Oxyanions

Oxyanions are negatively charged ions that contain oxygen bonded to another element, typically a nonmetal. They are the conjugate bases of oxoacids and occur in many inorganic compounds and aqueous solutions. In most cases, the central element is surrounded by several oxygen atoms in a geometry that is stabilized by resonance.

Naming follows a common pattern: the suffixes -ate or -ite indicate higher or lower numbers of oxygen atoms compared to a reference, while per- and hypo- prefixes indicate even more or fewer oxygens. The -ate form is usually the more oxidized and oxygen-rich member, the -ite form the related, slightly less oxygenated member, and hypo- or per- mark extremes such as hypochlorite ClO− and perchlorate ClO4−.

Common oxyanions include nitrate (NO3−), nitrite (NO2−), sulfate (SO4^2−), sulfite (SO3^2−), phosphate (PO4^3−), hydrogen phosphate (HPO4^2−), dihydrogen phosphate (H2PO4−), carbonate (CO3^2−), hydrogen carbonate (HCO3−), chlorate (ClO3−), chlorite (ClO2−), hypochlorite (ClO−), perchlorate (ClO4−), chromate (CrO4^2−), dichromate (Cr2O7^2−), and permanganate (MnO4−).

Oxyanions arise from oxidation of elements by oxygen and form salts with metal cations. They participate in acid-base equilibria: oxoanions are the conjugate bases of oxoacids such as nitric acid (HNO3), sulfuric acid (H2SO4), and phosphoric acid (H3PO4). They are important in environmental chemistry (nitrates, phosphates, sulfates) and biology (phosphate’s role in nucleic acids and energy transfer).