Helium compounds

Helium compounds are chemical species containing helium, from short-lived ions and weakly bound complexes to solids stabilized under extreme pressure. Their unusual bonding reflects helium’s closed electron shell and high ionization energy.

Helium: Helium is a chemical element with atomic number 2 and a closed 1s electron shell. Its exceptionally stable electron shell makes ordinary chemical bonding difficult.
Helium dimer cation: The helium dimer cation, He₂⁺, is a molecular ion bound by a shared electron across two helium nuclei. Ionization lets two helium atoms form a stable bond unavailable to neutral He₂.
Helium hydride ion: The helium hydride ion, HeH⁺, is a diatomic cation containing one helium atom and one hydrogen atom. Its detection in space made it a landmark example of naturally occurring helium chemistry.
Interstellar chemistry: Interstellar chemistry studies chemical reactions and molecular abundances in the gas and dust between stars. HeH⁺ provides a rare probe of chemistry in early and ionized cosmic environments.
Chemical bond: A chemical bond is an interaction that holds atoms or ions together in a molecule or solid. Helium species form through bonding mechanisms unlike ordinary valence-bond compounds.
Helium anion: A helium anion is a negatively charged helium-containing species with an electron temporarily or unusually bound. Its fragile electron attachment illustrates a less familiar route to helium-containing ions.
Helium dimer: The helium dimer, He₂, is a neutral pair of helium atoms bound by an extremely weak dispersion interaction. Its enormous size and fragile binding exemplify a neutral helium complex.
Planetary science: Planetary science investigates the formation, structure, composition, and evolution of planets and their systems. Pressure-stabilized helium compounds may affect models of giant-planet interiors.
Ionization energy: Ionization energy is the energy required to remove an electron from an atom or ion. Helium’s high first ionization energy helps explain its resistance to forming positive ions.
Charge-induced dipole: A charge-induced dipole interaction occurs when an electric charge distorts a nearby particle’s electron cloud. This attraction can stabilize complexes between helium and charged molecules.

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