The hydrogen anion, H⁻, is a negative ion of hydrogen, that is, a

hydrogen atom A hydrogen atom is an atom of the chemical element hydrogen. The electrically neutral atom contains a single positively charged proton and a single negatively charged electron bound to the nucleus by the Coulomb force. Atomic hydrogen cons ...

that has captured an extra electron. The hydrogen anion is an important constituent of the atmosphere of

star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

s, such as the Sun. In chemistry, this ion is called hydride. The ion has two electrons bound by the

electromagnetic force In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions o ...

to a nucleus containing one proton. The binding energy of H⁻ equals the binding energy of an extra electron to a hydrogen atom, called electron affinity of hydrogen. It is measured to be or (see Electron affinity (data page)). The total ground state energy thus becomes .

Occurrence

The hydrogen anion is the dominant bound-free opacity source at visible and near-infrared wavelengths in the

atmospheres The standard atmosphere (symbol: atm) is a unit of pressure defined as Pa. It is sometimes used as a ''reference pressure'' or ''standard pressure''. It is approximately equal to Earth's average atmospheric pressure at sea level. History The s ...

of stars like the Sun and cooler; its importance was first noted in the 1930s. The ion absorbs photons with energies in the range 0.75–4.0 eV, which ranges from the infrared into the visible spectrum. Most of the electrons in these negative ions come from the ionization of metals with low first ionization potentials, including the alkali metals and alkali earths. The process which ejects the electron from the ion is properly called ''photodetachment'' rather than ''photoionization'' because the result is a neutral atom (rather than an ion) and a free electron. H⁻ also occurs in the Earth's ionosphere and can be produced in

particle accelerator A particle accelerator is a machine that uses electromagnetic fields to propel charged particles to very high speeds and energies, and to contain them in well-defined beams. Large accelerators are used for fundamental research in particle ...

s. Its existence was first proven theoretically by

Hans Bethe Hans Albrecht Bethe (; July 2, 1906 – March 6, 2005) was a German-American theoretical physicist who made major contributions to nuclear physics, astrophysics, quantum electrodynamics, and solid-state physics, and who won the 1967 Nobel ...

in 1929. H⁻ is unusual because, in its free form, it has no bound excited states, as was finally proven in 1977. In chemistry, the

hydride In chemistry, a hydride is formally the anion of hydrogen( H−). The term is applied loosely. At one extreme, all compounds containing covalently bound H atoms are called hydrides: water (H2O) is a hydride of oxygen, ammonia is a hydride ...

anion is hydrogen that has the formal

oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...

−1. The term hydride is probably most often used to describe compounds of hydrogen with other elements in which the hydrogen is in the formal −1

. In most such compounds the bonding between the hydrogen and its nearest neighbor is covalent. An example of a hydride is the

borohydride Borohydride refers to the anion , which is also called tetrahydroborate, and its salts. Borohydride or hydroborate is also the term used for compounds containing , where ''n'' is an integer from 0 to 3, for example cyanoborohydride or cyanotrihyd ...

anion ().

References

{{reflist Hydrogen physics Astrophysics Anions

Occurrence

See also

References