{{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/compound/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 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 stars, such as the Sun. In chemistry, this ion is called '''hydride'''. The ion has 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}}<ref>{{Cite journal |last=Lykke |first=K. R. |last2=Murray |first2=K. K. |last3=Lineberger |first3=W. C. |date=1991-06-01 |title=<nowiki>Threshold photodetachment of ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$</nowiki> |url=https://journals.aps.org/pra/abstract/10.1103/PhysRevA.43.6104 |journal=Physical Review A |volume=43 |issue=11 |pages=6104–6107 |doi=10.1103/PhysRevA.43.6104|url-access=subscription }}</ref> or {{val|0.02771616|(70)|ul=hartree}} (see Electron affinity (data page)). The total ground state energy thus becomes {{val|-14.359888|u=eV}}.
By Lieb's inequality, H<sup>2−</sup> has no bound state, so it does not exist.<ref>{{Cite journal |last=Lieb |first=Elliott H. |author-link=Elliott H. Lieb |date=1984-06-01 |title=Bound on the maximum negative ionization of atoms and molecules |url=https://link.aps.org/doi/10.1103/PhysRevA.29.3018 |journal=Physical Review A |volume=29 |issue=6 |pages=3018–3028 |doi=10.1103/PhysRevA.29.3018|url-access=subscription }}</ref>
==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;<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 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<sup>−</sup> also occurs in the Earth's ionosphere<ref name="Rau"/> and can be produced in particle accelerators.<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> who used Hylleraas's variational method to show that H<sup>−</sup> 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<sup>−</sup> is unusual because, in its free form, it has no bound 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, 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|-}}).
==See also== * Hydron (hydrogen cation) * Electride, another very simple anion * Hydrogen ion
== References == {{reflist}}
Category:Hydrogen physics Category:Astrophysics Category:Anions