this article is about the chemistry of hydrogen. for other uses, see hydrogen (disambiguation).
"element 1" redirects here. for the racing team, see element one.
chemical element with atomic number 1
chemical element with atomic number 1
purple glow in its plasma state
standard atomic weightar, std(h)
[1.00784, 1.00811] conventional: 1.008
hydrogen in the periodic table
– ↑ h ↓ li
– ← hydrogen → helium
1: h and alkali metals
electrons per shell
(h2) 13.99 k (−259.16 °c, −434.49 °f)
(h2) 20.271 k (−252.879 °c, −423.182 °f)
when liquid (at m.p.)
0.07 g/cm3 (solid: 0.0763 g/cm3)
when liquid (at b.p.)
13.8033 k, 7.041 kpa
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)
−1, +1 (an amphoteric oxide)
pauling scale: 2.20
1st: 1312.0 kj/mol
van der waals radius
spectral lines of hydrogen
speed of sound
1310 m/s (gas, 27 °c)
−3.98·10−6 cm3/mol (298 k)
12385-13-6 1333-74-0 (h2)
henry cavendish (1766)
antoine lavoisier (1783)
main isotopes of hydrogen
hydrogen is the chemical element with the symbolh and atomic number 1. with a standard atomic weight of 1.008, hydrogen is the lightest element in the periodic table. hydrogen is the most abundant chemical substance in the universe, constituting roughly 75% of all baryonic mass.[note 1] non-remnantstars are mainly composed of hydrogen in the plasma state. the most common isotope of hydrogen, termed protium (name rarely used, symbol 1h), has one proton and no neutrons.
the universal emergence of atomic hydrogen first occurred during the recombination epoch (big bang). at standard temperature and pressure, hydrogen is a colorless, odorless, tasteless, non-toxic, nonmetallic, highly combustiblediatomicgas with the molecular formula h2. since hydrogen readily forms covalent compounds with most nonmetallic elements, most of the hydrogen on earth exists in molecular forms such as water or organic compounds. hydrogen plays a particularly important role in acid–base reactions because most acid-base reactions involve the exchange of protons between soluble molecules. in ionic compounds, hydrogen can take the form of a negative charge (i.e., anion) when it is known as a hydride, or as a positively charged (i.e., cation) species denoted by the symbol h+. the hydrogen cation is written as though composed of a bare proton, but in reality, hydrogen cations in ionic compounds are always more complex. as the only neutral atom for which the schrödinger equation can be solved analytically, study of the energetics and bonding of the hydrogen atom has played a key role in the development of quantum mechanics.
hydrogen gas was first artificially produced in the early 16th century by the reaction of acids on metals. in 1766–81, henry cavendish was the first to recognize that hydrogen gas was a discrete substance, and that it produces water when burned, the property for which it was later named: in greek, hydrogen means "water-former".
industrial production is mainly from steam reforming natural gas, and less often from more energy-intensive methods such as the electrolysis of water. most hydrogen is used near the site of its production, the two largest uses being fossil fuel processing (e.g., hydrocracking) and ammonia production, mostly for the fertilizer market. hydrogen is problematic in metallurgy because it can embrittle many metals, complicating the design of pipelines and storage tanks.
Hydrogen gas was first artificially produced in the early 16th century by the reaction of acids on metals. In 1766–81, Henry Cavendish was the first to recognize that hydrogen gas was a discrete substance, and that it produces water when burned, the property for which it was later named: in Greek, hydrogen means "water-former".