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. 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. Stars, including the Sun, mainly consist of hydrogen in a plasma state, while on Earth, hydrogen is found as the gas H₂ (dihydrogen) and in molecules, such as in water and organic compounds. The most common isotope of hydrogen, ¹H, consists of one proton, one electron, and no neutrons.

Hydrogen, ₁H

Purple glow in hydrogen's 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
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 – ↑ H ↓ Li (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	14.418 J/(mol·K) (H) 28.836 J/(mol·K) (H2)
Specific heat capacity	14303.571 J/(kg·K) (H)
Vapor pressure P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 15 20
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
Main isotopes Decay Isotope abun­dance half-life (t1/2) mode pro­duct 1H 99.9855% Preview warning: Infobox H isotopes: Abundance percentage not recognised "na=99.9855%" cat#% stable 2H 0.0115% Preview warning: Infobox H isotopes: Abundance percentage not recognised "na=0.0115%" cat#% stable 3H trace 12.32 y β− 3He Abundance of deuterium is highly variable
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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: this is the origin of hydrogen's name, which means 'water-former' (from Ancient Greek: ὕδωρ, romanized: húdōr, lit. 'water', and γεννάω, gennáō, 'I bring forth'). Understanding the colors of light absorbed and emitted by hydrogen was a crucial part of the development of 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⁺, hydron. Although tightly bonded to water molecules, hydrons 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. After stars began to form, most of the hydrogen in the intergalactic medium was re-ionized.

Nearly all hydrogen production is done by transforming fossil fuels, particularly steam reforming of natural gas. It can also be produced from water or saline by electrolysis, but this process is more expensive. Its main industrial uses include fossil fuel processing and ammonia production for fertilizer. Emerging uses for hydrogen include the use of fuel cells to generate electricity.