A branch of chemistry that deals with the reactivity of the superoxide ion (O₂⁻), a one-electron (e⁻) adduct of molecular oxygen (dioxygen; O₂) formed by the combination of O₂ and e⁻. Because 1–15% of the O₂ that is respired by mammals goes through the O₂⁻ oxidation state, the biochemistry and reaction chemistry of the species are important to those concerned with oxygen toxicity, carcinogenesis, and aging. Although the name superoxide has prompted many to assume an exceptional degree of reactivity for O₂⁻, the use of the prefix in fact was chosen to indicate stoichiometry. Superoxide was the name given in 1934 to the newly synthesized potassium salt (KO₂) to differentiate its two-oxygens-per-metal stoichiometry from that of most other metal–oxygen compounds (Na₂O, Na₂O₂, NaOH, Fe₂O₃).