Oxoanions

Oxoanions are polyatomic ions that contain oxygen atoms bonded to a central atom, typically in a relatively high oxidation state. They are usually the conjugate bases of oxoacids and carry a negative charge. Their chemistry is shaped by resonance among the surrounding oxygen atoms and by the oxidation state of the central atom.

Prominent families of oxoanions include nitrate (NO3−) and nitrite (NO2−); sulfate (SO4^2−) and sulfite (SO3^2−); phosphate (PO4^3−) and its protonated forms hydrogen phosphate (HPO4^2−) and dihydrogen phosphate (H2PO4−); carbonate (CO3^2−) and hydrogen carbonate (HCO3−); chlorate (ClO3−), chlorite (ClO2−), perchlorate (ClO4−), and hypochlorite (ClO−); chromate (CrO4^2−) and dichromate (Cr2O7^2−); and molybdate (MoO4^2−). Other examples include selenate and tellurate; many of these ions have several resonance structures that distribute the negative charge over multiple oxygen atoms.

Structurally, oxoanions often feature high-symmetry arrangements around the central atom (frequently tetrahedral or related geometries) and stabilize the negative charge through resonance. They are the conjugate bases of oxoacids; protonation yields the corresponding oxoacids, and deprotonation strengthens the oxide’s basic character. The tendency to form oxoanions generally increases with the central atom’s oxidation state and the number of oxygen atoms.

Oxoanions are widespread in minerals, salts, biology, and environmental chemistry. They are central to fertilizer use (nitrates and phosphates), biological phosphate cycles, and analytical chemistry, where oxoanions serve as reagents and indicators. Redox processes can interconvert oxoanions, such as nitrate and nitrite or permanganate reductions.