The hydrogen anion, H⁻, is a negative

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

of hydrogen, that is, a hydrogen atom that has captured an extra electron. The hydrogen anion is an important constituent of the atmosphere of stars, such as the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

. 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 The electron affinity (''E''ea) of an atom or molecule is defined as the amount of energy released when an electron attaches to a neutral atom or molecule in the gaseous state to form an anion. ::X(g) + e− → X−(g) + energy Note that this is ...

of hydrogen. It is measured to be or (see

Electron affinity (data page) This page deals with the electron affinity as a property of isolated atoms or molecules (i.e. in the gas phase). Solid state electron affinities are not listed here. Elements Electron affinity can be defined in two equivalent ways. First, a ...

). 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 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 The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names ...

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 Bound or bounds may refer to: Mathematics * Bound variable * Upper and lower bounds, observed limits of mathematical functions Physics * Bound state, a particle that has a tendency to remain localized in one or more regions of space Geography *B ...

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