Hydrogen, 1H
Purple glow in its plasma state
AppearanceColorless gas
Standard atomic weight Ar°(H)
Hydrogen in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson


(none) ← hydrogenhelium
Atomic number (Z)1
Groupgroup 1: hydrogen and alkali metals
Periodperiod 1
Block  s-block
Electron configuration1s1
Electrons per shell1
Physical properties
Phase at STPgas
Melting point(H2) 13.99 K ​(−259.16 °C, ​−434.49 °F)
Boiling point(H2) 20.271 K ​(−252.879 °C, ​−423.182 °F)
Density (at STP)0.08988 g/L
when liquid (at m.p.)0.07 g/cm3 (solid: 0.0763 g/cm3)[3]
when liquid (at b.p.)0.07099 g/cm3
Triple point13.8033 K, ​7.041 kPa
Critical point32.938 K, 1.2858 MPa
Heat of fusion(H2) 0.117 kJ/mol
Heat of vaporization(H2) 0.904 kJ/mol
Molar heat capacity(H2) 28.836 J/(mol·K)
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K) 15 20
Atomic properties
Oxidation states−1, 0, +1 (an amphoteric oxide)
ElectronegativityPauling scale: 2.20
Ionization energies
  • 1st: 1312.0 kJ/mol
Covalent radius31±5 pm
Van der Waals radius120 pm
Color lines in a spectral range
Spectral lines of hydrogen
Other properties
Natural occurrenceprimordial
Crystal structurehexagonal (hP4)
Lattice constants
Hexagonal crystal structure for hydrogen
a = 378.97 pm
c = 618.31 pm (at triple point)[4]
Thermal conductivity0.1805 W/(m⋅K)
Magnetic orderingdiamagnetic[5]
Molar magnetic susceptibility−3.98×10−6 cm3/mol (298 K)[6]
Speed of sound1310 m/s (gas, 27 °C)
CAS Number12385-13-6
1333-74-0 (H2)
DiscoveryHenry Cavendish[7][8] (1766)
Named byLouis-Bernard Guyton de Morveau
Antoine Lavoisier[9][10] (1787)
Isotopes of hydrogen
Main isotopes Decay
abun­dance half-life (t1/2) mode pro­duct
1H 99.9855%
Preview warning: Infobox H isotopes: Abundance percentage not recognised "na=99.9855%" cat#%
2H 0.0145%
Preview warning: Infobox H isotopes: Abundance percentage not recognised "na=0.0145%" cat#%
3H trace 12.32 y β 3He
 Category: Hydrogen
| references

Hydrogen is a chemical element; it has symbol H and atomic number 1. It is the lightest element and, at standard conditions, is a gas of diatomic molecules with the formula H2, sometimes called dihydrogen,[11] but more commonly called hydrogen gas, molecular hydrogen or simply hydrogen. It is colorless, odorless, tasteless,[12] non-toxic, and highly combustible. Constituting approximately 75% of all normal matter, hydrogen is the most abundant chemical substance in the universe.[13][note 1] Stars, including the Sun, primarily consist of hydrogen in a plasma state, while on Earth, hydrogen is found in water, organic compounds, and other molecular forms. The most common isotope of hydrogen (symbol 1H) consists of one proton, one electron, and no neutrons.

In the early universe, the formation of hydrogen's protons occurred during the first second following the Big Bang, with neutral hydrogen atoms only forming approximately 370,000 years later during the recombination epoch as the universe cooled and plasma had cooled enough for electrons to remain bound to protons.[14] Hydrogen, typically nonmetallic except under extreme pressures, 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 predominantly 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 denoted by the symbol H+. The cation, simply a proton (symbol p), exhibits specific behavior in aqueous solutions and in ionic compounds involves screening of its electric charge by surrounding polar molecules or anions. Hydrogen's unique position as the only neutral atom for which the Schrödinger equation can be directly solved has significantly contributed to the foundational principles of quantum mechanics through the exploration of its energetics and chemical bonding.[15]

Historically, hydrogen gas was first produced artificially in the early 16th century through the reaction of acids with metals. Henry Cavendish, between 1766 and 1781, identified hydrogen gas as a distinct substance[16] and discovered its property of producing water when burned—hence its name derived from the Greek "water-former".

Today, the majority of hydrogen production occurs through steam reforming of natural gas, with a smaller portion derived from energy-intensive methods such as the electrolysis of water.[17][18] Its primary industrial uses include fossil fuel processing, such as hydrocracking, and ammonia production, with emerging applications in fuel cells for electricity generation and as a heat source.[19] When used in fuel cells, hydrogen's only emission at the point of use is water vapor,[19] though combustion can produce nitrogen oxides.[19] Hydrogen's interaction with metals may cause embrittlement.[20]

  1. ^ "Standard Atomic Weights: Hydrogen". CIAAW. 2009.
  2. ^ Prohaska, Thomas; Irrgeher, Johanna; Benefield, Jacqueline; Böhlke, John K.; Chesson, Lesley A.; Coplen, Tyler B.; Ding, Tiping; Dunn, Philip J. H.; Gröning, Manfred; Holden, Norman E.; Meijer, Harro A. J. (4 May 2022). "Standard atomic weights of the elements 2021 (IUPAC Technical Report)". Pure and Applied Chemistry. doi:10.1515/pac-2019-0603. ISSN 1365-3075.
  3. ^ Wiberg, Egon; Wiberg, Nils; Holleman, Arnold Frederick (2001). Inorganic chemistry. Academic Press. p. 240. ISBN 978-0123526519.
  4. ^ Arblaster, John W. (2018). Selected Values of the Crystallographic Properties of Elements. Materials Park, Ohio: ASM International. ISBN 978-1-62708-155-9.
  5. ^ Lide, D. R., ed. (2005). "Magnetic susceptibility of the elements and inorganic compounds". CRC Handbook of Chemistry and Physics (PDF) (86th ed.). Boca Raton (FL): CRC Press. ISBN 978-0-8493-0486-6.
  6. ^ Weast, Robert (1984). CRC, Handbook of Chemistry and Physics. Boca Raton, Florida: Chemical Rubber Company Publishing. pp. E110. ISBN 978-0-8493-0464-4.
  7. ^ "Hydrogen". Van Nostrand's Encyclopedia of Chemistry. Wylie-Interscience. 2005. pp. 797–799. ISBN 978-0-471-61525-5.
  8. ^ Emsley, John (2001). Nature's Building Blocks. Oxford: Oxford University Press. pp. 183–191. ISBN 978-0-19-850341-5.
  9. ^ Miśkowiec, Paweł (April 2023). "Name game: The naming history of the chemical elements—part 1—from antiquity till the end of 18th century". Foundations of Chemistry. 25 (1): 29–51. doi:10.1007/s10698-022-09448-5.
  10. ^ Stwertka, Albert (1996). A Guide to the Elements. Oxford University Press. pp. 16–21. ISBN 978-0-19-508083-4.
  11. ^ "Dihydrogen". O=CHem Directory. University of Southern Maine. Archived from the original on 13 February 2009. Retrieved 6 April 2009.
  12. ^ "Hydrogen". Encyclopædia Britannica. Archived from the original on 24 December 2021. Retrieved 25 December 2021.
  13. ^ Boyd, Padi (19 July 2014). "What is the chemical composition of stars?". NASA. Archived from the original on 15 January 2015. Retrieved 5 February 2008.
  14. ^ Tanabashi, M.; et al. (2018). "Big-Bang Cosmology" (PDF). Physical Review Rev. D. 98 (3): 358. doi:10.1103/PhysRevD.98.030001. Archived (PDF) from the original on 29 June 2021 – via Particle Data Group at Lawrence Berkeley National Laboratory. Chapter 21.4.1 - This occurred when the age of the Universe was about 370,000 years. (Revised September 2017) by Keith A. Olive and John A. Peacock.
  15. ^ Laursen, S.; Chang, J.; Medlin, W.; Gürmen, N.; Fogler, H. S. (27 July 2004). "An extremely brief introduction to computational quantum chemistry". Molecular Modeling in Chemical Engineering. University of Michigan. Archived from the original on 20 May 2015. Retrieved 4 May 2015.
  16. ^ Presenter: Professor Jim Al-Khalili (21 January 2010). "Discovering the Elements". Chemistry: A Volatile History. 25:40 minutes in. BBC. BBC Four. Archived from the original on 25 January 2010. Retrieved 9 February 2010.
  17. ^ Dincer, Ibrahim; Acar, Canan (14 September 2015). "Review and evaluation of hydrogen production methods for better sustainability". International Journal of Hydrogen Energy. 40 (34): 11094–11111. Bibcode:2015IJHE...4011094D. doi:10.1016/j.ijhydene.2014.12.035. ISSN 0360-3199. Archived from the original on 15 February 2022. Retrieved 4 February 2022.
  18. ^ "Hydrogen Basics – Production". Florida Solar Energy Center. 2007. Archived from the original on 18 February 2008. Retrieved 5 February 2008.
  19. ^ a b c Lewis, Alastair C. (10 June 2021). "Optimising air quality co-benefits in a hydrogen economy: a case for hydrogen-specific standards for NO x emissions". Environmental Science: Atmospheres. 1 (5): 201–207. doi:10.1039/D1EA00037C. S2CID 236732702. This article incorporates text from this source, which is available under the CC BY 3.0 license.
  20. ^ Rogers, H. C. (1999). "Hydrogen Embrittlement of Metals". Science. 159 (3819): 1057–1064. Bibcode:1968Sci...159.1057R. doi:10.1126/science.159.3819.1057. PMID 17775040. S2CID 19429952.

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