A superoxide, also known by the obsolete name hyperoxide, is a compound that contains the superoxide anion with the chemical formula . The systematic name of the anion is dioxide(1-). Superoxide anion is particularly important as the product of the one-electron reduction of dioxygen O₂, which occurs widely in nature. Whereas molecular oxygen (dioxygen) is a diradical containing two unpaired electrons, the addition of a second electron fills one of its two degenerate molecular orbitals, leaving a charged ionic species with single unpaired electron and a net negative charge of -1. Both dioxygen and superoxide ion are free radicals that exhibit paramagnetism.