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Find out in here:  Hydrogen

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This article is about the chemistry of hydrogen. For the physics of atomic hydrogen, see Hydrogen atom. For other meanings, see Hydrogen (disambiguation).

Hydrogen
1H

-
↑
H
↓
Li

Periodic table

- ← hydrogen → helium

Appearance
colorless gas

Purple glow in its plasma state

Spectral lines of hydrogen
General properties
Name, symbol, number hydrogen, H, 1
Pronunciation /?ha?dr?d??n/[1] HYE-dro-jin
Element category nonmetal
Group, period, block 1, 1, s
Standard atomic weight 1.00794(7)
Electron configuration 1s1
1

History
Discovery H. Cavendish[2][3] (1766)
Named by A. Lavoisier[4] (1783)
Physical properties
Color colorless
Phase gas
Density (0 °C, 101.325 kPa)
0.08988 g/L
Liquid density at m.p. 0.07 (0.0763 solid)[5] g·cm−3
Liquid density at b.p. 0.07099 g·cm−3
Melting point 14.01 K, -259.14 °C, -434.45 °F
Boiling point 20.28 K, -252.87 °C, -423.17 °F
Triple point 13.8033 K (-259°C), 7.042 kPa
Critical point 32.97 K, 1.293 MPa
Heat of fusion (H2) 0.117 kJ·mol−1
Heat of vaporization (H2) 0.904 kJ·mol−1
Molar heat capacity (H2) 28.836 J·mol−1·K−1
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K) 15 20

Atomic properties
Oxidation states 1, -1
(amphoteric oxide)
Electronegativity 2.20 (Pauling scale)
Ionization energies 1st: 1312.0 kJ·mol−1
Covalent radius 31±5 pm
Van der Waals radius 120 pm
Miscellanea
Crystal structure hexagonal

Magnetic ordering diamagnetic[6]
Thermal conductivity 0.1805 W·m−1·K−1
Speed of sound (gas, 27 °C) 1310 m·s−1
CAS registry number 1333-74-0
Most stable isotopes
Main article: Isotopes of hydrogen
iso NA half-life DM DE (MeV) DP
1H 99.985% 1H is stable with 0 neutrons
2H 0.015% 2H is stable with 1 neutron
3H trace 12.32 y β− 0.01861 3He
4H syn 139(10) ys n 4.6(9) 3H

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Hydrogen ( /?ha?dr?d??n/ HY-dr?-j?n)[7] is a chemical element with symbol H and atomic number 1. With an average atomic weight of 1.00794 u (1.007825 u for hydrogen-1), hydrogen is the lightest element and its monatomic form (H1) is the most abundant chemical substance, constituting roughly 75% of the Universe's baryonic mass.[8] Non-remnant stars are mainly composed of hydrogen in its plasma state.

At standard temperature and pressure, hydrogen is a colorless, odorless, tasteless, non-toxic, nonmetallic, highly combustible diatomic gas with the molecular formula H2. Naturally occurring atomic hydrogen is rare on Earth because hydrogen readily forms covalent compounds with most elements and is present in the water molecule and in most organic compounds. Hydrogen plays a particularly important role in acid-base chemistry with many reactions exchanging protons between soluble molecules.

In ionic compounds, it can take a negative charge (an anion known as a hydride and written as H−), or as a positively charged species H+. The latter cation is written as though composed of a bare proton, but in reality, hydrogen cations in ionic compounds always occur as more complex species.

The most common isotope of hydrogen is protium (name rarely used, symbol 1H) with a single proton and no neutrons. As the simplest atom known, the hydrogen atom has been of theoretical use. For example, as the only neutral atom with an analytic solution to the Schrödinger equation, the study of the energetics and bonding of the hydrogen atom played a key role in the development of quantum mechanics.

Hydrogen gas was first artificially produced in the early 16th century, via the mixing of metals with strong acids. In 1766–81, Henry Cavendish was the first to recognize that hydrogen gas was a discrete substance,[9] and that it produces water when burned, a property which later gave it its name: in Greek, hydrogen means "water-former".

Industrial production is mainly from the steam reforming of natural gas, and less often from more energy-intensive hydrogen production methods like the electrolysis of water.[10] Most hydrogen is employed near its production site, with the two largest uses being fossil fuel processing (e.g., hydrocracking) and ammonia production, mostly for the fertilizer market.

Hydrogen is a concern in metallurgy as it can embrittle many metals,[11] complicating the design of pipelines and storage tanks