hydrogen anion

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;{{cite book |last=Mihalas |first=Dmitri |date=1978 |title=Stellar Atmospheres | publisher=W. H. Freeman |page=102 }} its importance was first noted in the 1930s.{{cite journal |last=Wildt |first=Rupert |date=1939 |journal=Astrophysical Journal |volume=90|page=611|bibcode = 1939ApJ....90..611W |doi = 10.1086/144125 |title=Negative Ions of Hydrogen and the Opacity of Stellar Atmospheres }} The ion absorbs photons with energies in the range 0.75–4.0 eV, which ranges from the infrared into the visible spectrum.{{cite journal |last=Rau |first=A. R. P. |date=1996 |url=http://www.ias.ac.in/jarch/jaa/17/113-145.pdf |title=The Negative Ion of Hydrogen |journal=Journal of Astrophysics and Astronomy |volume=17|issue=3 |pages=113–145|doi=10.1007/BF02702300 |bibcode = 1996JApA...17..113R |s2cid=56355519 }}{{cite book |last=Srinivasan |first=G. |date=1999 |title=From White Dwarfs to Black Holes: The Legacy of S. Chandrasekhar |location=Chicago |publisher=University of Chicago Press|chapter=Chapter 5}} Most of the electrons in these negative ions come from the ionization of metals with low first ionization potentials, including the alkali metals and alkaline 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 accelerators.{{cite journal |last1=Bryant |first1=H. C. |date=1977 |journal=Physical Review Letters |volume=38 |issue=5 |page=228|bibcode = 1977PhRvL..38..228B |doi = 10.1103/PhysRevLett.38.228 |title=Observation of Resonances near 11 eV in the Photodetachment Cross Section of the H⁻ Ion |last2=Dieterle |first2=B. D. |last3=Donahue |first3=J. |last4=Sharifian |first4=H. |last5=Tootoonchi |first5=H. |last6=Wolfe |first6=D. M. |last7=Gram |first7=P. A. M. |last8=Yates-Williams |first8=M. A. }}

Its existence was first proven theoretically by Hans Bethe in 1929,{{cite journal |last=Bethe |first=H. |date=1929 |journal=Zeitschrift für Physik |volume=57 |issue=11–12 |pages=815–821|bibcode = 1929ZPhy...57..815B |doi = 10.1007/BF01340659 |title=Berechnung der Elektronenaffinität des Wasserstoffs|s2cid=125100200 |language=de }} who used Hylleraas's variational method to show that H⁻ is bound. He estimated its ground-state energy as {{val|-1.0506||ul=Ry}} ({{val|-0.5253||ul=Hartree}}), placing it below the hydrogen atom's ground state energy ({{val|-0.5||ul=Hartree}}). H⁻ is unusual because, in its free form, it has no bound excited states, as was finally proven in 1977.{{cite journal |last=Hill |first=R. N. |date=1977 |journal=Physical Review Letters |volume=38 |issue=12 |page=643|bibcode = 1977PhRvL..38..643H |doi = 10.1103/PhysRevLett.38.643 |title=Proof that the H⁻ Ion Has Only One Bound State }}

In chemistry, hydrogen has the formal oxidation state −1 in the hydride 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 −1 oxidation state. In most such compounds the bonding between the hydrogen and its nearest neighbor is covalent. An example of a hydride is the borohydride anion ({{chem|BH|4|-}}).

• Hydron (hydrogen cation)
• Electride, another very simple anion
• Hydrogen ion

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Category:Hydrogen physics

Category:Astrophysics

Category:Anions