 | |
| Names | |
|---|---|
| Systematic IUPAC name Hydride[1] | |
| Identifiers | |
3D model (JSmol) | |
| ChEBI | |
| ChemSpider |
|
| 14911 | |
| |
| |
| Properties | |
| H− | |
| Molar mass | 1.009 g·mol−1 |
| Conjugate acid | Dihydrogen |
| Thermochemistry | |
Std molar entropy(S⦵298) | 108.96 J K−1 mol−1 |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Thehydrogen anion, H−, is anegativeion of hydrogen, that is, ahydrogen atom that has captured an extra electron. The hydrogen anion is an important constituent of the atmosphere ofstars, such as theSun. In chemistry, this ion is calledhydride. The ion hastwo electrons bound by theelectromagnetic force to a nucleus containing one proton.
The binding energy of H− equals the binding energy of an extra electron to a hydrogen atom, calledelectron affinity of hydrogen. It is measured to be0.754195(19) eV[2] or0.02771616(70) hartree (seeElectron affinity (data page)). The total ground state energy thus becomes−14.359888 eV.
ByLieb's inequality, H−− has no bound state, so it does not exist.[3]
The hydrogen anion is the dominantbound-freeopacity source at visible and near-infrared wavelengths in theatmospheres of stars like the Sun and cooler;[4] its importance was first noted in the 1930s.[5] The ion absorbs photons with energies in the range 0.75–4.0 eV, which ranges from the infrared into the visible spectrum.[6][7] Most of the electrons in these negative ions come from the ionization of metals with low first ionization potentials, including thealkali metals andalkaline earths. The process which ejects the electron from the ion is properly calledphotodetachment rather thanphotoionization because the result is a neutral atom (rather than an ion) and a free electron.
H− also occurs in the Earth's ionosphere[6] and can be produced inparticle accelerators.[8]
Its existence was first proven theoretically byHans Bethe in 1929,[9] who used Hylleraas's variational method to show that H− is bound. He estimated its ground-state energy as−1.0506 Ry (−0.5253 Eh), placing it below the hydrogen atom's ground state energy (−0.5 Eh). H− is unusual because, in its free form, it has noboundexcited states, as was finally proven in 1977.[10]
In chemistry, hydrogen has the formaloxidation state −1 in thehydride anion.
The term hydride is probably most often used to describe compounds of hydrogen with other elements in which the hydrogen is in the formal −1oxidation state. In most such compounds the bonding between the hydrogen and its nearest neighbor is covalent. An example of a hydride is theborohydride anion (BH−
4).
