Helium hydride ion
onium ion of helium; protonated helium
The helium hydride ion, hydridohelium(1+) ion, or helonium is a cation (positively charged ion) with chemical formula HeH+. It consists of a helium atom bonded to a hydrogen atom, with one electron removed. It can also be viewed as protonated helium. It is the lightest heteronuclear ion, and is believed to be the first compound formed in the Universe after the Big Bang.
The ion was first produced in a laboratory in 1925. It is stable in isolation, but extremely reactive, and cannot be prepared in bulk, because it would react with any other molecule with which it came into contact. Noted as the strongest known acid—stronger than even fluoroantimonic acid—its occurrence in the interstellar medium had been conjectured since the 1970s, and it was finally detected in April 2019 using the airborne SOFIA telescope.
Physical properties
The helium hydrogen ion is isoelectronic with molecular hydrogen (H2).
Unlike the dihydrogen ion H+2, the helium hydride ion has a permanent dipole moment, which makes its spectroscopic characterization easier. The calculated dipole moment of HeH+ is 2.26 or 2.84 D. The electron density in the ion is higher around the helium nucleus than the hydrogen.
Begin with the source’s own compact description: “Helium hydride ion” is onium ion of helium; protonated helium. The dossier treats that line as a proposition to test through Helium, hydride and onium, not as a finished interpretation.
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The citation trail is more important than the brevity of the summary: it shows where individual claims can be examined in context. The source revision retrieved here is dated Sep 6, 2026. The linked authority identifier is Q1188583. None of the 0 selected statements returned an explicit reference. The first chronological checks are 1925 and 2019.
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This entry incorporates text from “Helium hydride ion” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.