Editing Hydrogen (section)

===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
|}