Hydrogen

Hydrogen, ₁H
	Purple glow in its plasma state
Hydrogen
Appearance	Colorless gas
Standard atomic weight Ar°(H)
	[1.00784, 1.00811]; 1.0080±0.0002 (abridged);
Hydrogen in the periodic table
	(none) ← hydrogen → helium
Atomic number (Z)	1
Group	group 1: hydrogen and alkali metals
Period	period 1
Block	s-block
Electron configuration	1s1
Electrons per shell	1
Physical properties
Phase at STP	gas
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)
when liquid (at b.p.)	0.07099 g/cm3
Triple point	13.8033 K, 7.041 kPa
Critical point	32.938 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)
Atomic properties
Oxidation states	common: −1, +1;
Electronegativity	Pauling scale: 2.20
Ionization energies	1st: 1312.0 kJ/mol ; ;
Covalent radius	31±5 pm
Van der Waals radius	120 pm
	Spectral lines of hydrogen
Other properties
Natural occurrence	primordial
Crystal structure	hexagonal (hP4)
Lattice constants	a = 378.97 pm; c = 618.31 pm (at triple point)
Thermal conductivity	0.1805 W/(m⋅K)
Magnetic ordering	diamagnetic
Molar magnetic susceptibility	−3.98×10−6 cm3/mol (298 K)
Speed of sound	1310 m/s (gas, 27 °C)
CAS Number	12385-13-6 ; 1333-74-0 (H2)
History
Naming	name means 'water-former' in Greek
Discovery and first isolation	Henry Cavendish (1766)
Named by	Antoine Lavoisier (1783)
Isotopes of hydrogen v; e;
	Category: Hydrogen; view; talk; edit; \| references

Hydrogen is a chemical element; it has the symbol H and atomic number 1. It is the lightest and most abundant chemical element in the universe, constituting about 75% of all normal matter. Stars, including the Sun, mainly consist of hydrogen in a plasma state, while on Earth, hydrogen is found in water and organic compounds as the gas H₂ (dihydrogen), and in other molecular forms. The most common isotope of hydrogen (protium, ¹H) consists of one proton, one electron, and no neutrons.

Under standard conditions, hydrogen is a gas of diatomic molecules with the formula H₂, called dihydrogen, or sometimes hydrogen gas, molecular hydrogen, or simply hydrogen. Dihydrogen is colorless, odorless, non-toxic, and highly combustible.

Hydrogen gas was first produced artificially in the 17th century by the reaction of acids with metals. Henry Cavendish, in 1766–1781, identified hydrogen gas as a distinct substance 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 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⁺, called a proton. Although tightly bonded to water molecules, protons strongly affect the behavior of aqueous solutions, as reflected in the importance of pH. Hydride, on the other hand, is rarely observed because it tends to deprotonate solvents, yielding H₂.

In the early universe, neutral hydrogen atoms formed about 370,000 years after the Big Bang as the universe expanded and plasma had cooled enough for electrons to remain bound to protons. Once stars formed most of the atoms in the intergalactic medium re-ionized.

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. Its main industrial uses include fossil fuel processing and ammonia production. Emerging uses for hydrogen include the use of fuel cells to generate electricity.

^ "Standard Atomic Weights: Hydrogen". CIAAW. 2009.
^ 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.
^ Wiberg, Egon; Wiberg, Nils; Holleman, Arnold Frederick (2001). Inorganic chemistry. Academic Press. p. 240. ISBN 978-0123526519.
^ Arblaster, John W. (2018). Selected Values of the Crystallographic Properties of Elements. Materials Park, Ohio: ASM International. ISBN 978-1-62708-155-9.
^ 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.
^ Weast, Robert (1984). CRC, Handbook of Chemistry and Physics. Boca Raton, Florida: Chemical Rubber Company Publishing. pp. E110. ISBN 978-0-8493-0464-4.
^ ^a ^b 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.
^ Stwertka, Albert (1996). A Guide to the Elements. Oxford University Press. pp. 16–21. ISBN 978-0-19-508083-4.
^ Emsley, John (2001). Nature's Building Blocks. Oxford: Oxford University Press. pp. 183–191. ISBN 978-0-19-850341-5.
^ "Hydrogen". Van Nostrand's Encyclopedia of Chemistry. Wylie-Interscience. 2005. pp. 797–799. ISBN 978-0-471-61525-5.

[CIAAW-1] "Standard Atomic Weights: Hydrogen". CIAAW. 2009.

[CIAAW2021-2] 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.

[Arblaster_2018-4] 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.

[Miskowiec2023-7] 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.

[Stwertka-8] Stwertka, Albert (1996). A Guide to the Elements. Oxford University Press. pp. 16–21. ISBN 978-0-19-508083-4.

[nbb-9] Emsley, John (2001). Nature's Building Blocks. Oxford: Oxford University Press. pp. 183–191. ISBN 978-0-19-850341-5.

[Nostrand-10] "Hydrogen". Van Nostrand's Encyclopedia of Chemistry. Wylie-Interscience. 2005. pp. 797–799. ISBN 978-0-471-61525-5.

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