Editing Hydrogen (section)

== Properties ==
===Atomic hydrogen===
==== Electron energy levels ====
{{Main|Hydrogen atom}}
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}} Each energy level is further split by [[spin (physics)| spin]] interactions between the electron and proton into 4 [[Hyperfine_structure|hyperfine]] levels.<ref>{{Cite book |last1=Feynman |first1=Richard P. |url=https://www.worldcat.org/title/671704374 |title=The Feynman lectures on physics |last2=Leighton |first2=Robert B. |last3=Sands |first3=Matthew L. |date=2011 |publisher=Basic Books |isbn=978-0-465-02414-8 |edition=New millennium |location=New York |chapter=The Hyperfine Splitting in Hydrogen |oclc=671704374 |chapter-url=https://www.feynmanlectures.caltech.edu/III_12.html}}</ref> 

High precision values for the hydrogen atom energy levels are required for definitions of physical constants. Quantum calculations have identified 9 contributions to the energy levels. The eigenvalue from the [[Dirac equation]] is the largest contribution. Other terms include [[Relativistic quantum mechanics|relativistic]] recoil, the [[self-energy]], and the [[vacuum polarization]] terms.<ref>{{Cite journal |last1=Tiesinga |first1=Eite |last2=Mohr |first2=Peter J. |last3=Newell |first3=David B. |last4=Taylor |first4=Barry N. |date=2021-09-23 |title=CODATA Recommended Values of the Fundamental Physical Constants: 2018* |journal=Journal of Physical and Chemical Reference Data |volume=50 |issue=3 |pages=033105 |doi=10.1063/5.0064853 |issn=0047-2689 |pmc=9888147 |pmid=36726646|bibcode=2021JPCRD..50c3105T }}</ref>

==== Isotopes ====
{{Main|Isotopes of hydrogen}}
[[File:Blausen 0530 HydrogenIsotopes.png|thumb|left|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)]]
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
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|author7=Poroshin, N. O.
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|author9=Tel'kushev, M. V.
|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
|bibcode=2003PhRvL..90h2501K
|display-authors=8
|last1=Korsheninnikov
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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.<ref>{{NUBASE2020}}</ref>

'''{{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 nuclei in the universe is thought to have been produced at the time of the [[Big Bang]], and has endured since then.<ref>{{Cite journal |last1=Particle Data Group |last2=Workman |first2=R L |last3=Burkert |first3=V D |last4=Crede |first4=V |last5=Klempt |first5=E |last6=Thoma |first6=U |last7=Tiator |first7=L |last8=Agashe |first8=K |last9=Aielli |first9=G |last10=Allanach |first10=B C |last11=Amsler |first11=C |last12=Antonelli |first12=M |last13=Aschenauer |first13=E C |last14=Asner |first14=D M |last15=Baer |first15=H |date=2022-08-08 |title=Review of Particle Physics |url=https://academic.oup.com/ptep/article/doi/10.1093/ptep/ptac097/6651666 |journal=Progress of Theoretical and Experimental Physics |language=en |volume=2022 |issue=8 |doi=10.1093/ptep/ptac097 |issn=2050-3911|hdl=1854/LU-01HQG4F6CV7P2F3WWNH4RRN8HD |hdl-access=free }}</ref>{{rp|loc=24.2}} 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
|url=https://query.nytimes.com/gst/fullpage.html?res=9D0CE4D81030F932A25752C1A967958260
|access-date=12 February 2008
|archive-date=29 January 2021
|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>

[[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> The [[exotic atom]] [[muonium]] (symbol Mu), composed of an anti[[muon]] and an [[electron]], is analogous hydrogen 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]]
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|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>

===Dihydrogen===

Under [[standard conditions]], hydrogen is a [[gas]] of [[diatomic molecule]]s with the [[chemical formula|formula]] {{chem2|H2}}, officially called "dihydrogen",<ref>[http://old.iupac.org/publications/books/rbook/Red_Book_2005.pdf Nomenclature of Inorganic Chemistry IUPAC Recommendations 2005] - Full text (PDF)<br />2004 version with separate chapters as pdf: [http://www.iupac.org/reports/provisional/abstract04/connelly_310804.html IUPAC Provisional Recommendations for the Nomenclature of Inorganic Chemistry (2004)] {{webarchive|url=https://web.archive.org/web/20080219122415/http://www.iupac.org/reports/provisional/abstract04/connelly_310804.html |date=2008-02-19 }}</ref>{{rp|308}} but also called "molecular hydrogen",<ref>{{Cite encyclopedia|title=Hydrogen|url=https://www.britannica.com/science/hydrogen|url-status=live|access-date=25 December 2021|encyclopedia=[[Encyclopædia Britannica]]|archive-date=24 December 2021|archive-url=https://web.archive.org/web/20211224165150/https://www.britannica.com/science/hydrogen}}</ref> or simply hydrogen. Dihydrogen is a colorless, odorless, flammable gas.<ref>{{Cite encyclopedia|title=Hydrogen|url=https://www.britannica.com/science/hydrogen|url-status=live|access-date=25 December 2021|encyclopedia=[[Encyclopædia Britannica]]|archive-date=24 December 2021|archive-url=https://web.archive.org/web/20211224165150/https://www.britannica.com/science/hydrogen}}</ref>

==== Combustion ====
[[File:19. Експлозија на смеса од водород и воздух.webm|thumb|left|Combustion of hydrogen with the oxygen in the air. When the bottom cap is removed, allowing air to enter, hydrogen in the container rises and burns as it mixes with the air.]]

Hydrogen gas is highly flammable, reacting with [[oxygen]] in air, to produce liquid water:
:{{chem2|2 H2(g) + O2(g) → 2 H2O(l)}} 
The [[Enthalpy of combustion|amount of heat released]] per [[mole (unit)|mole]] of hydrogen is −286&nbsp;kJ or 141.865 MJ for a kilogram mass.<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 |last1=Yamada |first1=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 |bibcode=2011IJHE...36.2560Y |issn=0360-3199}}</ref>

Hydrogen flames emit faint blue and [[ultraviolet]] light.<ref>{{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="spinoff-2016" />

==== 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:Phase diagram of hydrogen.png|thumb|left|[[Phase diagram]] of hydrogen with a [[logarithmic scale]] The left edge corresponds about one atmosphere.<ref>{{Cite journal |last=Stevenson |first=D J |date=May 1982 |title=Interiors of the Giant Planets |url=https://www.annualreviews.org/doi/10.1146/annurev.ea.10.050182.001353 |journal=Annual Review of Earth and Planetary Sciences |language=en |volume=10 |issue=1 |pages=257–295 |doi=10.1146/annurev.ea.10.050182.001353 |bibcode=1982AREPS..10..257S |issn=0084-6597}}</ref>|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]].<ref>{{cite journal |author1=James Dewar |author1-link=James Dewar |title=Liquid Hydrogen |journal=Science |date=1900 |volume=11 |issue=278 |pages=641–651 |doi=10.1126/science.11.278.641 |pmid=17813562 |bibcode=1900Sci....11..641D |language=en}}</ref>

Liquid hydrogen becomes [[solid hydrogen]] at [[standard pressure]] below hydrogen's [[melting point]] of {{convert|14.01|K}}. 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.<ref>{{Cite book|last=Phillips |first=K. J. H. |date=1995 |title=Guide to the Sun |page=|publisher=[[Cambridge University Press]] |url=https://books.google.com/books?id=idwBChjVP0gC&q=Guide+to+the+Sun+phillips |isbn=978-0-521-39788-9 |url-status=live |archive-url= https://web.archive.org/web/20180115215631/https://books.google.com/books?id=idwBChjVP0gC&printsec=frontcover&dq=Guide+to+the+Sun+phillips&hl=en&sa=X&ved=0ahUKEwiBj4Gbj5bXAhXrrVQKHfnAAKUQ6AEIKDAA |archive-date=15 January 2018 }}</ref>

==== Thermal and physical properties ====
{|class="wikitable mw-collapsible mw-collapsed"
|- 
! colspan{{=}}"8" |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>
|-
|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
|}