View source for Hydrogen anion

{{About|the anion of hydrogen|binary compounds of hydrogen with another element|Hydride}}
{{Short description|Negative ion of hydrogen}}
{{Chembox
| ImageFile      = Hydride-anion.png
| SystematicName = Hydride<ref>{{Cite web|url = https://pubchem.ncbi.nlm.nih.gov/summary/summary.cgi?cid=166653|title = Hydride - PubChem Public Chemical Database|work = The PubChem Project|location = USA|publisher = [[National Center for Biotechnology Information]]}}</ref>
| Section1       = {{Chembox Identifiers
| CASNo = 12184-88-2
| PubChem = 166653
| ChemSpiderID = 145831
| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
| ChEBI = 29239
| Gmelin = 14911
| SMILES = [H-]
| StdInChI = 1S/H/q-1
| StdInChI_Ref = {{stdinchicite|correct|chemspider}}
| StdInChIKey = KLGZELKXQMTEMM-UHFFFAOYSA-N
| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
}}
| Section2       = {{Chembox Properties
| H=1 | Formula_Charge = -
| ConjugateAcid = [[Dihydrogen]]
}}
| Section3       = {{Chembox Thermochemistry
| Entropy = 108.96 J K<sup>−1</sup> mol<sup>−1</sup>
}}
}}
The '''hydrogen anion''', H<sup>−</sup>, is a [[Electric charge|negative]] [[ion]] of hydrogen, <!-- don’t link as [[hydrogen ion]], it’s a set index! -->that is, a [[hydrogen atom]] that has captured an extra electron. The hydrogen anion is an important constituent of the atmosphere of [[star]]s, such as the [[Sun]].  In chemistry, this ion is called '''hydride'''. The ion has [[Two-electron atom|two electrons]] bound by the [[electromagnetic force]] to a nucleus containing one proton.

The binding energy of H<sup>−</sup> equals the binding energy of an extra electron to a hydrogen atom, called [[electron affinity]] of hydrogen. It is measured to be {{val|0.754195|(19)|ul=eV}} or {{val|0.0277161|(62)|ul=hartree}} (see [[Electron affinity (data page)]]). The total ground state energy thus becomes {{val|-14.359888|u=eV}}.

==Occurrence==
The hydrogen anion is the dominant [[Kramers'_opacity_law|bound-free]] [[Opacity_(optics)|opacity]] source at visible and near-infrared wavelengths in the [[stellar atmosphere|atmospheres]] of stars like the Sun and cooler;<ref>{{cite book |last=Mihalas |first=Dmitri |date=1978 |title=Stellar Atmospheres | publisher=W. H. Freeman |page=102 }}</ref> its importance was first noted in the 1930s.<ref>{{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 }} </ref> The ion absorbs photons with energies in the range 0.75–4.0 eV, which ranges from the infrared into the visible spectrum.<ref name="Rau">{{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 }}</ref><ref>{{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}}</ref>  Most of the electrons in these negative ions come from the ionization of metals with low first ionization potentials, including the [[alkali metal|alkali metals]] and [[alkaline earth metal|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<sup>−</sup> also occurs in the Earth's ionosphere<ref name="Rau"/> and can be produced in [[particle accelerator]]s.<ref>{{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<sup>−</sup> 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. }}</ref> 

Its existence was first proven theoretically by [[Hans Bethe]] in 1929.<ref>{{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 }}</ref> H<sup>−</sup> is unusual because, in its free form, it has no [[bound state|bound]] [[energy level|excited states]], as was finally proven in 1977.<ref>{{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<sup>−</sup> Ion Has Only One Bound State }}</ref>

In chemistry, the [[hydride]] anion is hydrogen that has the formal [[oxidation state]] −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 [[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|-}}).

==See also==
* [[Hydron (chemistry)|Hydron]] (hydrogen cation)
* [[Electride]], another very simple anion
* [[Hydrogen ion]]

== References ==
{{reflist}}

[[Category:Hydrogen physics]]
[[Category:Astrophysics]]
[[Category:Anions]]