url=http://www.iki.rssi.ru/mirrors/stern/stargaze/Q5.htm
|title=The Atomic Nucleus and Bohr's Early Model of the Atom
|publisher=NASA Goddard Space Flight Center (mirror)
|access-date=20 December 2007
|archive-url=https://web.archive.org/web/20081017073826/http://www.iki.rssi.ru/mirrors/stern/stargaze/Q5.htm
|archive-date=17 October 2008
}}</ref>

An accurate description of the hydrogen atom comes from a quantum analysis that uses the [[Schrödinger equation]], [[Dirac equation]] or [[Richard Feynman|Feynman]] [[path integral formulation]] to calculate the [[probability amplitude|probability density]] of the electron around the proton.<ref>{{cite web| last=Stern| first=D. P.| date=13 February 2005| url=http://www-spof.gsfc.nasa.gov/stargaze/Q7.htm| title=Wave Mechanics| publisher=NASA Goddard Space Flight Center| access-date=16 April 2008| archive-url=https://web.archive.org/web/20080513195241/http://www-spof.gsfc.nasa.gov/stargaze/Q7.htm| archive-date=13 May 2008| url-status=live}}</ref> The most complex formulas include the small effects of [[special relativity]] and [[vacuum polarization]]. In the quantum mechanical treatment, the electron in a ground state hydrogen atom has no angular momentum—illustrating how the "planetary orbit" differs from electron motion.{{citation needed|date=January 2025}}

=== Spin isomers ===
{{Main|Spin isomers of hydrogen}}
Molecular {{chem2|H2}} exists as two [[nuclear isomer]]s that differ in the [[Spin (physics)|spin states]] of their nuclei.<ref name="uigi">{{cite web|author=Staff|date=2003|url=http://www.uigi.com/hydrogen.html|title=Hydrogen (H<sub>2</sub>) Properties, Uses, Applications: Hydrogen Gas and Liquid Hydrogen|publisher=Universal Industrial Gases, Inc.|access-date=5 February 2008|archive-url=https://web.archive.org/web/20080219073329/http://www.uigi.com/hydrogen.html|archive-date=19 February 2008|url-status=live}}</ref> In the '''orthohydrogen''' form, the spins of the two nuclei are parallel, forming a spin [[triplet state]] having a [[Spin quantum number#Total spin of an atom or molecule|total molecular spin]] <math>S = 1</math>; in the '''parahydrogen''' form the spins are antiparallel and form a spin [[singlet state]] having spin <math>S = 0</math>. The equilibrium ratio of ortho- to para-hydrogen depends on temperature. At room temperature or warmer, equilibrium hydrogen gas contains about 25% of the para form and 75% of the ortho form.<ref name="Green2012">{{cite journal |last1=Green |first1=Richard A. |display-authors=etal |title=The theory and practice of hyperpolarization in magnetic resonance using ''para''hydrogen |journal=Prog. Nucl. Magn. Reson. Spectrosc. |date=2012 |volume=67 |pages=1–48 |doi=10.1016/j.pnmrs.2012.03.001 |pmid=23101588 |bibcode=2012PNMRS..67....1G |url=https://www.sciencedirect.com/science/article/abs/pii/S0079656512000477 |access-date=28 August 2021 |archive-date=28 August 2021 |archive-url=https://web.archive.org/web/20210828222611/https://www.sciencedirect.com/science/article/abs/pii/S0079656512000477 |url-status=live }}</ref> The ortho form is an [[excited state]], having higher energy than the para form by 1.455&nbsp;kJ/mol,<ref name="PlanckInstitut">{{cite web |url=https://www.mpibpc.mpg.de/146336/para-Wasserstoff |language=de |website=Max-Planck-Institut für Biophysikalische Chemie |title=Die Entdeckung des para-Wasserstoffs (The discovery of para-hydrogen) |access-date=9 November 2020 |archive-date=16 November 2020 |archive-url=https://web.archive.org/web/20201116064055/https://www.mpibpc.mpg.de/146336/para-Wasserstoff |url-status=live }}</ref> and it converts to the para form over the course of several minutes when cooled to low temperature.<ref>{{cite journal|last1=Milenko|first1=Yu. Ya.|last2=Sibileva|first2=R. M.|last3=Strzhemechny|first3=M. A.|title=Natural ortho-para conversion rate in liquid and gaseous hydrogen|journal=Journal of Low Temperature Physics|date=1997|volume=107|issue=1–2|pages=77–92
|doi=10.1007/BF02396837|bibcode = 1997JLTP..107...77M |s2cid=120832814}}</ref> The thermal properties of these isomers differ because each has distinct [[Rotational–vibrational spectroscopy|rotational quantum states]].<!-- This link is less direct than [[Rotational spectroscopy]] but presently the subject better (June 2021).--><ref name="NASA">{{cite web|last=Hritz|first=J.|date=March 2006|url=http://smad-ext.grc.nasa.gov/gso/manual/chapter_06.pdf|title=CH. 6&nbsp;– Hydrogen|work=NASA Glenn Research Center Glenn Safety Manual, Document GRC-MQSA.001|publisher=NASA|access-date=5 February 2008|archive-url=https://web.archive.org/web/20080216050326/http://smad-ext.grc.nasa.gov/gso/manual/chapter_06.pdf|archive-date=16 February 2008}}</ref>

The ortho-to-para ratio in {{chem2|H2}} is an important consideration in the [[liquefaction]] and storage of [[liquid hydrogen]]: the conversion from ortho to para is [[exothermic]] and produces sufficient heat to evaporate most of the liquid if not converted first to parahydrogen during the cooling process.<ref name="Amos98">{{cite web|url=http://www.nrel.gov/docs/fy99osti/25106.pdf|title=Costs of Storing and Transporting Hydrogen|publisher=National Renewable Energy Laboratory|date=1 November 1998|first1=Wade A.|last1=Amos|pages=6–9|access-date=19 May 2015|archive-url=https://web.archive.org/web/20141226131234/http://www.nrel.gov/docs/fy99osti/25106.pdf|archive-date=26 December 2014|url-status=live}}</ref> [[Catalyst]]s for the ortho-para interconversion, such as [[ferric oxide]] and [[activated carbon]] compounds, are used during hydrogen cooling to avoid this loss of liquid.<ref name="Svadlenak">{{cite journal|last1=Svadlenak|first1=R. E.|last2=Scott|first2=A. B.|title=The Conversion of Ortho- to Parahydrogen on Iron Oxide-Zinc Oxide Catalysts|journal=Journal of the American Chemical Society|date=1957|volume=79|issue=20|pages=5385–5388|doi=10.1021/ja01577a013|bibcode=1957JAChS..79.5385S }}</ref>

=== Phases ===
[[File:Hydrogen ampoule.jpg|thumb|Hydrogen gas is colorless and transparent, here contained in a glass [[ampoule]].]]
[[File:Phase diagram of hydrogen.png|thumb|[[Phase diagram]] of hydrogen. The temperature and pressure scales are [[logarithmic scale|logarithmic]], so one unit corresponds to a 10× change. The left edge corresponds to 10{{sup|5}} Pa, or about one atmosphere.{{image reference needed|date=December 2022}}|alt=Phase diagram of hydrogen on logarithmic scales. Lines show boundaries between phases, with the end of the liquid-gas line indicating the critical point. The triple point of hydrogen is just off-scale to the left.]]
[[Liquid hydrogen]] can exist at temperatures below hydrogen's [[critical point (thermodynamics)|critical point]] of 33&nbsp;[[Kelvins|K]].<ref>{{cite web|url=https://webbook.nist.gov/cgi/cbook.cgi?ID=C1333740&Mask=4 |title=Hydrogen |website=NIST Chemistry WebBook, SRD 69 |publisher=[[National Institute of Standards and Technology]] |access-date=2025-01-14 |year=2023}}</ref> However, for it to be in a fully liquid state at [[atmospheric pressure]], H<sub>2</sub> needs to be cooled to {{convert|20.28|K|C F}}. Hydrogen was liquefied by [[James Dewar]] in 1898 by using [[regenerative cooling]] and his invention, the [[vacuum flask]].

[[Solid hydrogen]] can be made at [[standard pressure]], by decreasing the temperature below hydrogen's [[melting point]] of {{convert|14.01|K}}. It was collected for the first time by [[James Dewar]] in 1899.{{citation needed|date=January 2025}} Multiple distinct solid phases exist, known as Phase I through Phase V, each exhibiting a characteristic molecular arrangement.<ref name="Helled2020">{{cite journal|first1=Ravit |last1=Helled |first2=Guglielmo |last2=Mazzola |first3=Ronald |last3=Redmer |title=Understanding dense hydrogen at planetary conditions |date=2020-09-01 |journal=Nature Reviews Physics |volume=2 |issue=10 |pages=562–574 |doi=10.1038/s42254-020-0223-3 |arxiv=2006.12219|bibcode=2020NatRP...2..562H }}</ref> Liquid and solid phases can exist in combination at the [[triple point]], a substance known as [[slush hydrogen]].<ref>{{cite book |last=Ohira |first=K. |chapter=Slush hydrogen production, storage, and transportation |date=2016 |title=Compendium of Hydrogen Energy |pages=53–90 |publisher=Elsevier |doi=10.1016/b978-1-78242-362-1.00003-1 |isbn=978-1-78242-362-1}}</ref>

[[Metallic hydrogen]], a phase obtained at extremely high pressures (in excess of {{convert|400|GPa|atm psi}}), is an electrical conductor. It is believed to exist deep within [[giant planet]]s like [[Jupiter]].<ref name="Helled2020"/><ref>{{cite book|last1=Frankoi |first1=A. |display-authors=etal |title=Astronomy 2e |year=2022 |publisher=OpenStax |chapter-url=https://openstax.org/books/astronomy-2e/pages/11-2-the-giant-planets |chapter=11.2 The Giant Planets |page=370 |isbn=978-1-951693-50-3}}</ref>

When [[ionization|ionized]], hydrogen becomes a [[plasma (physics)|plasma]]. This is the form in which hydrogen exists within [[star]]s.

=== Isotopes ===
{{Main|Isotopes of hydrogen}}
[[File:Blausen 0530 HydrogenIsotopes.png|thumb|The three naturally-occurring isotopes of hydrogen: hydrogen-1 (protium), hydrogen-2 (deuterium), and hydrogen-3 (tritium)|alt=Diagram showing the structure of each of Hydrogen-1 (mass number 1, 1 electron, 1 proton), Hydrogen-2 or deuterium (mass number 2, 1 electron, 1 proton, 1 neutron), and Hydrogen-3 or tritium (mass number 3, 1 electron, 1 proton, 2 neutrons)]]
[[File:Hydrogen discharge tube.jpg|thumb|Hydrogen discharge (spectrum) tube]]
[[File:Deuterium discharge tube.jpg|thumb|Deuterium discharge (spectrum) tube]]
Hydrogen has three naturally occurring isotopes, denoted {{chem|1|H}}, {{chem|2|H}} and {{chem|3|H}}. Other, highly unstable nuclei ({{chem|4|H}} to {{chem|7|H}}) have been synthesized in the laboratory but not observed in nature.<ref name="Gurov">{{cite journal
|author=Gurov, Y. B.
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|author10=Chernyshev, B. A.
|author11=Tschurenkova, T. D.
|title=Spectroscopy of superheavy hydrogen isotopes in stopped-pion absorption by nuclei
|journal=Physics of Atomic Nuclei
|date=2004|volume=68|issue=3|pages=491–97
|doi=10.1134/1.1891200
|bibcode = 2005PAN....68..491G |s2cid=122902571
}}</ref><ref name="Korsheninnikov">{{cite journal
|title=Experimental Evidence for the Existence of <sup>7</sup>H and for a Specific Structure of <sup>8</sup>He
|journal=Physical Review Letters
|date=2003|volume=90|issue=8|page=082501
|doi=10.1103/PhysRevLett.90.082501|pmid=12633420
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|first2=E.
|last3=Kuzmin
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|first8=I.
|last9=Timofeyuk
|first9=N.}}</ref>
* '''{{chem|1|H}}''' is the most common hydrogen isotope, with an abundance of >99.98%. Because the [[atomic nucleus|nucleus]] of this isotope consists of only a single proton, it is given the descriptive but rarely used formal name ''protium''.<ref>{{cite journal
|last1=Urey|first1=H. C.
|last2=Brickwedde|first2=F. G.|last3=Murphy|first3=G. M.
|title=Names for the Hydrogen Isotopes
|journal=Science|date=1933|volume=78
|issue=2035|pages=602–603
|doi=10.1126/science.78.2035.602
|pmid=17797765|bibcode = 1933Sci....78..602U }}</ref> It is the only stable isotope with no neutrons; see [[Isotopes of helium#Helium-2 (diproton)|diproton]] for a discussion of why others do not exist.
* '''{{chem|2|H}}''', the other stable hydrogen isotope, is known as [[deuterium]] and contains one proton and one [[neutron]] in the nucleus. Nearly all deuterium in the universe is thought to have been produced at the time of the [[Big Bang]], and has endured since then. Deuterium is not radioactive, and is not a significant toxicity hazard. Water enriched in molecules that include deuterium instead of normal hydrogen is called [[heavy water]]. Deuterium and its compounds are used as a non-radioactive label in chemical experiments and in solvents for {{chem|1|H}}-[[NMR spectroscopy]].<ref>{{cite journal
|author=Oda, Y.
|author2=Nakamura, H.
|author3=Yamazaki, T.
|author4=Nagayama, K.
|author5=Yoshida, M.
|author6=Kanaya, S.
|author7=Ikehara, M.
|title=1H NMR studies of deuterated ribonuclease HI selectively labeled with protonated amino acids
|journal=[[Journal of Biomolecular NMR]]
|date=1992|volume=2|issue=2|pages=137–47
|doi=10.1007/BF01875525
|pmid=1330130|s2cid=28027551
}}</ref> Heavy water is used as a [[neutron moderator]] and coolant for nuclear reactors. Deuterium is also a potential fuel for commercial [[nuclear fusion]].<ref>{{cite news
|last=Broad
|first=W. J.
|date=11 November 1991
|title=Breakthrough in Nuclear Fusion Offers Hope for Power of Future
|work=The New York Times
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|access-date=12 February 2008
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|archive-url=https://web.archive.org/web/20210129015717/https://www.nytimes.com/1991/11/11/us/breakthrough-in-nuclear-fusion-offers-hope-for-power-of-future.html
|url-status=live
}}</ref>
* '''{{chem|3|H}}''' is known as [[tritium]] and contains one proton and two neutrons in its nucleus. It is radioactive, decaying into [[helium-3]] through [[beta decay]] with a [[half-life]] of 12.32 years.<ref name="Miessler" /> It is radioactive enough to be used in [[Radioluminescent paint|luminous paint]] to enhance the visibility of data displays, such as for painting the hands and dial-markers of watches. The watch glass prevents the small amount of radiation from escaping the case.<ref name="Traub95">{{cite web|last1=Traub|first1=R. J.|last2=Jensen|first2=J. A.|title=Tritium radioluminescent devices, Health and Safety Manual|url=http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/27/001/27001618.pdf|publisher=International Atomic Energy Agency|access-date=20 May 2015|page=2.4|date=June 1995|archive-url=https://web.archive.org/web/20150906043743/http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/27/001/27001618.pdf|archive-date=6 September 2015|url-status=live}}</ref> Small amounts of tritium are produced naturally by cosmic rays striking atmospheric gases; tritium has also been released in [[nuclear testing|nuclear weapons tests]].<ref>{{cite web| author=Staff| date=15 November 2007| url=http://www.epa.gov/rpdweb00/radionuclides/tritium.html| publisher=U.S. Environmental Protection Agency| title=Tritium| access-date=12 February 2008| archive-url=https://web.archive.org/web/20080102171148/http://www.epa.gov/rpdweb00/radionuclides/tritium.html| archive-date=2 January 2008| url-status=live}}</ref> It is used in nuclear fusion,<ref>{{cite web| last=Nave| first=C. R.| title=Deuterium-Tritium Fusion| work=HyperPhysics| publisher=Georgia State University| date=2006| url=http://hyperphysics.phy-astr.gsu.edu/Hbase/nucene/fusion.html| access-date=8 March 2008| archive-url=https://web.archive.org/web/20080316055852/http://hyperphysics.phy-astr.gsu.edu/Hbase/nucene/fusion.html| archive-date=16 March 2008| url-status=live}}</ref> as a tracer in [[isotope geochemistry]],<ref>{{cite journal| first1=C.| last1=Kendall| first2=E.| last2=Caldwell| journal=Isotope Tracers in Catchment Hydrology| title=Chapter 2: Fundamentals of Isotope Geochemistry| editor1=C. Kendall| editor2=J. J. McDonnell| publisher=US Geological Survey| date=1998| doi=10.1016/B978-0-444-81546-0.50009-4| url=http://wwwrcamnl.wr.usgs.gov/isoig/isopubs/itchch2.html#2.5.1| access-date=8 March 2008| archive-url=https://web.archive.org/web/20080314192517/http://wwwrcamnl.wr.usgs.gov/isoig/isopubs/itchch2.html#2.5.1| archive-date=14 March 2008| pages=51–86}}</ref> and in specialized [[self-powered lighting]] devices.<ref>{{cite web| title=The Tritium Laboratory| publisher=University of Miami| date=2008| url=http://www.rsmas.miami.edu/groups/tritium/| access-date=8 March 2008| archive-url=https://web.archive.org/web/20080228061358/http://www.rsmas.miami.edu/groups/tritium/| archive-date=28 February 2008| df=dmy-all}}</ref> Tritium has also been used in chemical and biological labeling experiments as a [[radiolabel]].<ref name="holte">{{cite journal| last1=Holte| first1=A. E.| last2=Houck| first2=M. A.| last3=Collie| first3=N. L.| title=Potential Role of Parasitism in the Evolution of Mutualism in Astigmatid Mites| journal=Experimental and Applied Acarology| volume=25| issue=2| pages=97–107| date=2004|doi=10.1023/A:1010655610575| pmid=11513367| s2cid=13159020}}</ref>

Unique among the elements, distinct names are assigned to its isotopes in common use. During the early study of radioactivity, heavy radioisotopes were given their own names, but these are mostly no longer used. The symbols D and T (instead of {{chem|2|H}} and {{chem|3|H}}) are sometimes used for deuterium and tritium, but the symbol P was already used for [[phosphorus]] and thus was not available for protium.<ref>{{cite web|last=van der Krogt|first=P.|date=5 May 2005|url=http://elements.vanderkrogt.net/element.php?sym=H|publisher=Elementymology & Elements Multidict|title=Hydrogen|access-date=20 December 2010|archive-url=https://web.archive.org/web/20100123001440/http://elements.vanderkrogt.net/element.php?sym=H|archive-date=23 January 2010}}</ref> In its [[IUPAC nomenclature|nomenclatural]] guidelines, the [[International Union of Pure and Applied Chemistry]] (IUPAC) allows any of D, T, {{chem|2|H}}, and {{chem|3|H}} to be used, though {{chem|2|H}} and {{chem|3|H}} are preferred.<ref>§ IR-3.3.2, [http://old.iupac.org/reports/provisional/abstract04/RB-prs310804/Chap3-3.04.pdf Provisional Recommendations] {{Webarchive|url=https://web.archive.org/web/20160209043933/http://old.iupac.org/reports/provisional/abstract04/RB-prs310804/Chap3-3.04.pdf |date=9 February 2016 }}, Nomenclature of Inorganic Chemistry, Chemical Nomenclature and Structure Representation Division, IUPAC. Accessed on line 3 October 2007.</ref>

The [[exotic atom]] [[muonium]] (symbol Mu), composed of an anti[[muon]] and an [[electron]], can also be considered a light radioisotope of hydrogen.<ref name="Gold">{{cite book|author=IUPAC|title=Compendium of Chemical Terminology|title-link=Compendium of Chemical Terminology|publisher=[[Blackwell Scientific Publications]]|year=1997|isbn=978-0-86542-684-9|editor=A. D. McNaught, A. Wilkinson|edition=2nd|chapter=Muonium|doi=10.1351/goldbook.M04069|author-link=International Union of Pure and Applied Chemistry|access-date=15 November 2016|chapter-url=http://goldbook.iupac.org/M04069.html|archive-url=https://web.archive.org/web/20080313121643/http://goldbook.iupac.org/M04069.html|archive-date=13 March 2008|url-status=live}}</ref> Because muons decay with lifetime {{val|2.2|u=[[Microsecond|µs]]}}, muonium is too unstable for observable chemistry.<ref name="Hughes">{{cite journal|author=V. W. Hughes|display-authors=etal|year=1960|title=Formation of Muonium and Observation of its Larmor Precession|journal=[[Physical Review Letters]]|volume=5|issue=2|pages=63–65|bibcode=1960PhRvL...5...63H|doi=10.1103/PhysRevLett.5.63}}</ref> Nevertheless, muonium compounds are important test cases for [[Quantum simulator|quantum simulation]], due to the mass difference between the antimuon and the proton,<ref>{{Cite journal|last1=Bondi|first1=D. K.|last2=Connor|first2=J. N. L.|last3=Manz|first3=J.|last4=Römelt|first4=J.|date=20 October 1983|title=Exact quantum and vibrationally adiabatic quantum, semiclassical and quasiclassical study of the collinear reactions Cl + MuCl, Cl + HCl, Cl + DCl|journal=Molecular Physics|volume=50|issue=3|pages=467–488|doi=10.1080/00268978300102491|bibcode=1983MolPh..50..467B |issn=0026-8976}}</ref> and IUPAC nomenclature incorporates such hypothetical compounds as muonium chloride (MuCl) and sodium muonide (NaMu), analogous to [[hydrogen chloride]] and [[sodium hydride]] respectively.<ref name="iupac">{{cite journal
 |doi=10.1351/pac200173020377
 |author=W. H. Koppenol
 |author2=IUPAC
 |author2-link=International Union of Pure and Applied Chemistry
 |year=2001
 |title=Names for muonium and hydrogen atoms and their ions
 |url=http://www.iupac.org/publications/pac/2001/pdf/7302x0377.pdf
 |journal=[[Pure and Applied Chemistry]]
 |volume=73
 |issue=2
 |pages=377–380
 |s2cid=97138983
 |access-date=15 November 2016
|archive-url=https://web.archive.org/web/20110514000319/http://www.iupac.org/publications/pac/2001/pdf/7302x0377.pdf
 |archive-date=14 May 2011
|url-status=live
 }}</ref>

[[Antihydrogen]] ({{physics particle|anti=yes|H}}) is the [[antimatter]] counterpart to hydrogen. It consists of an [[antiproton]] with a [[positron]]. Antihydrogen is the only type of antimatter atom to have been produced {{as of|2015|lc=y}}.<ref name="char15">{{cite journal|last1=Charlton|first1=Mike|last2=Van Der Werf|first2=Dirk Peter|title=Advances in antihydrogen physics|journal=Science Progress|date=1 March 2015|volume=98|issue=1|pages=34–62|doi=10.3184/003685015X14234978376369|pmid=25942774|pmc=10365473 |s2cid=23581065}}</ref><ref name="Keller15">{{cite journal|last1=Kellerbauer|first1=Alban|title=Why Antimatter Matters|journal=European Review|date=29 January 2015|volume=23|issue=1|pages=45–56|doi=10.1017/S1062798714000532|s2cid=58906869}}</ref>

=== Thermal and physical properties ===
Table of thermal and physical properties of hydrogen (H{{sub|2}}) at atmospheric pressure:<ref>{{Cite book |last=Holman |first=Jack P. |url=https://www.worldcat.org/oclc/46959719 |title=Heat transfer |date=2002 |publisher=McGraw-Hill |isbn=0-07-240655-0 |edition=9th |location=New York, NY |pages=600–606 |language=English |oclc=46959719}}</ref><ref>{{cite book |author-link1=Frank P. Incropera |last1=Incropera |last2=Dewitt |last3=Bergman |last4=Lavigne |first1=Frank P. |first2=David P. |first3=Theodore L. |first4=Adrienne S. |url=https://www.worldcat.org/oclc/62532755 |title=Fundamentals of heat and mass transfer |date=2007 |publisher=John Wiley and Sons, Inc |isbn=978-0-471-45728-2 |edition=6th |location=Hoboken, NJ |pages=941–950 |language=English |oclc=62532755 }}</ref>
{|class="wikitable mw-collapsible mw-collapsed"
|Temperature (K)
|Density (kg/m^3)
|Specific heat (kJ/kg K)
|Dynamic viscosity (kg/m s)
|Kinematic viscosity (m^2/s)
|Thermal conductivity (W/m K)
|Thermal diffusivity (m^2/s)
|Prandtl Number
|-
|100
|0.24255
|11.23
|4.21E-06
|1.74E-05
|6.70E-02
|2.46E-05
|0.707
|-
|150
|0.16371
|12.602
|5.60E-06
|3.42E-05
|0.0981
|4.75E-05
|0.718
|-
|200
|0.1227
|13.54
|6.81E-06
|5.55E-05
|0.1282
|7.72E-05
|0.719
|-
|250
|0.09819
|14.059
|7.92E-06
|8.06E-05
|0.1561
|1.13E-04
|0.713
|-
|300
|0.08185
|14.314
|8.96E-06
|1.10E-04
|0.182
|1.55E-04
|0.706
|-
|350
|0.07016
|14.436
|9.95E-06
|1.42E-04
|0.206
|2.03E-04
|0.697
|-
|400
|0.06135
|14.491
|1.09E-05
|1.77E-04
|0.228
|2.57E-04
|0.69
|-
|450
|0.05462
|14.499
|1.18E-05
|2.16E-04
|0.251
|3.16E-04
|0.682
|-
|500
|0.04918
|14.507
|1.26E-05
|2.57E-04
|0.272
|3.82E-04
|0.675
|-
|550
|0.04469
|14.532
|1.35E-05
|3.02E-04
|0.292
|4.52E-04
|0.668
|-
|600
|0.04085
|14.537
|1.43E-05
|3.50E-04
|0.315
|5.31E-04
|0.664
|-
|700
|0.03492
|14.574
|1.59E-05
|4.55E-04
|0.351
|6.90E-04
|0.659
|-
|800
|0.0306
|14.675
|1.74E-05
|5.69E-04
|0.384
|8.56E-04
|0.664
|-
|900
|0.02723
|14.821
|1.88E-05
|6.90E-04
|0.412
|1.02E-03
|0.676
|-
|1000
|0.02424
|14.99
|2.01E-05
|8.30E-04
|0.448
|1.23E-03
|0.673
|-
|1100
|0.02204
|15.17
|2.13E-05
|9.66E-04
|0.488
|1.46E-03
|0.662
|-
|1200
|0.0202
|15.37
|2.26E-05
|1.12E-03
|0.528
|1.70E-03
|0.659
|-
|1300
|0.01865
|15.59
|2.39E-05
|1.28E-03
|0.568
|1.96E-03
|0.655
|-
|1400
|0.01732
|15.81
|2.51E-05
|1.45E-03
|0.61
|2.23E-03
|0.65
|-
|1500
|0.01616
|16.02
|2.63E-05
|1.63E-03
|0.655
|2.53E-03
|0.643
|-
|1600
|0.0152
|16.28
|2.74E-05
|1.80E-03
|0.697
|2.82E-03
|0.639
|-
|1700
|0.0143
|16.58
|2.85E-05
|1.99E-03
|0.742
|3.13E-03
|0.637
|-
|1800
|0.0135
|16.96
|2.96E-05
|2.19E-03
|0.786
|3.44E-03
|0.639
|-
|1900
|0.0128
|17.49
|3.07E-05
|2.40E-03
|0.835
|3.73E-03
|0.643
|-
|2000
|0.0121
|18.25
|3.18E-05
|2.63E-03
|0.878
|3.98E-03
|0.661
|}

== History ==
{{Main|Timeline of hydrogen technologies}}

=== 18th century ===
[[File:Portret van Robert Boyle, RP-P-OB-4578 (cropped).jpg|thumb|[[Robert Boyle]], who discovered the reaction between [[iron filings]] and dilute acids]]
In 1671, Irish scientist [[Robert Boyle]] discovered and described the reaction between [[iron]] filings and dilute [[acid]]s, which results in the production of hydrogen gas.<ref>{{Cite book |last=Boyle |first=R. |url=https://quod.lib.umich.edu/e/eebo2/A29057.0001.001?rgn=main;view=fulltext |title=Tracts written by the Honourable Robert Boyle containing new experiments, touching the relation betwixt flame and air, and about explosions, an hydrostatical discourse occasion'd by some objections of Dr. Henry More against some explications of new experiments made by the author of these tracts: To which is annex't, an hydrostatical letter, dilucidating an experiment about a way of weighing water in water, new experiments, of the positive or relative levity of bodies under water, of the air's spring on bodies under water, about the differing pressure of heavy solids and fluids |publisher=Printed for Richard Davis |year=1672 |pages=64–65}}</ref><ref>{{cite web
|first=M.
|last=Winter
|date=2007
|url=http://education.jlab.org/itselemental/ele001.html
|title=Hydrogen: historical information
|publisher=WebElements Ltd
|access-date=5 February 2008
|archive-url=https://web.archive.org/web/20080410102154/http://education.jlab.org/itselemental/ele001.html
|archive-date=10 April 2008
}}</ref>
Boyle did not note that the gas was inflammable, but hydrogen would play a key role in overturning the [[phlogiston theory]] of combustion.<ref name=Ramsay-1896>{{Cite book |last=Ramsay |first=W. |url=https://www.gutenberg.org/files/52778/52778-h/52778-h.htm |title=The gases of the atmosphere: The history of their discovery |publisher=Macmillan |year=1896 |pages=19}}</ref>

In 1766, [[Henry Cavendish]] was the first to recognize hydrogen gas as a discrete substance, by naming the gas from a [[metal-acid reaction]] "inflammable air". He speculated that "inflammable air" was in fact identical to the hypothetical substance "[[Phlogiston theory|phlogiston]]"<ref>{{cite book |last = Musgrave
 |first = A.
 |chapter = Why did oxygen supplant phlogiston? Research programmes in the Chemical Revolution
 |title = Method and appraisal in the physical sciences
 |series = The Critical Background to Modern Science, 1800–1905
 |editor = Howson, C.
 |year = 1976
 |publisher = Cambridge University Press
 |access-date = 22 October 2011
 |chapter-url = http://ebooks.cambridge.org/chapter.jsf?bid=CBO9780511760013&cid=CBO9780511760013A009
 |doi = 10.1017/CBO9780511760013
 |isbn = 978-0-521-21110-9
 |url-access = registration
 |url = https://archive.org/details/methodappraisali0000unse
}}</ref><ref name="cav766">{{cite journal|last1=Cavendish|first1=Henry|title=Three Papers, Containing Experiments on Factitious Air, by the Hon. Henry Cavendish, F. R. S.|journal=Philosophical Transactions|date=12 May 1766|volume=56|pages=141–184|jstor=105491|bibcode=1766RSPT...56..141C|doi=10.1098/rstl.1766.0019|doi-access=free}}</ref> and further finding in 1781 that the gas produces water when burned. He is usually given credit for the discovery of hydrogen as an element.<ref name="Nostrand">{{cite encyclopedia| title=Hydrogen| encyclopedia=Van Nostrand's Encyclopedia of Chemistry| pages=797–799| publisher=Wylie-Interscience| year=2005| isbn=978-0-471-61525-5}}</ref><ref name="nbb">{{cite book| last=Emsley| first=John| title=Nature's Building Blocks| publisher=Oxford University Press| year=2001| location=Oxford| pages=183–191| isbn=978-0-19-850341-5}}</ref>

[[File:Antoine-Laurent Lavoisier by Louis Jean Desire Delaistre (cropped).jpg|thumb|[[Antoine Lavoisier]], who identified the element that came to be known as hydrogen]]
In 1783, [[Antoine Lavoisier]] identified the element that came to be known as hydrogen<ref>{{cite book| last=Stwertka| first=Albert| title=A Guide to the Elements| url=https://archive.org/details/guidetoelements00stwe| url-access=registration| publisher=Oxford University Press| year=1996| pages=[https://archive.org/details/guidetoelements00stwe/page/16 16–21]| isbn=978-0-19-508083-4}}</ref> when he and [[Pierre-Simon Laplace|Laplace]] reproduced Cavendish's finding that water is produced when hydrogen is burned.<ref name="nbb" /> Lavoisier produced hydrogen for his experiments on mass conservation by treating metallic [[iron]] with a steam of H<sub>2</sub> through an incandescent iron tube heated in a fire. Anaerobic oxidation of iron by the protons of water at high temperature can be schematically represented by the set of following reactions:
:1) {{chem2|Fe + H2O -> FeO + H2}}

:2) {{chem2|Fe + 3 H2O -> Fe2O3 + 3 H2}}

:3) {{chem2|Fe + 4 H2O -> Fe3O4 + 4 H2}}

Many metals react similarly with water leading to the production of hydrogen.<ref>{{Cite journal |last1=Northwood |first1=D. O. |last2=Kosasih |first2=U. |date=1983 |title=Hydrides and delayed hydrogen cracking in zirconium and its alloys |url=https://journals.sagepub.com/doi/full/10.1179/imtr.1983.28.1.92 |journal=International Metals Reviews |language=en |volume=28 |issue=1 |pages=92–121 |doi=10.1179/imtr.1983.28.1.92 |issn=0308-4590}}</ref>  In some situations, this H<sub>2</sub>-producing process is problematic as is the case of zirconium cladding on nuclear fuel rods.<ref>{{cite journal |doi=10.1016/j.jnucmat.2019.02.042 |title=Hydrogen in zirconium alloys: A review |date=2019 |last1=Motta |first1=Arthur T. |last2=Capolungo |first2=Laurent |last3=Chen |first3=Long-Qing |last4=Cinbiz |first4=Mahmut Nedim |last5=Daymond |first5=Mark R. |last6=Koss |first6=Donald A. |last7=Lacroix |first7=Evrard |last8=Pastore |first8=Giovanni |last9=Simon |first9=Pierre-Clément A. |last10=Tonks |first10=Michael R. |last11=Wirth |first11=Brian D. |last12=Zikry |first12=Mohammed A. |journal=Journal of Nuclear Materials |volume=518 |pages=440–460 |bibcode=2019JNuM..518..440M }}</ref>

===19th century===

By 1806 hydrogen was used to fill balloons.<ref>{{Cite journal |last=Szydło |first=Z. A. |date=2020 |title=Hydrogen - Some Historical Highlights |journal=Chemistry-Didactics-Ecology-Metrology |volume=25 |issue=1–2 |pages=5–34|doi=10.2478/cdem-2020-0001 |s2cid=231776282 |doi-access=free }}</ref>
[[François Isaac de Rivaz]] built the first [[de Rivaz engine]], an internal combustion engine powered by a mixture of hydrogen and oxygen in 1806. [[Edward Daniel Clarke]] invented the hydrogen gas blowpipe in 1819. The [[Döbereiner's lamp]] and [[limelight]] were invented in 1823.Hydrogen was [[Liquid hydrogen|liquefied]] for the first time by [[James Dewar]] in 1898 by using [[regenerative cooling]] and his invention, the [[vacuum flask]]. He produced [[solid hydrogen]] the next year.<ref name="nbb" />

One of the first [[quantum mechanics|quantum]] effects to be explicitly noticed (but not understood at the time) was [[James Clerk Maxwell]]'s observation that the [[specific heat capacity]] of {{chem2|H2}} unaccountably departs from that of a [[diatomic]] gas below room temperature and begins to increasingly resemble that of a monatomic gas at cryogenic temperatures. According to quantum theory, this behavior arises from the spacing of the (quantized) rotational energy levels, which are particularly wide-spaced in {{chem2|H2}} because of its low mass. These widely spaced levels inhibit equal partition of heat energy into rotational motion in hydrogen at low temperatures. Diatomic gases composed of heavier atoms do not have such widely spaced levels and do not exhibit the same effect.<ref name="Berman">{{cite journal
|last1=Berman|first1=R.|last2=Cooke|first2=A. H.|last3=Hill|first3=R. W.
|title=Cryogenics|journal=Annual Review of Physical Chemistry
|date=1956|volume=7|pages=1–20
|doi=10.1146/annurev.pc.07.100156.000245|bibcode = 1956ARPC....7....1B }}</ref>

===20th century===
The existence of the [[hydride|hydride anion]] was suggested by [[Gilbert N. Lewis]] in 1916 for group 1 and 2 salt-like compounds.  In 1920, Moers electrolyzed molten [[lithium hydride]] (LiH), producing a [[Stoichiometry|stoichiometric]] quantity of hydrogen at the anode.<ref name="Moers">{{cite journal|last=Moers|first=K.|title=Investigations on the Salt Character of Lithium Hydride|journal=Zeitschrift für Anorganische und Allgemeine Chemie|date=1920|volume=113|issue=191|pages=179–228|doi=10.1002/zaac.19201130116|url=https://zenodo.org/record/1428170|access-date=24 August 2019|archive-url=https://web.archive.org/web/20190824162148/https://zenodo.org/record/1428170/files/article.pdf|archive-date=24 August 2019|url-status=live}}</ref>

[[File:Emission_spectrum-H_labeled.svg|thumb|Hydrogen emission spectrum lines in the four visible lines of the [[Balmer series]]|alt=A line spectrum showing black background with narrow lines superimposed on it: one violet, one blue, one cyan, and one red.]]
Because of its simple atomic structure, consisting only of a proton and an electron, the [[hydrogen atom]], together with the spectrum of light produced from it or absorbed by it, has been central to the development of the theory of [[atom]]ic structure.<ref>{{cite book |last=Crepeau |first=R.
|title=Niels Bohr: The Atomic Model |series=Great Scientific Minds
|date=1 January 2006 |isbn=978-1-4298-0723-4
}}</ref> Hydrogen's unique position as the only neutral atom for which the [[Schrödinger equation]] can be directly solved, has significantly contributed to the understanding of quantum mechanics through the exploration of its energetics.<ref name="Laursen04">{{cite web|last1=Laursen|first1=S.|last2=Chang|first2=J.|last3=Medlin|first3=W.|last4=Gürmen|first4=N.|last5=Fogler|first5=H. S.|title=An extremely brief introduction to computational quantum chemistry|url=http://www.umich.edu/~elements/5e/web_mod/quantum/introduction_3.htm|website=Molecular Modeling in Chemical Engineering|publisher=University of Michigan|access-date=4 May 2015|date=27 July 2004|archive-url=https://web.archive.org/web/20150520061846/http://www.umich.edu/~elements/5e/web_mod/quantum/introduction_3.htm|archive-date=20 May 2015|url-status=live}}</ref> Furthermore, study of the corresponding simplicity of the hydrogen molecule and the corresponding cation [[H2+|{{chem2|H2+}}]] brought understanding of the nature of the chemical bond, which followed shortly after the quantum mechanical treatment of the hydrogen atom had been developed in the mid-1920s.{{citation needed|date=January 2025}}

==== Hydrogen-lifted airship ====
[[File:Hindenburg over New York 1937 (cropped).jpg|alt=Airship Hindenburg over New York|thumb|The [[Hindenburg-class airship|Hindenburg]] over [[New York City]] in 1937]]
Because {{chem2|H2}} is only 7% the density of air, it was once widely used as a [[lifting gas]] in balloons and [[airship]]s.<ref name="Almqvist03">{{cite book |last1=Almqvist |first1=Ebbe |url={{Google books|OI0fTJhydh4C|page=|keywords=|text=|plainurl=yes}} |title=History of industrial gases |date=2003 |publisher=Kluwer Academic/Plenum Publishers |isbn=978-0-306-47277-0 |location=New York, N.Y. |pages=47–56 |access-date=20 May 2015}}</ref> The first hydrogen-filled [[balloon]] was invented by [[Jacques Charles]] in 1783. Hydrogen provided the lift for the first reliable form of air-travel following the 1852 invention of the first hydrogen-lifted [[airship]] by [[Henri Giffard]]. German count [[Ferdinand von Zeppelin]] promoted the idea of rigid airships lifted by hydrogen that later were called [[Zeppelin]]s; the first of which had its maiden flight in 1900.<ref name="nbb" /> Regularly scheduled flights started in 1910 and by the outbreak of World War I in August 1914, they had carried 35,000 passengers without a serious incident. Hydrogen-lifted airships were used as observation platforms and bombers during the war.{{citation needed|date=January 2025}}

The first non-stop transatlantic crossing was made by the British airship ''[[R34 (airship)|R34]]'' in 1919. Regular passenger service resumed in the 1920s and the discovery of [[helium]] reserves in the United States promised increased safety, but the U.S. government refused to sell the gas for this purpose. Therefore, hydrogen was used in the [[LZ 129 Hindenburg|''Hindenburg'']] airship, which caught fire over [[New Jersey]] on 6 May 1937.<ref name="nbb" />  The hydrogen that filled the airship was ignited, possibly by static electricity, and burst into flames.<ref>{{Cite web |last=Follows |first=Mike |date=July 2, 2015 |title=What ignited the Hindenburg? |url=https://edu.rsc.org/feature/what-ignited-the-hindenburg/2000137.article |access-date=2025-02-19 |website=RSC Education |language=en}}</ref> The visible flames in the photographs of the disaster were the result of the airship skin burning.<ref>{{Cite web |title=Myths about the Hindenburg Crash |url=https://www.airships.net/hindenburg/disaster/myths/ |url-status=live |archive-url=https://web.archive.org/web/20210420055020/https://www.airships.net/hindenburg/disaster/myths/ |archive-date=20 April 2021 |access-date=29 March 2021 |website=Airships.net |language=en-US}}</ref> Following the [[Hindenburg disaster|Hindenberg disaster]], commercial hydrogen airship travel [[Rigid airship#Demise|ceased]]. Hydrogen is still used, in preference to non-flammable but more expensive helium, as a lifting gas for [[Weather balloon#Materials and equipment|weather balloons]].<ref>{{Cite web |last=Rappe |first=Mollie |last2= |first2= |date=May 9, 2023 |title=Researchers switch from helium to hydrogen weather balloons |url=https://phys.org/news/2023-05-helium-hydrogen-weather-balloons.html |access-date=2025-02-19 |website=phys.org |language=en}}</ref>

==== Deuterium and tritium ====
[[Deuterium]] was discovered in December 1931 by [[Harold Urey]], and [[tritium]] was prepared in 1934 by [[Ernest Rutherford]], [[Mark Oliphant]], and [[Paul Harteck]].<ref name="Nostrand" /> [[Heavy water]], which consists of deuterium in the place of regular hydrogen, was discovered by Urey's group in 1932.<ref name="nbb" />

==== Hydrogen-cooled turbogenerator ====
The first [[hydrogen-cooled turbogenerator]] went into service using gaseous hydrogen as a [[coolant]] in the rotor and the stator in 1937 at [[Dayton, Ohio|Dayton]], Ohio, owned by the Dayton Power & Light Co.<ref>{{cite book|url=https://archive.org/stream/chronologicalhis00natirich/chronologicalhis00natirich_djvu.txt|title=A chronological history of electrical development from 600 B.C|author=National Electrical Manufacturers Association|year=1946|page=102|publisher=New York, N.Y., National Electrical Manufacturers Association|access-date=9 February 2016|archive-url=https://web.archive.org/web/20160304141424/http://www.archive.org/stream/chronologicalhis00natirich/chronologicalhis00natirich_djvu.txt|archive-date=4 March 2016|url-status=live}}</ref> This was justified by the high thermal conductivity and very low viscosity of hydrogen gas, thus lower drag than air. This is the most common coolant used for generators 60 MW and larger; smaller generators are usually [[air cooling|air-cooled]].{{citation needed|date=January 2025}}

==== Nickel–hydrogen battery ====
The [[nickel–hydrogen battery]] was used for the first time in 1977 aboard the U.S. Navy's Navigation technology satellite-2 (NTS-2).<ref>{{Cite journal|title=NTS-2 Nickel-Hydrogen Battery Performance 31|journal=Journal of Spacecraft and Rockets|volume=17|pages=31–34|doi=10.2514/3.57704|year=1980|last1=Stockel|first1=J.F|last2=j.d. Dunlop|last3=Betz|first3=F|bibcode=1980JSpRo..17...31S}}</ref> The [[International Space Station]],<ref>{{cite conference|url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20020070612_2002115777.pdf|work=IECEC '02. 2002 37th Intersociety Energy Conversion Engineering Conference, 2002|pages=45–50|date=July 2002|access-date=11 November 2011|doi=10.1109/IECEC.2002.1391972|title=Validation of international space station electrical performance model via on-orbit telemetry|last1=Jannette|first1=A. G.|last2=Hojnicki|first2=J. S.|last3=McKissock|first3=D. B.|last4=Fincannon|first4=J.|last5=Kerslake|first5=T. W.|last6=Rodriguez|first6=C. D.|isbn=0-7803-7296-4|archive-url=https://web.archive.org/web/20100514100504/http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20020070612_2002115777.pdf|archive-date=14 May 2010|url-status=live|hdl=2060/20020070612|hdl-access=free}}</ref> [[2001 Mars Odyssey|Mars Odyssey]]<ref>{{cite book|doi=10.1109/AERO.2002.1035418 |date=2002|last1=Anderson|first1=P. M.|last2=Coyne|first2=J. W.|title=Proceedings, IEEE Aerospace Conference |chapter=A lightweight, high reliability, single battery power system for interplanetary spacecraft |isbn=978-0-7803-7231-3|volume=5|pages=5–2433|s2cid=108678345}}</ref> and the [[Mars Global Surveyor]]<ref>{{cite web|url=http://www.astronautix.com/craft/marveyor.htm|title=Mars Global Surveyor|publisher=Astronautix.com|access-date=6 April 2009|archive-url=https://web.archive.org/web/20090810180658/http://www.astronautix.com/craft/marveyor.htm|archive-date=10 August 2009}}</ref> are equipped with nickel-hydrogen batteries. In the dark part of its orbit, the [[Hubble Space Telescope]] is also powered by nickel-hydrogen batteries, which were finally replaced in May 2009,<ref>{{cite web|url=http://www.nasa.gov/mission_pages/hubble/servicing/SM4/main/SM4_Essentials.html|title=Hubble servicing mission 4 essentials|date=7 May 2009|editor=Lori Tyahla|access-date=19 May 2015|publisher=NASA|archive-url=https://web.archive.org/web/20150313073737/http://www.nasa.gov/mission_pages/hubble/servicing/SM4/main/SM4_Essentials.html|archive-date=13 March 2015|url-status=live}}</ref> more than 19 years after launch and 13 years beyond their design life.<ref>{{cite web|url=http://www.nasa.gov/mission_pages/hubble/servicing/series/battery_story.html|title=Extending Hubble's mission life with new batteries|date=25 November 2008|first1=Susan|last1=Hendrix|editor=Lori Tyahla|access-date=19 May 2015|publisher=NASA|archive-url=https://web.archive.org/web/20160305002850/http://www.nasa.gov/mission_pages/hubble/servicing/series/battery_story.html|archive-date=5 March 2016|url-status=live}}</ref>

==Chemistry==
===Laboratory syntheses===
{{chem2|H2}} is produced in labs, often as a by-product of other reactions. Many metals react with water to produce {{chem2|H2}}, but the rate of hydrogen evolution depends on the metal, the pH, and the presence of alloying agents. Most often, hydrogen evolution is induced by acids. The alkali and alkaline earth metals, aluminium, zinc, manganese, and iron react readily with aqueous acids. This reaction is the basis of the [[Kipp's apparatus]], which once was used as a laboratory gas source:
:{{chem2|Zn + 2 H+ → Zn(2+) + H2}}

In the absence of acid, the evolution of {{chem2|H2}} is slower. Because iron is widely used structural material, its [[anaerobic corrosion]] is of technological significance:

:{{chem2|Fe + 2 H2O → Fe(OH)2 + H2}}

Many metals, such as [[aluminium]], are slow to react with water because they form passivated oxide coatings of oxides. An alloy of aluminium and [[gallium]], however, does react with water.<ref>{{cite journal|doi= 10.1016/j.ijhydene.2008.02.025|title= Activation of aluminium metal to evolve hydrogen from water|year= 2008|last1= Parmuzina|first1= A.V.|last2= Kravchenko|first2= O.V.|journal= International Journal of Hydrogen Energy|volume= 33|issue= 12|pages= 3073–3076|bibcode= 2008IJHE...33.3073P}}</ref> At high pH, aluminium can produce {{chem2|H2}}:

:{{chem2|2 Al + 6 H2O + 2 OH- → 2 [Al(OH)4]- + 3 H2}}

===Reactions of H<sub>2</sub>===
[[File:HFe H2 dppe 2.svg|thumb|right|A [[dihydrogen complex]] of iron, [HFe(H<sub>2</sub>)(dppe)<sub>2</sub>]<sup>+</sup>.]]

{{chem2|H2}} is relatively unreactive. The thermodynamic basis of this low reactivity is the very strong H–H bond, with a [[bond dissociation energy]] of 435.7 kJ/mol.<ref>{{RubberBible87th}}</ref>  It does form coordination complexes called [[dihydrogen complex]]es.  These species provide insights into the early steps in the interactions of hydrogen with metal catalysts.  According to [[neutron diffraction]], the metal and two H atoms form a triangle in these complexes.  The H-H bond remains intact but is elongated.  They are acidic.<ref>{{Cite book
| edition = 1
| publisher = Springer
| isbn = 0-306-46465-9
| last = Kubas
| first = Gregory J.
| title = Metal Dihydrogen and σ-Bond Complexes: Structure, Theory, and Reactivity
| date = 2001-08-31
}}</ref>

Although exotic on Earth, the {{chem2|H3+}} ion is common in the universe. It is a triangular species, like the aforementioned dihydrogen complexes. It is known as [[Trihydrogen cation|protonated molecular hydrogen]] or the trihydrogen cation.<ref name="Carrington">{{cite journal
|last1=Carrington|first1=A.|last2=McNab|first2=I. R.
|title=The infrared predissociation spectrum of triatomic hydrogen cation (H<sub>3</sub><sup>+</sup>)
|journal=Accounts of Chemical Research
|date=1989|volume=22|issue=6|pages=218–222
|doi=10.1021/ar00162a004}}</ref>

Hydrogen directly reacts with chlorine, fluorine and bromine to give [[Hydrogen fluoride|HF]], [[HCl]], and [[Hydrogen bromide|HBr]], respectively.  The conversion involves a [[radical chain mechanism]].  With heating, H<sub>2</sub> reacts efficiently with the alkali and alkaline earth metals to give the [[saline hydride]]s of the formula MH and MH<sub>2</sub>, respectively.<ref name=UllmannH2/>  One of the striking properties of H<sub>2</sub> is its inertness toward unsaturated organic compounds, such as [[alkene]]s and [[alkyne]]s.  These species only react with H<sub>2</sub> in the presence of catalysts.  Especially active catalysts are the [[platinum metal]]s ([[platinum]], [[rhodium]], [[palladium]], etc.).  A major driver for the mining of these rare and expensive elements is their use as catalysts.<ref name=UllmannH2/>

===Hydrogen-containing compounds===
{{Main|Hydrogen compounds}}
Most known compounds contain hydrogen, not as H<sub>2</sub>, but as covalently bonded H atoms.  This interaction is the basis of [[organic chemistry]] and [[biochemistry]].Hydrogen forms many compounds with [[carbon]], called the [[hydrocarbon]]s. Hydrocarbons are called [[organic compound]]s. In nature, they almost always contain "[[heteroatom]]s" such as nitrogen, oxygen, and sulfur.<ref name="hydrocarbon">{{cite web| title=Structure and Nomenclature of Hydrocarbons| publisher=Purdue University| url=http://chemed.chem.purdue.edu/genchem/topicreview/bp/1organic/organic.html| access-date=23 March 2008| archive-url=https://web.archive.org/web/20120611084045/http://chemed.chem.purdue.edu/genchem/topicreview/bp/1organic/organic.html| archive-date=11 June 2012}}</ref> The study of their properties is known as [[organic chemistry]]<ref>{{March6th}}</ref> and their study in the context of living [[organism]]s is called [[biochemistry]].<ref>{{Lehninger4th}}</ref> By some definitions, "organic" compounds are only required to contain carbon. However, most of them also contain hydrogen, and because it is the carbon-hydrogen bond that gives this class of compounds most of its particular chemical characteristics, carbon-hydrogen bonds are required in some definitions of the word "organic" in chemistry.<ref name="hydrocarbon" /> Millions of [[hydrocarbon]]s are known, and they are usually formed by complicated pathways that seldom involve elemental hydrogen.{{citation needed|date=January 2025}}

=== Hydrides ===
{{Main|Hydride}}
[[File:NaH.jpg|thumb|A sample of [[sodium hydride]]]]
Hydrogen forms compounds with less electronegative elements, such as [[metal]]s and [[main group]] elements.  In these compounds, hydrogen takes on a partial negative charge. The term "hydride" suggests that the H atom has acquired a negative or anionic character, denoted {{chem2|H−}}.  Usually hydride refers to hydrogen in a compound with a more [[electropositive]] element.  For hydrides other than group 1 and 2 metals, the term can be misleading, considering the low electronegativity of hydrogen. A well known hydride is [[lithium aluminium hydride]], the {{chem2|[AlH4]-}} anion carries hydridic centers firmly attached to the Al(III).<ref>{{Greenwood&Earnshaw2nd|page=228}}</ref>  Perhaps the most extensive series of hydrides are the [[boranes]], compounds consisting only of boron and hydrogen.<ref name="Downs">{{cite journal
|last1=Downs|first1=A. J.
|last2=Pulham|first2=C. R.
|title=The hydrides of aluminium, gallium, indium, and thallium: a re-evaluation
|journal=Chemical Society Reviews
|date=1994|volume=23|pages=175–184
|doi=10.1039/CS9942300175
|issue=3
}}</ref>

Hydrides can bond to these electropositive elements not only as a terminal [[ligand]] but also as [[bridging ligand]]s.  In diborane ({{chem2|B2H6}}), four H's are terminal and two bridge between the two B atoms.<ref name="Miessler" />

===Protons and acids ===
{{Further|Acid–base reaction}}
[[File:Base pair AT.svg|282px|thumb|right| An "A-T base pair" in DNA illustrating how hydrogen bonds are critical to the [[genetic code]].  The drawing illustrates that in many chemical depictions, C-H bonds are not always shown explicitly, an indication of their pervasiveness.]]

When bonded to a more electronegative element, particularly [[fluorine]], [[oxygen]], or [[nitrogen]], hydrogen can participate in a form of medium-strength noncovalent bonding with another electronegative element with a lone pair, a phenomenon called [[hydrogen bond]]ing that is critical to the stability of many biological molecules.<ref>{{cite web|last=Kimball|first=J. W.|title=Hydrogen|work=Kimball's Biology Pages|date=7 August 2003|url=http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/H/HydrogenBonds.html|access-date=4 March 2008|archive-url=https://web.archive.org/web/20080304040611/http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/H/HydrogenBonds.html|archive-date=4 March 2008|url-status=live}}</ref><ref>IUPAC Compendium of Chemical Terminology, Electronic version, [http://goldbook.iupac.org/H02899.html Hydrogen Bond] {{Webarchive|url=https://web.archive.org/web/20080319045705/http://goldbook.iupac.org/H02899.html |date=19 March 2008 }}</ref> 
{{chem2|H+}} can also be obtained by oxidation of H<sub>2</sub>. Under the [[Brønsted–Lowry acid–base theory]], acids are proton donors, while bases are proton acceptors.{{citation needed|date=January 2025}}

A bare proton, {{chem2|H+}} essentially cannot exist in anything other than a vacuum. Otherwise it attaches to other atoms, ions, or molecules.  Even species as inert as methane can be protonated. The term 'proton' is used loosely and metaphorically to refer to refer to solvated {{chem2|H+}}" without any implication that any single protons exist freely as a species.  To avoid the implication of the naked proton in solution, acidic aqueous solutions are sometimes considered to contain the "[[hydronium]] ion" ({{chem2|[H3O]+}}) or still more accurately, {{chem2|[H9O4]+}}.<ref name="Okumura">{{cite journal
|last1=Okumura|first1=A. M.
|last2=Yeh|first2=L. I.|last3=Myers|first3=J. D.|last4=Lee|first4=Y. T.
|title=Infrared spectra of the solvated hydronium ion: vibrational predissociation spectroscopy of mass-selected H<sub>3</sub>O+•(H<sub>2</sub>O<sub>)n</sub>•(H<sub>2</sub>)<sub>m</sub>
|journal=Journal of Physical Chemistry
|date=1990|volume=94|issue=9|pages=3416–3427|doi=10.1021/j100372a014
}}</ref> Other [[oxonium ion]]s are found when water is in acidic solution with other solvents.<ref name="Perdoncin">{{cite journal
|last1=Perdoncin|first1=G.|last2=Scorrano|first2=G.
|title=Protonation Equilibria in Water at Several Temperatures of Alcohols, Ethers, Acetone, Dimethyl Sulfide, and Dimethyl Sulfoxide
|journal=Journal of the American Chemical Society
|date=1977|volume=99|issue=21|pages=6983–6986
|doi=10.1021/ja00463a035
|bibcode=1977JAChS..99.6983P }}</ref>

==Occurrence==
===Cosmic===
[[File:Nursery of New Stars - GPN-2000-000972.jpg|right|thumb|[[NGC 604]], a giant [[H II region|region of ionized hydrogen]] in the [[Triangulum Galaxy]]|alt=A white-green cotton-like clog on black background.]]
Hydrogen, as atomic H, is the most [[Natural abundance|abundant]] [[chemical element]] in the universe, making up 75% of [[Baryon|normal matter]] by [[mass]] and >90% by number of atoms.<ref>{{cite book
|last=Clayton|first=D. D.
|title=Handbook of Isotopes in the Cosmos: Hydrogen to Gallium
|date=2003
|publisher=[[Cambridge University Press]]
|isbn=978-0-521-82381-4
}}</ref> In astrophysics, neutral hydrogen in the [[interstellar medium]] is called ''H I'' and ionized hydrogen is called ''H II''.<ref>{{Cite book |last1=Kaplan |first1=S. A. |url=https://www.degruyter.com/document/doi/10.4159/harvard.9780674493988/html |title=The Interstellar Medium |last2=Pikelner |first2=S. B. |date=1970-12-31 |publisher=Harvard University Press |isbn=978-0-674-49397-1 |pages=1–77 |chapter=1. Interstellar Hydrogen |doi=10.4159/harvard.9780674493988}}</ref> Radiation from stars ionizes H I to H II, creating [[Strömgren sphere|spheres]] of ionized H II around stars. In the [[chronology of the universe]] neutral hydrogen dominated until the birth of stars during the era of [[reionization]] led to bubbles of ionized hydrogen that grew and merged over 500  million of years.<ref>{{Cite journal |last=Dijkstra |first=Mark |date=January 2014 |title=Lyα Emitting Galaxies as a Probe of Reionisation |url=https://www.cambridge.org/core/journals/publications-of-the-astronomical-society-of-australia/article/ly-emitting-galaxies-as-a-probe-of-reionisation/51F95FB047C1F0418D1DA56D39470C22 |journal=Publications of the Astronomical Society of Australia |language=en |volume=31 |pages=e040 |doi=10.1017/pasa.2014.33 |arxiv=1406.7292 |bibcode=2014PASA...31...40D |issn=1323-3580}}</ref>
They are the source of the 21-cm [[hydrogen line]] at 1420&nbsp;MHz that is detected in order to probe primordial hydrogen. The large amount of neutral hydrogen found in the [[damped Lyman-alpha system]]s is thought to dominate the [[Physical cosmology|cosmological]] [[baryon]]ic density of the universe up to a [[redshift]] of ''z''&nbsp;=&nbsp;4.<ref>{{cite journal
|last1=Storrie-Lombardi|first1=L. J.
|last2=Wolfe|first2=A. M.
|title=Surveys for z > 3 Damped Lyman-alpha Absorption Systems: the Evolution of Neutral Gas
|journal=Astrophysical Journal
|date=2000|volume=543|pages=552–576
|arxiv=astro-ph/0006044
|doi=10.1086/317138
|bibcode=2000ApJ...543..552S
|issue=2|s2cid=120150880
}}</ref>

Hydrogen is found in great abundance in stars and [[gas giant]] planets. [[Molecular cloud]]s of {{chem2|H2}} are associated with [[star formation]]. Hydrogen plays a vital role in powering [[star]]s through the [[proton-proton reaction]] in lower-mass stars, and through the [[CNO cycle]] of [[nuclear fusion]] in case of stars more massive than the [[Sun]].<ref>{{cite web
|last1=Haubold|first1=H.|last2=Mathai|first2=A. M.
|date=15 November 2007|url=http://neutrino.aquaphoenix.com/un-esa/sun/sun-chapter4.html |archive-url =https://web.archive.org/web/20111211073137/http://neutrino.aquaphoenix.com/un-esa/sun/sun-chapter4.html
 |archive-date=11 December 2011 |title=Solar Thermonuclear Energy Generation
|publisher=[[Columbia University]]|access-date=12 February 2008
}}</ref>

Hydrogen [[Plasma (physics)|plasma]] states have properties quite distinct from those of molecular or atomic hydrogen. As a plasma, hydrogen's electron and proton are not bound together, resulting in very high electrical conductivity and high emissivity (producing the light from the Sun and other stars). The charged particles are highly influenced by magnetic and electric fields. For example, in the [[solar wind]] they interact with the Earth's [[magnetosphere]] giving rise to [[Birkeland current]]s and the [[aurora]].{{citation needed|date=January 2025}}

A molecular form called [[protonated molecular hydrogen]] ({{chem2|H3+}}) is found in the interstellar medium, where it is generated by ionization of molecular hydrogen from [[cosmic ray]]s. This ion has also been observed in the [[Primary atmosphere|upper atmosphere of Jupiter]]. The ion is long-lived in outer space due to the low temperature and density. {{chem2|H3+}} is one of the most abundant ions in the universe, and it plays a notable role in the chemistry of the interstellar medium.<ref>{{cite web|author=McCall Group|author2=Oka Group|date=22 April 2005|url=http://h3plus.uiuc.edu/|title=H3+ Resource Center|publisher=Universities of Illinois and Chicago|access-date=5 February 2008|archive-url=https://web.archive.org/web/20071011211244/http://h3plus.uiuc.edu/|archive-date=11 October 2007}}</ref> Neutral [[triatomic hydrogen]] {{chem2|H3}} can exist only in an excited form and is unstable.<ref name="couple">{{citation|year=2003|publisher=Department of Molecular and Optical Physics, University of Freiburg, Germany|author=Helm, H.|display-authors=etal|title=Dissociative Recombination of Molecular Ions with Electrons|pages=275–288|doi=10.1007/978-1-4615-0083-4_27|chapter=Coupling of Bound States to Continuum States in Neutral Triatomic Hydrogen|isbn=978-1-4613-4915-0}}</ref> By contrast, the positive [[hydrogen molecular ion]] ({{chem2|H2+}}) is a rare in the universe.{{citation needed|date=January 2025}}

===Terrestrial===
Hydrogen is the third most abundant element on the Earth's surface,<ref name="ArgonneBasic">{{cite journal
|author=Dresselhaus, M.
|author-link=Mildred Dresselhaus
|display-authors=etal
|date=15 May 2003
|url=http://www.sc.doe.gov/bes/hydrogen.pdf
|title=Basic Research Needs for the Hydrogen Economy
|journal=APS March Meeting Abstracts
|volume=2004
|pages=m1.001
|publisher=Argonne National Laboratory, U.S. Department of Energy, Office of Science Laboratory
|access-date=5 February 2008
|archive-url=https://web.archive.org/web/20080213144956/http://www.sc.doe.gov/bes/hydrogen.pdf
|archive-date=13 February 2008
|bibcode=2004APS..MAR.m1001D
}}</ref> mostly in the form of [[chemical compound]]s such as [[hydrocarbon]]s and water.<ref name="Miessler">{{cite book|first1=G. L.|last1=Miessler|last2=Tarr|first2=D. A.|date=2003|title=Inorganic Chemistry|edition=3rd|publisher=Prentice Hall|isbn=978-0-13-035471-6|url-access=registration|url=https://archive.org/details/inorganicchemist03edmies}}</ref>  Elemental hydrogen is normally in the form of a gas, {{chem2|H2}}. It is present in a very low concentration in Earth's atmosphere (around 0.53 [[part per million|ppm]] on a molar basis<ref name="Grinter">{{cite journal |last1=Rhys Grinter |last2=Kropp |first2=A. |last3=Venugopal |display-authors=etal |date=2023 |title=Structural basis for bacterial energy extraction from atmospheric hydrogen |journal=Nature |volume=615 |issue=7952 |pages=541–547 |bibcode=2023Natur.615..541G |doi=10.1038/s41586-023-05781-7 |pmc=10017518 |pmid=36890228}}</ref>) because of its light weight, which enables it to [[atmospheric escape|escape the atmosphere]] more rapidly than heavier gases. Despite its low concentration in our atmosphere, terrestrial hydrogen is sufficiently abundant to support the metabolism of several bacteria.<ref>{{cite journal |doi=10.1042/BST20230120 |title=Developing high-affinity, oxygen-insensitive &#91;NiFe&#93;-hydrogenases as biocatalysts for energy conversion |date=2023 |last1=Greening |first1=Chris |last2=Kropp |first2=Ashleigh |last3=Vincent |first3=Kylie |last4=Grinter |first4=Rhys |journal=Biochemical Society Transactions |volume=51 |issue=5 |pages=1921–1933 |pmid=37743798 |pmc=10657181 }}</ref>

Large underground deposits of hydrogen gas have been discovered in several countries including [[Mali]], [[France]] and [[Australia]].<ref name=":0">{{Cite web |last=Pearce |first=Fred |date=January 25, 2024 |title=Natural Hydrogen: A Potential Clean Energy Source Beneath Our Feet |url=https://e360.yale.edu/features/natural-geologic-hydrogen-climate-change |access-date=2024-01-27 |website=[[Yale Environment 360|Yale E360]] |language=en-US}}</ref> As of 2024, it is uncertain how much underground hydrogen can be extracted economically.<ref name=":0" />

== Production and storage==
{{Main|Hydrogen production}}

===Industrial routes===
Nearly all of the world's current supply of hydrogen gas ({{chem2|H2}}) is created from fossil fuels.<ref>{{cite news |last1=Reed |first1=Stanley |last2=Ewing |first2=Jack |date=13 July 2021 |title=Hydrogen Is One Answer to Climate Change. Getting It Is the Hard Part |url=https://www.nytimes.com/2021/07/13/business/hydrogen-climate-change.html |work=The New York Times}}</ref><ref name="rosenow-2022">{{cite journal |last1=Rosenow |first1=Jan |date=27 September 2022 |title=Is heating homes with hydrogen all but a pipe dream? An evidence review |journal=Joule |volume=6 |issue=10 |pages=2225–2228 |bibcode=2022Joule...6.2225R |doi=10.1016/j.joule.2022.08.015 |s2cid=252584593 |doi-access=free}}  Article in press.</ref>{{rp|1}} Many methods exist for producing H<sub>2</sub>, but three dominate commercially: steam reforming often coupled to water-gas shift, partial oxidation of hydrocarbons, and water electrolysis.<ref name=KO/>

====Steam reforming====
[[File:SMR+WGS-1.png|thumb|Inputs and outputs of steam reforming (SMR) and water gas shift (WGS) reaction of natural gas, a process used in hydrogen production]]
Hydrogen is mainly produced by [[steam reforming|steam methane reforming]] (SMR), the reaction of water and methane.<ref>{{cite web |last1=Freyermuth |first1=George H |title=1934 Patent: "The manufacture of hydrogen from methane hydrocarbons by the action of steam at elevated temperature" |url=http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&p=1&u=/netahtml/PTO/srchnum.html&r=1&f=G&l=50&d=PALL&s1=1970695.PN. |website=Patent Full-Text Databases |publisher=United States Patent and Trademark Office |access-date=30 October 2020 |archive-date=1 October 2021 |archive-url=https://web.archive.org/web/20211001062417/https://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.html&r=1&f=G&l=50&d=PALL&s1=1970695.PN. |url-status=live }}</ref><ref name="rotech">{{cite book |last1=Press |first1=Roman J. |url=https://archive.org/details/introductiontohy0000unse/page/249/mode/2up |title=Introduction to Hydrogen Technology |last2=Santhanam |first2=K. S. V. |last3=Miri |first3=Massoud J. |last4=Bailey |first4=Alla V. |last5=Takacs |first5=Gerald A. |publisher=John Wiley & Sons |year=2008 |isbn=978-0-471-77985-8 |pages=249 |url-access=registration}}</ref><!--update?--> <ref name="Oxtoby">{{cite book
| first=D. W.|last=Oxtoby|date=2002
| title=Principles of Modern Chemistry
| edition=5th|publisher=Thomson Brooks/Cole
| isbn=978-0-03-035373-4}}</ref> Thus, at high temperature (1000–1400&nbsp;K, 700–1100&nbsp;°C or 1300–2000&nbsp;°F), steam (water vapor) reacts with [[methane]] to yield [[carbon monoxide]] and {{chem2|H2}}.

:{{chem2|CH4 + H2O → CO + 3 H2}}
Producing one tonne of hydrogen through this process emits 6.6–9.3 tonnes of carbon dioxide.<ref name="Bonheure-2021">{{Cite web |last1=Bonheure |first1=Mike |last2=Vandewalle |first2=Laurien A. |last3=Marin |first3=Guy B. |last4=Van Geem |first4=Kevin M. |date=March 2021 |title=Dream or Reality? Electrification of the Chemical Process Industries |url=https://www.aiche-cep.com/cepmagazine/march_2021/MobilePagedArticle.action?articleId=1663852 |url-status=live |archive-url=https://web.archive.org/web/20210717132733/https://www.aiche-cep.com/cepmagazine/march_2021/MobilePagedArticle.action?articleId=1663852 |archive-date=17 July 2021 |access-date=6 July 2021 |website=CEP Magazine |publisher=[[American Institute of Chemical Engineers]]}}</ref> The production of natural gas feedstock also produces emissions such as [[Gas venting|vented]] and [[Fugitive gas emissions|fugitive]] [[methane]], which further contributes to the overall carbon footprint of hydrogen.<ref name="Griffiths-20212">{{Cite journal |last1=Griffiths |first1=Steve |last2=Sovacool |first2=Benjamin K. |last3=Kim |first3=Jinsoo |last4=Bazilian |first4=Morgan |last5=Uratani |first5=Joao M. |display-authors=4 |date=2021 |title=Industrial decarbonization via hydrogen: A critical and systematic review of developments, socio-technical systems and policy options |url=https://www.sciencedirect.com/science/article/pii/S2214629621003017?dgcid=coauthor |url-status=live |journal=[[Energy Research & Social Science]] |volume=80 |page=39 |bibcode=2021ERSS...8002208G |doi=10.1016/j.erss.2021.102208 |issn=2214-6296 |archive-url=https://web.archive.org/web/20211016205152/https://www.sciencedirect.com/science/article/abs/pii/S2214629621003017?dgcid=coauthor |archive-date=16 October 2021 |access-date=11 September 2021}}</ref>

This reaction is favored at low pressures, Nonetheless, conducted at high pressures (2.0&nbsp;MPa, 20&nbsp;atm or 600&nbsp;[[inHg]]) because high-pressure {{chem2|H2}} is the most marketable product, and [[pressure swing adsorption]] (PSA) purification systems work better at higher pressures. The product mixture is known as "[[synthesis gas]]" because it is often used directly for the production of [[methanol]] and many other compounds. [[Hydrocarbon]]s other than methane can be used to produce synthesis gas with varying product ratios. One of the many complications to this highly optimized technology is the formation of coke or carbon:
:{{chem2|CH4 → C + 2 H2}}

Therefore, steam reforming typically employs an excess of {{chem2|H2O}}. Additional hydrogen can be recovered from the steam by using carbon monoxide through the [[water gas shift reaction]] (WGS).  This process requires an [[iron oxide]] catalyst:<ref name="Oxtoby" />
:{{chem2|CO + H2O → CO2 + H2}}

Hydrogen is sometimes produced and consumed in the same industrial process, without being separated. In the [[Haber process]] for [[ammonia production]], hydrogen is generated from natural gas.<ref>{{cite web| last=Funderburg| first=E.| title=Why Are Nitrogen Prices So High?| publisher=The Samuel Roberts Noble Foundation| date=2008| url=http://www.noble.org/Ag/Soils/NitrogenPrices/Index.htm| access-date=11 March 2008| archive-url=https://web.archive.org/web/20010509065844/http://www.noble.org/ag/Soils/NitrogenPrices/Index.htm| archive-date=9 May 2001| df=dmy-all}}</ref>

====Partial oxidation of hydrocarbons====
Other methods for CO and {{chem2|H2}} production include partial oxidation of hydrocarbons:<ref name="uigi"/>

:{{chem2|2 CH4 + O2 → 2 CO + 4 H2}}

Although less important commercially, coal can serve as a prelude to the shift reaction above:<ref name="Oxtoby" />

:{{chem2|C + H2O → CO + H2}}

Olefin production units may produce substantial quantities of byproduct hydrogen particularly from cracking light feedstocks like [[ethane]] or [[propane]].<ref>{{Cite journal |last=Hannula |first=Ilkka |date=2015 |title=Co-production of synthetic fuels and district heat from biomass residues, carbon dioxide and electricity: Performance and cost analysis |url=http://dx.doi.org/10.1016/j.biombioe.2015.01.006 |journal=Biomass and Bioenergy |volume=74 |pages=26–46 |doi=10.1016/j.biombioe.2015.01.006 |bibcode=2015BmBe...74...26H |issn=0961-9534}}</ref>

====Water electrolysis ====
[[File:Hydrogen production via Electrolysis.png|thumb|Inputs and outputs of the electrolysis of water production of hydrogen]]
[[Electrolysis of water]] is a conceptually simple method of producing hydrogen.  
:{{chem2|2 H2O(l) → 2 H2(g) + O2(g)}}
Commercial [[electrolyzer]]s use [[nickel]]-based catalysts in strongly alkaline solution. Platinum is a better catalyst but is expensive.<ref>{{cite journal |doi=10.1038/ncomms5695 |title=Nanoscale nickel oxide/Nickel heterostructures for active hydrogen evolution electrocatalysis |date=2014 |last1=Gong |first1=Ming |last2=Zhou |first2=Wu |last3=Tsai |first3=Mon-Che |last4=Zhou |first4=Jigang |last5=Guan |first5=Mingyun |last6=Lin |first6=Meng-Chang |last7=Zhang |first7=Bo |last8=Hu |first8=Yongfeng |last9=Wang |first9=Di-Yan |last10=Yang |first10=Jiang |last11=Pennycook |first11=Stephen J. |last12=Hwang |first12=Bing-Joe |last13=Dai |first13=Hongjie |journal=Nature Communications |volume=5 |page=4695 |pmid=25146255 |bibcode=2014NatCo...5.4695G |s2cid=205329127 |doi-access=free }}</ref> The hydrogen created through electrolysis using renewable energy is commonly referred to as "[[green hydrogen]]".<ref>{{Cite web |date=31 August 2021 |title=Hydrogen industry must clean itself up before expanding into new… |url=https://www.canarymedia.com/articles/hydrogen/hydrogen-industry-must-clean-itself-up-before-expanding-into-new-uses-report-finds |access-date=2023-04-05 |website=Canary Media |language=en}}</ref>

[[Electrolysis]] of [[brine]] to yield [[chlorine]]<ref>{{cite web| last=Lees| first=A.| title=Chemicals from salt| publisher=BBC|date=2007|url=http://www.bbc.co.uk/schools/gcsebitesize/chemistry/usefulproductsrocks/chemicals_saltrev3.shtml|access-date=11 March 2008|archive-url = https://web.archive.org/web/20071026052022/http://www.bbc.co.uk/schools/gcsebitesize/chemistry/usefulproductsrocks/chemicals_saltrev3.shtml |archive-date = 26 October 2007}}</ref> also produces high purity hydrogen as a co-product, which is used for a variety of transformations such as [[hydrogenation]]s.<ref>{{Cite book |last1=Schmittinger |first1=Peter |chapter=Chlorine |date=2006-01-15 |title=Ullmann's Encyclopedia of Industrial Chemistry |place=Weinheim, Germany |publisher=Wiley-VCH Verlag GmbH & Co. KGaA |language=en |doi=10.1002/14356007.a06_399.pub2 |isbn=978-3-527-30673-2 |last2=Florkiewicz |first2=Thomas |last3=Curlin |first3=L. Calvert |last4=Lüke |first4=Benno |last5=Scannell |first5=Robert |last6=Navin |first6=Thomas |last7=Zelfel |first7=Erich |last8=Bartsch |first8=Rüdiger}}</ref>

The [[electrolysis]] process is more expensive than producing hydrogen from methane without [[carbon capture and storage]].<ref name="Evans-2020">{{Cite web |last1=Evans |first1=Simon |last2=Gabbatiss |first2=Josh |date=30 November 2020 |title=In-depth Q&A: Does the world need hydrogen to solve climate change? |url=https://www.carbonbrief.org/in-depth-qa-does-the-world-need-hydrogen-to-solve-climate-change |url-status=live |archive-url=https://web.archive.org/web/20201201155033/https://www.carbonbrief.org/in-depth-qa-does-the-world-need-hydrogen-to-solve-climate-change |archive-date=1 December 2020 |access-date=1 December 2020 |website=[[Carbon Brief]]}}</ref>

Innovation in [[Electrolysis of water|hydrogen electrolyzers]] could make large-scale production of hydrogen from electricity more cost-competitive.<ref>{{Cite book|author1-link=International Energy Agency|last1=IEA|title=Net Zero by 2050: A Roadmap for the Global Energy Sector|year=2021|url=https://iea.blob.core.windows.net/assets/ad0d4830-bd7e-47b6-838c-40d115733c13/NetZeroby2050-ARoadmapfortheGlobalEnergySector.pdf|archive-date=23 May 2021|archive-url=https://web.archive.org/web/20210523155010/https://iea.blob.core.windows.net/assets/ad0d4830-bd7e-47b6-838c-40d115733c13/NetZeroby2050-ARoadmapfortheGlobalEnergySector.pdf|url-status=live
|pages=15, 75–76}}</ref>

==== Methane pyrolysis ====

Hydrogen can be produced by [[pyrolysis]] of natural gas (methane), producing hydrogen gas and solid carbon with the aid a catalyst and 74 kJ/mol input heat:
:{{chem2|CH4(g) → C(s) + 2 H2(g)}} (ΔH° = 74 kJ/mol)
The carbon may be sold as a manufacturing feedstock or fuel, or landfilled.
This route could have a lower carbon footprint than existing hydrogen production processes, but mechanisms for removing the carbon and preventing it from reacting with the catalyst remain obstacles for industrial scale use.<ref>{{Cite journal |last=Rasul |first=M. G. |last2=Hazrat |first2=M. A |last3=Sattar |first3=M. A. |last4=Jahirul |first4=M. I. |last5=Shearer |first5=M. J. |date=2022-11-15 |title=The future of hydrogen: Challenges on production, storage and applications |url=https://linkinghub.elsevier.com/retrieve/pii/S0196890422011049 |journal=Energy Conversion and Management |volume=272 |pages=116326 |doi=10.1016/j.enconman.2022.116326 |issn=0196-8904}}</ref>{{rp|17}}<ref>{{cite journal |last1=Schneider |first1=Stefan |title=State of the Art of Hydrogen Production via Pyrolysis of Natural Gas |journal=ChemBioEng Reviews |year=2020 |volume=7 |issue=5 |pages=150–158 |publisher=Wiley Online Library |doi=10.1002/cben.202000014 |doi-access=free }}</ref>

==== Thermochemical ====
[[Water splitting]] is the process by which water is decomposed into its components.  Relevant to the biological scenario is this simple equation:
:{{chem2|2 H2O  ->  4 H+ + O2 + 4e-}}
The reaction occurs in the [[Light-dependent reactions|light reactions]] in all [[photosynthetic]] organisms. A few organisms, including the alga ''[[Chlamydomonas reinhardtii]]'' and [[cyanobacteria]], have evolved a second step in the [[dark reaction]]s in which protons and electrons are reduced to form {{chem2|H2}} gas by specialized hydrogenases in the [[chloroplast]].<ref>{{cite journal|last1=Kruse|first1=O.|last2=Rupprecht|first2=J.|last3=Bader|first3=K.|last4=Thomas-Hall|first4=S.|last5=Schenk|first5=P. M.|last6=Finazzi|first6=G.|last7=Hankamer|first7=B.|title=Improved photobiological H<sub>2</sub> production in engineered green algal cells|journal=The Journal of Biological Chemistry|date=2005|volume=280|issue=40|pages=34170–7|doi=10.1074/jbc.M503840200|pmid=16100118|s2cid=5373909|url=http://espace.library.uq.edu.au/view/UQ:75490/UQ75490_OA.pdf|access-date=24 August 2019|archive-date=29 January 2021|archive-url=https://web.archive.org/web/20210129015735/https://espace.library.uq.edu.au/data/UQ_75490/UQ75490_OA.pdf?Expires=1611885542&Key-Pair-Id=APKAJKNBJ4MJBJNC6NLQ&Signature=Qmpjq4YH0rwOJNqiSZ5M7-E5cYH~Dm2B-4kasb1eH66pVWPlvPNRj7TfcTKR1lDhF0--bkJdtE~yrSWwcZAA8FzxAA3MXY99mHTIOxyD3s73Dai1bwrLNuOkibXTVo6WbY5RKv7JAhXJ2sUV~TDIphC4Qikr0AWk5z-dwdY997n0NzcdTlqr0sn5n9WsOari3pJ0wRuL0w6Ged~HhrQ6ClrheilhtRo43U6HuaATFKEAuUM682rv4gvRCEVR1ljVOW0jwruB0SAJszTOZAbqNtb3V0SJh0x7wI8~ZZrp-XYqqzLDsWOB9w3ttyGSpLjcE2LvI7ty5vUljlfBGbnnLg__|url-status=live|doi-access=free}}</ref>

Efforts have been undertaken to genetically modify cyanobacterial hydrogenases to more efficiently generate {{chem2|H2}} gas even in the presence of oxygen.<ref>{{cite web
|first1= Hamilton O.
|last1= Smith
|last2= Xu
|first2= Qing
|date= 2005
|url= http://www.hydrogen.energy.gov/pdfs/progress05/iv_e_6_smith.pdf
|title= IV.E.6 Hydrogen from Water in a Novel Recombinant Oxygen-Tolerant Cyanobacteria System
|work= FY2005 Progress Report
|publisher= United States Department of Energy
|access-date= 6 August 2016
|archive-url= https://web.archive.org/web/20161229231756/https://www.hydrogen.energy.gov/pdfs/progress05/iv_e_6_smith.pdf
|archive-date= 29 December 2016
|url-status= live
}}</ref> Efforts have also been undertaken with genetically modified [[Biological hydrogen production (Algae)|alga in a bioreactor]].<ref>{{cite news| last=Williams| first=C.| title=Pond life: the future of energy| work=Science| publisher=The Register| date=24 February 2006| url=https://www.theregister.co.uk/2006/02/24/pond_scum_breakthrough/| access-date=24 March 2008| archive-url=https://web.archive.org/web/20110509143055/http://www.theregister.co.uk/2006/02/24/pond_scum_breakthrough/| archive-date=9 May 2011| url-status=live}}</ref>

Relevant to the thermal water-splitting scenario is this simple equation:
:{{chem2|2 H2O  ->  2 H2 + O2}}
More than 200 thermochemical cycles can be used for [[water splitting]]. Many of these cycles such as the [[iron oxide cycle]], [[cerium(IV) oxide–cerium(III) oxide cycle]], [[zinc zinc-oxide cycle]], [[sulfur-iodine cycle]], [[copper-chlorine cycle]] and [[hybrid sulfur cycle]] have been evaluated for their commercial potential to produce hydrogen and oxygen from water and heat without using electricity.<ref>{{cite web|url=http://www.hydrogen.energy.gov/pdfs/review05/pd28_weimer.pdf|title=Development of solar-powered thermochemical production of hydrogen from water|first1=Al|last1=Weimer|date=25 May 2005|publisher=Solar Thermochemical Hydrogen Generation Project|access-date=21 December 2008|archive-url=https://web.archive.org/web/20070417134156/http://www.hydrogen.energy.gov/pdfs/review05/pd28_weimer.pdf|archive-date=17 April 2007|url-status=live}}</ref> A number of labs (including in [[France]], [[Germany]], [[Greece]], [[Japan]], and the [[United States]]) are developing thermochemical methods to produce hydrogen from solar energy and water.<ref>{{cite web|url=http://www.hydrogen.energy.gov/pdfs/progress07/ii_f_1_perret.pdf|title=Development of Solar-Powered Thermochemical Production of Hydrogen from Water, DOE Hydrogen Program, 2007|author=Perret, R.|access-date=17 May 2008|archive-url=https://web.archive.org/web/20100527212241/http://www.hydrogen.energy.gov/pdfs/progress07/ii_f_1_perret.pdf|archive-date=27 May 2010}}</ref>

===Natural routes===
====Biohydrogen====
{{Further|Biohydrogen|Biological hydrogen production (Algae)}}
{{chem2|H2}} is produced by enzymes called [[hydrogenase]]s. This  process allows the host organism to use [[fermentation]] as a source of energy.<ref>{{cite journal |doi=10.1021/cr050186q |title=&#91;NiFe&#93; and &#91;FeFe&#93; Hydrogenases Studied by Advanced Magnetic Resonance Techniques |date=2007 |last1=Lubitz |first1=Wolfgang |last2=Reijerse |first2=Eduard |last3=Van Gastel |first3=Maurice |journal=Chemical Reviews |volume=107 |issue=10 |pages=4331–4365 |pmid=17845059 }}</ref>  These same enzymes also can oxidize H<sub>2</sub>, such that the host organisms can subsist by reducing oxidized substrates using electrons extracted from H<sub>2</sub>.{{citation needed|date=January 2025}}

The hydrogenase enzyme feature [[iron]] or [[nickel]]-iron centers at their [[active site]]s.<ref>{{cite book|first1=R.|last1=Cammack|url=https://books.google.com/books?id=GTzajKoBoNwC&pg=PA202|last2=Robson|first2=R. L.|date=2001|pages=202–203|title=Hydrogen as a Fuel: Learning from Nature|publisher=Taylor & Francis Ltd|isbn=978-0-415-24242-4|access-date=3 September 2020|archive-date=29 January 2021|archive-url=https://web.archive.org/web/20210129015731/https://books.google.com/books?id=GTzajKoBoNwC&pg=PA202|url-status=live}}</ref> The natural cycle of hydrogen production and consumption by organisms is called the [[hydrogen cycle]].<ref name="Rhee6">{{cite journal|last1=Rhee|first1=T. S.|last2=Brenninkmeijer|first2=C. A. M.|last3=Röckmann|first3=T.|title=The overwhelming role of soils in the global atmospheric hydrogen cycle|journal=Atmospheric Chemistry and Physics|date=19 May 2006|volume=6|issue=6|pages=1611–1625|doi=10.5194/acp-6-1611-2006|bibcode=2006ACP.....6.1611R|url=https://hal.archives-ouvertes.fr/hal-00301903/file/acpd-5-11215-2005.pdf|access-date=24 August 2019|archive-url=https://web.archive.org/web/20190824162153/https://hal.archives-ouvertes.fr/hal-00301903/file/acpd-5-11215-2005.pdf|archive-date=24 August 2019|url-status=live|doi-access=free}}</ref>

Some bacteria such as ''[[Mycobacterium smegmatis]]'' can use the small amount of hydrogen in the atmosphere as a source of energy when other sources are lacking. Their hydrogenase are designed with small channels that exclude oxygen and so permits the reaction to occur even though the hydrogen concentration is very low and the oxygen concentration is as in normal air.<ref name=Grinter/><ref>{{cite journal |last1=Alex Wilkins |title=Soil bacteria enzyme generates electricity from hydrogen in the air |journal=New Scientist |date=Mar 8, 2023 |volume=257 |issue=3430 |page=13 |doi=10.1016/S0262-4079(23)00459-1 |bibcode=2023NewSc.257...13W |s2cid=257625443 |url=https://www.newscientist.com/article/2363552-soil-bacteria-enzyme-generates-electricity-from-hydrogen-in-the-air/}}</ref>

Confirming the existence of hydrogenases in the human gut, {{chem2|H2}} occurs in human breath. The concentration in the breath of fasting people at rest is typically less than 5 [[parts per million]] (ppm) but can be 50&nbsp;ppm when people with intestinal disorders consume molecules they cannot absorb during diagnostic [[hydrogen breath test]]s.<ref>{{cite journal|doi=10.1088/1752-7155/2/4/046002|title=Implementation and interpretation of hydrogen breath tests|year=2008|last1=Eisenmann|first1=Alexander|last2=Amann|first2=Anton|last3=Said|first3=Michael|last4=Datta|first4=Bettina|last5=Ledochowski|first5=Maximilian|journal=Journal of Breath Research|volume=2|issue=4|page=046002|pmid=21386189|bibcode=2008JBR.....2d6002E|s2cid=31706721|url=http://pdfs.semanticscholar.org/2f16/5a981d54c41da92c1ae81af44021a88f1b95.pdf|access-date=26 December 2020|archive-date=29 January 2021|archive-url=https://web.archive.org/web/20210129015732/http://pdfs.semanticscholar.org/2f16/5a981d54c41da92c1ae81af44021a88f1b95.pdf}}</ref>

====Serpentinization====
[[Serpentinization]] is a geological mechanism that produce highly [[Reduction (chemistry)|reducing]] conditions.<ref name=FrostBeard2007/> Under these conditions, water is capable of oxidizing ferrous ({{chem|Fe|2+}}) ions in fayalite. The process is of interest because it generates hydrogen gas:<ref name="Dincer-2015">{{Cite journal |last1=Dincer |first1=Ibrahim |last2=Acar |first2=Canan |date=14 September 2015 |title=Review and evaluation of hydrogen production methods for better sustainability |url=https://www.sciencedirect.com/science/article/pii/S0360319914034119 |url-status=live |journal=International Journal of Hydrogen Energy |language=en |volume=40 |issue=34 |pages=11094–11111 |bibcode=2015IJHE...4011094D |doi=10.1016/j.ijhydene.2014.12.035 |issn=0360-3199 |archive-url=https://web.archive.org/web/20220215183915/https://www.sciencedirect.com/science/article/abs/pii/S0360319914034119 |archive-date=15 February 2022 |access-date=4 February 2022}}</ref><ref>{{cite web| title = Methane and hydrogen formation from rocks – Energy sources for life| url = http://www.lostcity.washington.edu/science/chemistry/methane.html| access-date = 2011-11-06}}</ref><ref>{{Cite journal| last = Sleep| first = N.H.| author2 = A. Meibom, Th. Fridriksson, R.G. Coleman, D.K. Bird| year = 2004| title = H<sub>2</sub>-rich fluids from serpentinization: Geochemical and biotic implications| journal = Proceedings of the National Academy of Sciences of the United States of America| volume = 101| issue = 35| pages = 12818–12823| doi = 10.1073/pnas.0405289101|bibcode = 2004PNAS..10112818S| pmid=15326313| pmc=516479| doi-access = free}}</ref>
:{{chem2|Fe2SiO4 + H2O → 2 Fe3O4 + SiO2 +H2}}

Closely related to this geological process is the [[Schikorr reaction]]:
:{{chem2|3 Fe(OH)2 → Fe3O4 + 2 H2O + H2}}
This process also is relevant to the corrosion of [[iron]] and [[steel]] in [[Anoxic waters|oxygen-free]] [[groundwater]] and in reducing [[soil]]s below the [[water table]].{{citation needed|date=January 2025}}

===Storage===
If H<sub>2</sub> is to used as an energy source, its storage is important. It dissolves only poorly in solvents.  For example, at room temperature and 0.1 M[[Pascal (unit)|pascal]], ca. 0.05 moles dissolves in one kilogram of [[diethyl ether]].<ref name="UllmannH2">{{cite book |doi=10.1002/14356007.a13_297.pub3 |chapter=Hydrogen, 1. Properties and Occurrence |title=Ullmann's Encyclopedia of Industrial Chemistry |date=2013 |last1=Lauermann |first1=Gerhard |last2=Häussinger |first2=Peter |last3=Lohmüller |first3=Reiner |last4=Watson |first4=Allan M. |pages=1–15 |isbn=978-3-527-30673-2 }}</ref> The H<sub>2</sub> can be stored in compressed form, although compressing costs energy.  Liquifaction is impractical given its low [[critical temperature]].  In contrast, ammonia and many hydrocarbons can be liquified at room temperature under pressure.  For these reasons, hydrogen ''carriers'' - materials that reversibly bind H<sub>2</sub> - have attracted much attention.  The key question is then the weight percent of H<sub>2</sub>-equivalents within the carrier material.  For example, hydrogen can be reversibly absorbed into many [[Rare earth element|rare earth]] and [[transition metal]]s<ref name="Takeshita">{{cite journal
|last1=Takeshita|first1=T.
|last2=Wallace|first2=W. E.
|last3=Craig|first3=R. S.
|title=Hydrogen solubility in 1:5 compounds between yttrium or thorium and nickel or cobalt
|journal=[[Inorganic Chemistry (journal)|Inorganic Chemistry]]
|volume=13|issue=9|pages=2282–2283
|date=1974
|doi=10.1021/ic50139a050
}}</ref> and is soluble in both nanocrystalline and [[amorphous metal]]s.<ref name="Kirchheim1">{{cite journal |last1=Kirchheim |first1=R. |last2=Mutschele |first2=T. |last3=Kieninger |first3=W. |title=Hydrogen in amorphous and nanocrystalline metals |journal=Materials Science and Engineering |date=1988 |volume=99 |issue=1–2 |pages=457–462 |doi=10.1016/0025-5416(88)90377-1 |last4=Gleiter |first4=H. |last5=Birringer |first5=R. |last6=Koble |first6=T.}}</ref> Hydrogen [[solubility]] in metals is influenced by local distortions or impurities in the [[crystal lattice]].<ref name="Kirchheim2">{{cite journal
|last=Kirchheim|first=R.
|title=Hydrogen solubility and diffusivity in defective and amorphous metals
|journal=[[Progress in Materials Science]]
|volume=32|issue=4|pages=262–325
|date=1988
|doi=10.1016/0079-6425(88)90010-2
}}</ref> These properties may be useful when hydrogen is purified by passage through hot [[palladium]] disks, but the gas's high solubility is also a metallurgical problem, contributing to the [[hydrogen embrittlement|embrittlement]] of many metals,<ref name="Rogers 1999 1057–1064">{{cite journal |last=Rogers|first=H. C. |title=Hydrogen Embrittlement of Metals |journal=[[Science (journal)|Science]] |volume=159|issue=3819|pages=1057–1064 |date=1999 |doi=10.1126/science.159.3819.1057 |pmid=17775040 |bibcode=1968Sci...159.1057R |s2cid=19429952}}</ref> complicating the design of pipelines and storage tanks.<ref name="Christensen">{{cite news
|last1=Christensen
|first1=C. H.
|last2=Nørskov
|first2=J. K.
|last3=Johannessen
|first3=T.
|date=9 July 2005
|title=Making society independent of fossil fuels&nbsp;– Danish researchers reveal new technology
|publisher=[[Technical University of Denmark]]
|url=http://news.mongabay.com/2005/0921-hydrogen_tablet.html
|access-date=19 May 2015
|archive-url=https://web.archive.org/web/20150521085421/http://news.mongabay.com/2005/0921-hydrogen_tablet.html
|archive-date=21 May 2015
|url-status=live
}}</ref>

The most problematic aspect of metal hydrides for storage is their modest H<sub>2</sub> content, often on the order of 1%.  For this reason, there is interest in storage of H<sub>2</sub> in compounds of low [[molecular weight]].  For example, [[ammonia borane]] ({{chem2|H3N\sBH3}}) contains 19.8 weight percent of H<sub>2</sub>.  The problem with this material is that after release of H<sub>2</sub>, the resulting boron nitride does not re-add H<sub>2</sub>, i.e. ammonia borane is an irreversible hydrogen carrier.<ref>{{cite journal |doi=10.1016/j.gee.2022.03.011 |title=Ammonia borane-enabled hydrogen transfer processes: Insights into catalytic strategies and mechanisms |date=2023 |last1=Zhao |first1=Wenfeng |last2=Li |first2=Hu |last3=Zhang |first3=Heng |last4=Yang |first4=Song |last5=Riisager |first5=Anders |journal=Green Energy & Environment |volume=8 |issue=4 |pages=948–971 |bibcode=2023GrEE....8..948Z |doi-access=free }}</ref>  More attractive, somewhat ironically, are [[hydrocarbon]]s such as [[tetrahydroquinoline]], which reversibly release some H<sub>2</sub> when heated in the presence of a catalyst:<ref>{{cite journal |doi=10.1021/acscatal.7b03547 |title=NHC-Based Iridium Catalysts for Hydrogenation and Dehydrogenation of N-Heteroarenes in Water under Mild Conditions |date=2018 |last1=Vivancos |first1=Ángela |last2=Beller |first2=Matthias |last3=Albrecht |first3=Martin |journal=ACS Catalysis |volume=8 |pages=17–21 }}</ref>
:{{chem2|C9H10NH <-> C9H7N  +  2H2}}

== Applications ==

{{See also|Hydrogen economy}}
[[File:The_Hydrogen_Ladder,_Version_5.0.jpg|thumb|520px|Some projected uses in the medium term, but analysts disagree<ref>{{Cite web |last=Barnard |first=Michael |date=2023-10-22 |title=What's New On The Rungs Of Liebreich's Hydrogen Ladder? |url=https://cleantechnica.com/2023/10/22/whats-new-on-the-rungs-of-liebreichs-hydrogen-ladder/ |access-date=2024-03-10 |website=CleanTechnica |language=en-US}}</ref>]]

=== Petrochemical industry ===
Large quantities of {{chem2|H2}} are used in the "upgrading" of [[fossil fuels]]. Key consumers of {{chem2|H2}} include [[hydrodesulfurization]], and [[hydrocracking]]. Many of these reactions can be classified as [[hydrogenolysis]], i.e., the cleavage of bonds by hydrogen. Illustrative is the separation of sulfur from liquid fossil fuels:<ref name=KO>{{cite book |doi=10.1002/0471238961.0825041803262116.a01.pub2 |chapter=Hydrogen |title=Kirk-Othmer Encyclopedia of Chemical Technology |date=2001 |last1=Baade |first1=William F. |last2=Parekh |first2=Uday N. |last3=Raman |first3=Venkat S. |isbn=9780471484943 }}</ref>
:{{chem2|R2S + 2 H2 → H2S + 2 RH}}

=== Hydrogenation ===
[[Hydrogenation]], the addition of {{chem2|H2}} to various substrates, is done on a large scale. Hydrogenation of {{chem2|N2}} to produce ammonia by the [[Haber process]], consumes a few percent of the energy budget in the entire industry. The resulting ammonia is used to supply most of the protein consumed by humans.<ref name="Smil_2004_Enriching">{{cite book |last1=Smil |first1=Vaclav |title=Enriching the Earth: Fritz Haber, Carl Bosch, and the Transformation of World Food Production |date=2004 |publisher=MIT |location=Cambridge, MA |isbn=978-0-262-69313-4 |edition=1st}}</ref> Hydrogenation is used to convert [[unsaturated fat]]s and [[vegetable oil|oils]] to saturated fats and oils. The major application is the production of [[margarine]]. [[Methanol]] is produced by hydrogenation of carbon dioxide. It is similarly the source of hydrogen in the manufacture of [[hydrochloric acid]]. {{chem2|H2}} is also used as a [[reducing agent]] for the conversion of some [[ore]]s to the metals.<ref>{{cite web|author=Chemistry Operations|date=15 December 2003|url=http://periodic.lanl.gov/1.shtml|title=Hydrogen|publisher=Los Alamos National Laboratory|access-date=5 February 2008|archive-url=https://web.archive.org/web/20110304203439/http://periodic.lanl.gov/1.shtml|archive-date=4 March 2011}}</ref>

=== Coolant ===
{{Main|Hydrogen-cooled turbo generator}}
Hydrogen is commonly used in power stations as a coolant in generators due to a number of favorable properties that are a direct result of its light diatomic molecules. These include low [[density]], low [[viscosity]], and the highest [[specific heat]] and [[thermal conductivity]] of all gases.{{citation needed|date=January 2025}}

=== Fuel ===
The potential for using hydrogen (H<sub>2</sub>)  as a fuel has been widely discussed. Hydrogen can be used in [[fuel cells]] to produce electricity, or burned to generate heat.<ref name=":0422">{{Cite journal |last=Lewis |first=Alastair C. |date=10 June 2021 |title=Optimising air quality co-benefits in a hydrogen economy: a case for hydrogen-specific standards for NO x emissions |journal=Environmental Science: Atmospheres |language=en |volume=1 |issue=5 |pages=201–207 |bibcode=2021ESAt....1..201L |doi=10.1039/D1EA00037C |doi-access=free}}{{Creative Commons text attribution notice|cc=by3|from this source=yes|url=|authors=|vrt=}}</ref> When hydrogen is consumed in fuel cells, the only emission at the point of use is water vapour.<ref name=":0422" /> When burned, hydrogen produces relatively little pollution at the point of combustion, but can lead to thermal formation of harmful [[NOx|nitrogen oxides]].<ref name=":0422" />

If hydrogen is produced with low or zero greenhouse gas emissions ([[green hydrogen]]), it can play a significant role in decarbonizing energy systems where there are challenges and limitations to replacing fossil fuels with direct use of electricity.<ref name="IPCC-20222">{{Cite book |author=IPCC |author-link=IPCC |url=https://ipcc.ch/report/ar6/wg3/downloads/report/IPCC_AR6_WGIII_FullReport.pdf |title=Climate Change 2022: Mitigation of Climate Change |publisher=Cambridge University Press (In Press) |year=2022 |isbn=9781009157926 |editor1-last=Shukla |editor1-first=P.R. |series=Contribution of Working Group III to the [[IPCC Sixth Assessment Report|Sixth Assessment Report]] of the Intergovernmental Panel on Climate Change |place=Cambridge, UK and New York, NY, US |pages=91–92 |doi=10.1017/9781009157926 |ref={{harvid|IPCC AR6 WG3|2022}} |editor2-last=Skea |editor2-first=J. |editor3-last=Slade |editor3-first=R. |editor4-last=Al Khourdajie |editor4-first=A. |editor5-last=van Diemen |editor5-first=R. |editor6-last=McCollum |editor6-first=D. |editor7-last=Pathak |editor7-first=M. |editor8-last=Some |editor8-first=S. |editor9-last=Vyas |editor9-first=P. |display-editors=4 |editor10-first=R. |editor10-last=Fradera |editor11-first=M. |editor11-last=Belkacemi |editor12-first=A. |editor12-last=Hasija |editor13-first=G. |editor13-last=Lisboa |editor14-first=S. |editor14-last=Luz |editor15-first=J. |editor15-last=Malley}}</ref><ref name="Evans-2020" />

Hydrogen fuel can produce the intense heat required for industrial production of steel, cement, glass, and chemicals, thus contributing to the decarbonisation of industry alongside other technologies, such as [[electric arc furnace]]s for steelmaking.<ref>{{Cite web |last=Kjellberg-Motton |first=Brendan |date=2022-02-07 |title=Steel decarbonisation gathers speed {{!}} Argus Media |url=https://www.argusmedia.com/en//news/2299399-steel-decarbonisation-gathers-speed |access-date=2023-09-07 |website=www.argusmedia.com |language=en}}</ref> However, it is likely to play a larger role in providing industrial feedstock for cleaner production of ammonia and organic chemicals.<ref name="IPCC-2022">{{Cite book |author=IPCC |author-link=IPCC |url=https://ipcc.ch/report/ar6/wg3/downloads/report/IPCC_AR6_WGIII_FullReport.pdf |title=Climate Change 2022: Mitigation of Climate Change |publisher=Cambridge University Press (In Press) |year=2022 |isbn=9781009157926 |editor1-last=Shukla |editor1-first=P.R. |series=Contribution of Working Group III to the [[IPCC Sixth Assessment Report|Sixth Assessment Report]] of the Intergovernmental Panel on Climate Change |place=Cambridge, UK and New York, NY, US |pages=91–92 |doi=10.1017/9781009157926 |ref={{harvid|IPCC AR6 WG3|2022}} |editor2-last=Skea |editor2-first=J. |editor3-last=Slade |editor3-first=R. |editor4-last=Al Khourdajie |editor4-first=A. |editor5-last=van Diemen |editor5-first=R. |editor6-last=McCollum |editor6-first=D. |editor7-last=Pathak |editor7-first=M. |editor8-last=Some |editor8-first=S. |editor9-last=Vyas |editor9-first=P. |display-editors=4 |editor10-first=R. |editor10-last=Fradera |editor11-first=M. |editor11-last=Belkacemi |editor12-first=A. |editor12-last=Hasija |editor13-first=G. |editor13-last=Lisboa |editor14-first=S. |editor14-last=Luz |editor15-first=J. |editor15-last=Malley}}</ref> For example, in [[steelmaking]], hydrogen could function as a clean fuel and also as a low-carbon catalyst, replacing coal-derived [[Coke (fuel)|coke]] (carbon):<ref>{{Cite web |last1=Blank |first1=Thomas |last2=Molly |first2=Patrick |date=January 2020 |title=Hydrogen's Decarbonization Impact for Industry |url=https://rmi.org/wp-content/uploads/2020/01/hydrogen_insight_brief.pdf |url-status=live |archive-url=https://web.archive.org/web/20200922115313/https://rmi.org/wp-content/uploads/2020/01/hydrogen_insight_brief.pdf |archive-date=22 September 2020 |access-date= |publisher=[[Rocky Mountain Institute]] |pages=2, 7, 8}}</ref>
:{{chem2|2FeO  +  C -> 2Fe  +  CO2}}
:::vs
:{{chem2|FeO  +  H2 -> Fe  +  H2O}}
Hydrogen used to decarbonise transportation is likely to find its largest applications in shipping, aviation and, to a lesser extent, heavy goods vehicles, through the use of hydrogen-derived synthetic fuels such as [[Green ammonia|ammonia]] and [[Green methanol|methanol]] and fuel cell technology.<ref name="IPCC-2022" /> For light-duty vehicles including cars, hydrogen is far behind other [[alternative fuel vehicle]]s, especially compared with the rate of adoption of [[battery electric vehicles]], and may not play a significant role in future.<ref>{{Cite journal |last=Plötz |first=Patrick |date=2022-01-31 |title=Hydrogen technology is unlikely to play a major role in sustainable road transport |url=https://www.nature.com/articles/s41928-021-00706-6 |journal=Nature Electronics |language=en |volume=5 |issue=1 |pages=8–10 |doi=10.1038/s41928-021-00706-6 |s2cid=246465284 |issn=2520-1131}}</ref>

[[Liquid hydrogen]] and [[liquid oxygen]] together serve as [[cryogenic propellant]]s in [[liquid-propellant rocket]]s, as in the [[RS-25|Space Shuttle main engines]]. [[NASA]] has investigated the use of [[rocket propellant]] made from atomic hydrogen, boron or carbon that is frozen into solid molecular hydrogen particles suspended in liquid helium. Upon warming, the mixture vaporizes to allow the atomic species to recombine, heating the mixture to high temperature.<ref>{{Cite web |url=https://ntrs.nasa.gov/api/citations/20030005922/downloads/20030005922.pdf |title=NASA/TM—2002-211915: Solid Hydrogen Experiments for Atomic Propellants |access-date=2 July 2021 |archive-date=9 July 2021 |archive-url=https://web.archive.org/web/20210709183557/https://ntrs.nasa.gov/api/citations/20030005922/downloads/20030005922.pdf |url-status=live }}</ref>

Hydrogen produced when there is a surplus of [[Variable renewable energy|variable renewable electricity]] could in principle be stored and later used to generate heat or to re-generate electricity.<ref>{{Cite journal |last1=Palys |first1=Matthew J. |last2=Daoutidis |first2=Prodromos |date=2020 |title=Using hydrogen and ammonia for renewable energy storage: A geographically comprehensive techno-economic study |journal=[[Computers & Chemical Engineering]] |volume=136 |pages=106785 |doi=10.1016/j.compchemeng.2020.106785 |issn=0098-1354 |doi-access=free}}</ref>  It can be further transformed into [[synthetic fuel]]s such as [[ammonia]] and [[methanol]].<ref>{{cite book |author=[[IRENA]] |url=https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2021/March/IRENA_World_Energy_Transitions_Outlook_2021.pdf |title=World Energy Transitions Outlook: 1.5°C Pathway |year=2021 |isbn=978-92-9260-334-2 |pages=12, 22 |archive-url=https://web.archive.org/web/20210611230855/https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2021/March/IRENA_World_Energy_Transitions_Outlook_2021.pdf |archive-date=11 June 2021 |url-status=live}}</ref> Disadvantages of hydrogen fuel include high costs of storage and distribution due to hydrogen's explosivity, its large volume compared to other fuels, and its tendency to make pipes brittle.<ref name="Griffiths-20212"/>

=== Semiconductor industry ===
Hydrogen is employed to saturate broken ("dangling") bonds of [[amorphous silicon]] and [[amorphous carbon]] that helps stabilizing material properties.<ref>{{cite journal
|last1=Le Comber| first1=P. G.
|title=Hall effect and impurity conduction in substitutionally doped amorphous silicon
|journal=Philosophical Magazine|doi=10.1080/14786437708232943
|volume=35
|issue=5
|pages=1173–1187
|date=1977
|last2=Jones
|first2=D. I.
|last3=Spear
|first3=W. E.|bibcode = 1977PMag...35.1173C }}</ref> It is also a potential [[electron donor]] in various oxide materials, including [[zinc oxide|ZnO]],<ref>{{cite journal|last=Van de Walle|first=C. G.|title=Hydrogen as a cause of doping in zinc oxide|journal=Physical Review Letters|volume=85|issue=5|doi=10.1103/PhysRevLett.85.1012|pages=1012–1015|date=2000|pmid=10991462|bibcode=2000PhRvL..85.1012V|hdl=11858/00-001M-0000-0026-D0E6-E|url=http://pubman.mpdl.mpg.de/pubman/item/escidoc:741885/component/escidoc:932688/PRL-85-1012-2000.pdf|access-date=1 August 2018|archive-url=https://web.archive.org/web/20170815000602/http://pubman.mpdl.mpg.de/pubman/item/escidoc:741885/component/escidoc:932688/PRL-85-1012-2000.pdf|archive-date=15 August 2017|url-status=live|hdl-access=free}}</ref><ref>{{cite journal
|last1=Janotti|first1= A.
|title=Hydrogen multicentre bonds|doi=10.1038/nmat1795
|journal=Nature Materials
|volume=6|pages=44–47
|date=2007
|pmid=17143265
|last2=Van De Walle
|first2=C. G.
|issue=1|bibcode = 2007NatMa...6...44J }}</ref> [[Tin dioxide|{{chem2|SnO2}}]], [[Cadmium oxide|CdO]], [[Magnesium oxide|MgO]],<ref>{{cite journal|last1=Kilic|first1=C.|title=n-type doping of oxides by hydrogen|doi=10.1063/1.1482783|journal=Applied Physics Letters|volume=81|issue=1|pages=73–75|date=2002|last2=Zunger|first2=Alex|bibcode=2002ApPhL..81...73K|s2cid=96415065}}</ref> [[Zirconium dioxide|{{chem2|ZrO2}}]], [[Hafnium(IV) oxide|{{chem2|HfO2}}]], [[Lanthanum(III) oxide|{{chem2|La2O3}}]], [[Yttrium(III) oxide|{{chem2|Y2O3}}]], [[Titanium dioxide|{{chem2|TiO2}}]], [[Strontium titanate|{{chem2|SrTiO3}}]], [[Lanthanum aluminate|{{chem2|LaAlO3}}]], [[Silicon dioxide|{{chem2|SiO2}}]], [[Aluminium oxide|{{chem2|Al2O3}}]], [[Zircon|{{chem2|ZrSiO4}}]], [[Hafnon|{{chem2|HfSiO4}}]], and [[Strontium zirconate|{{chem2|SrZrO3}}]].<ref>{{cite journal
|last1=Peacock| first1=P. W.|doi=10.1063/1.1609245
|title=Behavior of hydrogen in high dielectric constant oxide gate insulators
|journal=Applied Physics Letters
|volume=83
|issue=10
|pages=2025–2027
|date=2003
|last2=Robertson
|first2=J.
|bibcode = 2003ApPhL..83.2025P }}</ref>

=== Niche and evolving uses ===
*Shielding gas: Hydrogen is used as a [[shielding gas]] in [[welding]] methods such as [[atomic hydrogen welding]].<ref>{{cite journal
|last=Durgutlu| first=A.
|title=Experimental investigation of the effect of hydrogen in argon as a shielding gas on TIG welding of austenitic stainless steel
|journal=Materials & Design
|volume=25
|issue=1
|pages=19–23
|date=2003
|doi=10.1016/j.matdes.2003.07.004}}</ref><ref>{{cite web
|title=Atomic Hydrogen Welding| publisher=Specialty Welds
|date=2007
|url=http://www.specialwelds.com/underwater-welding/atomic-hydrogen-welding.htm|archive-url=https://web.archive.org/web/20110716115120/http://www.specialwelds.com/underwater-welding/atomic-hydrogen-welding.htm|archive-date=16 July 2011}}</ref>

*Cryogenic research: Liquid {{chem2|H2}} is used in [[cryogenic]] research, including [[superconductivity]] studies.<ref>{{cite journal
|last=Hardy
|first=W. N.
|title=From H2 to cryogenic H masers to HiTc superconductors: An unlikely but rewarding path
|journal=Physica C: Superconductivity
|volume=388–389
|pages=1–6
|date=2003
|doi=10.1016/S0921-4534(02)02591-1|bibcode = 2003PhyC..388....1H }}</ref>
*Leak detection: Pure or mixed with nitrogen (sometimes called [[forming gas]]), hydrogen is a [[tracer gas]] for [[Leak detection|detection]] of minute leaks. Applications can be found in the automotive, chemical, power generation, aerospace, and telecommunications industries.<ref>{{cite conference
|first=M.
|last=Block
|title=Hydrogen as Tracer Gas for Leak Detection
|work=16th WCNDT 2004
|publisher=Sensistor Technologies
|date=3 September 2004
|location=Montreal, Canada
|url=http://www.ndt.net/abstract/wcndt2004/523.htm
|access-date=25 March 2008
|archive-url=https://web.archive.org/web/20090108102521/http://www.ndt.net/abstract/wcndt2004/523.htm
|archive-date=8 January 2009
}}</ref> Hydrogen is an authorized food additive (E 949) that allows food package leak testing, as well as having anti-oxidizing properties.<ref>{{cite web
|url=http://ec.europa.eu/food/fs/sfp/addit_flavor/flav15_en.pdf
|title=Report from the Commission on Dietary Food Additive Intake
|publisher=[[European Union]]
|access-date=5 February 2008
|archive-url=https://web.archive.org/web/20080216050325/http://ec.europa.eu/food/fs/sfp/addit_flavor/flav15_en.pdf
|archive-date=16 February 2008
|url-status=live
}}</ref>

*Neutron moderation: [[Deuterium]] (hydrogen-2) is used in [[CANDU reactor|nuclear fission applications]] as a [[neutron moderator|moderator]] to slow [[neutron]]s.
*Nuclear fusion fuel: Deuterium is used in [[nuclear fusion]] reactions.<ref name="nbb" />
*Isotopic labeling: Deuterium compounds have applications in chemistry and biology in studies of [[Kinetic isotope effect|isotope effects]] on reaction rates.<ref>{{cite journal|last1=Reinsch| first1=J.|first2=A. |last2=Katz|first3=J.|last3=Wean|first4=G.|last4=Aprahamian|first5=J. T.|last5=MacFarland
|title=The deuterium isotope effect upon the reaction of fatty acyl-CoA dehydrogenase and butyryl-CoA| journal=J. Biol. Chem.|volume=255
|issue=19|pages=9093–97|date=1980| doi=10.1016/S0021-9258(19)70531-6|pmid=7410413|doi-access=free}}</ref>

*Tritium uses: [[Tritium]] (hydrogen-3), produced in [[nuclear reactor]]s, is used in the production of [[hydrogen bomb]]s,<ref>{{cite journal| last=Bergeron| first=K. D.| title=The Death of no-dual-use| journal=Bulletin of the Atomic Scientists| volume=60| issue=1| pages=15–17| date=2004| url=http://find.galegroup.com/itx/start.do?prodId=SPJ.SP06| doi=10.2968/060001004| access-date=13 April 2008| archive-url=https://web.archive.org/web/20080419051641/http://find.galegroup.com/itx/start.do?prodId=SPJ.SP06| archive-date=19 April 2008| url-status=live| bibcode=2004BuAtS..60a..15B}}</ref> as an isotopic label in the biosciences,<ref name="holte" /> and as a source of [[beta particle|beta radiation]] in [[Tritium radioluminescence|radioluminescent paint]] for instrument dials and emergency signage.<ref name="Traub95" />

== Safety and precautions ==
{{Main|Hydrogen safety}}
{{Chembox
| container_only = yes
|Section7={{Chembox Hazards
| ExternalSDS =
| GHSPictograms = {{GHS02}}
| GHSSignalWord = Danger
| HPhrases = {{H-phrases|220}}
| PPhrases = {{P-phrases|202|210|271|403|377|381}}<ref>{{Cite web | url=http://isolab.ess.washington.edu/isolab/images/documents/msds_sds/hydrogen.pdf | title=MyChem: Chemical | access-date=1 October 2018 | archive-url=https://web.archive.org/web/20181001070437/http://isolab.ess.washington.edu/isolab/images/documents/msds_sds/hydrogen.pdf | archive-date=1 October 2018 }}</ref>
| NFPA-H = 0
| NFPA-F = 4
| NFPA-R = 0
| NFPA-S =
| NFPA_ref =
 }}
}}
In hydrogen pipelines and steel storage vessels, hydrogen molecules are prone to react with metals, causing [[hydrogen embrittlement]] and leaks in the pipeline or storage vessel.<ref name=":2">{{Cite journal |last=Li |first=Hao |last2=Cao |first2=Xuewen |last3=Liu |first3=Yang |last4=Shao |first4=Yanbo |last5=Nan |first5=Zilong |last6=Teng |first6=Lin |last7=Peng |first7=Wenshan |last8=Bian |first8=Jiang |date=2022-11-01 |title=Safety of hydrogen storage and transportation: An overview on mechanisms, techniques, and challenges |url=https://linkinghub.elsevier.com/retrieve/pii/S2352484722008332 |journal=Energy Reports |volume=8 |pages=6258–6269 |doi=10.1016/j.egyr.2022.04.067 |issn=2352-4847}}Text was copied from this source, which is available under a [[creativecommons:by/4.0/|Creative Commons Attribution 4.0 International License]]</ref> Since it is lighter than air, hydrogen does not easily accumulate to form a combustible gas mixture.<ref name=":2" />  However, even without ignition sources, high-pressure hydrogen leakage may cause spontaneous combustion and [[detonation]].<ref name=":2" />

Hydrogen is flammable when mixed even in small amounts with air. Ignition can occur at a volumetric ratio of hydrogen to air as low as 4%.<ref>{{Cite journal |last=Yang |first=Fuyuan |last2=Wang |first2=Tianze |last3=Deng |first3=Xintao |last4=Dang |first4=Jian |last5=Huang |first5=Zhaoyuan |last6=Hu |first6=Song |last7=Li |first7=Yangyang |last8=Ouyang |first8=Minggao |date=2021-09-03 |title=Review on hydrogen safety issues: Incident statistics, hydrogen diffusion, and detonation process |url=https://linkinghub.elsevier.com/retrieve/pii/S0360319921025520 |journal=International Journal of Hydrogen Energy |volume=46 |issue=61 |pages=31467–31488 |doi=10.1016/j.ijhydene.2021.07.005 |issn=0360-3199}}</ref> In approximately 70% of hydrogen ignition accidents, the ignition source cannot be found, and it is widely believed by scholars that spontaneous ignition of hydrogen occurs.<ref name=":2" />

Hydrogen fire, while being extremely hot, is almost invisible, and thus can lead to accidental burns.<ref name=":3">{{Cite web |date=2016 |title=Hydrogen Detection Tape Saves Time and Lives {{!}} NASA Spinoff |url=https://spinoff.nasa.gov/Spinoff2016/ps_5.html |access-date=2025-02-23 |website=spinoff.nasa.gov}}</ref>  Hydrogen is non-toxic,<ref>{{Cite journal |last=Abohamzeh |first=Elham |last2=Salehi |first2=Fatemeh |last3=Sheikholeslami |first3=Mohsen |last4=Abbassi |first4=Rouzbeh |last5=Khan |first5=Faisal |date=2021-09-01 |title=Review of hydrogen safety during storage, transmission, and applications processes |url=https://linkinghub.elsevier.com/retrieve/pii/S0950423021001790 |journal=Journal of Loss Prevention in the Process Industries |volume=72 |pages=104569 |doi=10.1016/j.jlp.2021.104569 |issn=0950-4230}}</ref> but like most gases it can cause [[asphyxiation]] in the absence of adequate ventilation.<ref>{{Cite web |last=U.S. Department of Energy |title=Current Safe Operating Practices |url=https://www.energy.gov/eere/fuelcells/current-safe-operating-practices |access-date=2025-02-24 |website=Energy.gov |language=en}}</ref>

== See also ==
{{div col}}
* [[Combined cycle hydrogen power plant]]
* {{annotated link|Hydrogen economy}}
* {{annotated link|Hydrogen production}}
* {{annotated link|Hydrogen safety}}
* {{annotated link|Hydrogen technologies}}
* {{annotated link|Hydrogen transport}}
* {{annotated link|Methane pyrolysis}} (for hydrogen)
* {{annotated link|Natural hydrogen}}
* {{annotated link|Pyrolysis}}
{{div col end}}

== Notes ==
<references group="note" />

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

== Further reading ==
{{Library resources box
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* [https://www.feynmanlectures.caltech.edu/III_12.html The Hyperfine Splitting in Hydrogen - The Feynman Lectures on Physics]
* {{cite book| title=Chart of the Nuclides| edition=17th| publisher= Knolls Atomic Power Laboratory|date=2010| url=http://www.nuclidechart.com/|isbn=978-0-9843653-0-2}}
* {{cite book|last=Newton|first=David E.|date=1994|title=The Chemical Elements|publisher=Franklin Watts|location=New York|isbn=978-0-531-12501-4|url=https://archive.org/details/chemicalelements00newt}}
* {{cite book|last=Rigden|first=John S.|date=2002|title=Hydrogen: The Essential Element|publisher=Harvard University Press|location=Cambridge, Massachusetts|isbn=978-0-531-12501-4|url=https://archive.org/details/chemicalelements00newt}}
* {{cite book|author=Romm, Joseph J.|title=The Hype about Hydrogen, Fact and Fiction in the Race to Save the Climate|publisher=Island Press|date=2004|isbn=978-1-55963-703-9|title-link=The Hype about Hydrogen}}
* {{cite book|last=Scerri|first=Eric|date=2007|title=The Periodic System, Its Story and Its Significance|publisher=Oxford University Press|location=New York|isbn=978-0-19-530573-9|url-access=registration|url=https://archive.org/details/periodictableits0000scer}}

== External links ==
{{Spoken Wikipedia|date=28 October 2006|En-Hydrogen (part 1).ogg|En-Hydrogen (part 2).ogg}}
* [https://web.archive.org/web/20060612225336/http://www.physics.drexel.edu/~tim/open/hydrofin/ Basic Hydrogen Calculations of Quantum Mechanics]
* [http://www.periodicvideos.com/videos/001.htm Hydrogen] at ''[[The Periodic Table of Videos]]'' (University of Nottingham)
<!-- access forbidden *[http://www.astro.washington.edu/users/larson/Astro150b/Lectures/JupSatUraNep/hydrogen_phase.gif Hydrogen phase diagram] -->
* [http://militzer.berkeley.edu/diss/node5.html High temperature hydrogen phase diagram]
* [http://hyperphysics.phy-astr.gsu.edu/Hbase/quantum/hydwf.html#c3 Wavefunction of hydrogen]

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