Under pressure, helium forms stable molecules

Abstract

Chemistry textbook authors may soon have to rewrite sections covering noble gases and chemical inertness. An international research team has reported the synthesis of a helium-sodium compound that’s stable at high pressures (Nat. Chem. 2017, DOI: 10.1038/nchem.2716). Helium’s best known feature is its unwillingness to react. With a stable closed-shell electron configuration, zero electron affinity, and an ionization energy that is higher than that of all other elements, helium defines chemical inertness. To examine the scope of the element’s low reactivity, scientists have searched for years—theoretically and experimentally—for helium-containing molecules. But they have turned up very little: for example, unusual species such as the HeH+ radical, which is stable only in its charged form, and HHeF, a metastable compound. In contrast, helium’s noble gas cousins xenon and krypton long ago showed themselves capable of forming a variety of stable compounds. Still, a team of researchers that includes Artem R. O...