Anions

Anions are negatively charged ions. They form when atoms or molecules gain extra electrons (reduction) or, for polyatomic species, by loss of protons from acids to give conjugate bases, leaving an overall negative charge. The negative charge is balanced by positively charged ions (cations) in salts or solutions.

Common simple anions include fluoride (F−), chloride (Cl−), oxide (O2−), sulfide (S2−), and hydroxide (OH−). Polyatomic anions include nitrate (NO3−), nitrite (NO2−), carbonate (CO3 2−), sulfate (SO4 2−), phosphate (PO4 3−), and acetate (C2H3O2−). Oxyanions are often named with -ate or -ite endings, with per- and hypo- forms in some series.

In chemistry, anions form ionic bonds with cations to produce neutral salts. In aqueous solution they behave as electrolytes, conducting electricity and affecting pH and conductivity. The size and charge of an anion influence properties such as lattice energy of solids, solubility, and reactivity. Anions participate in acid–base chemistry as conjugate bases and, for examples like hydroxide or carbonate, can accept protons.

Anions play important roles across disciplines. In biology, chloride helps regulate fluid balance, while phosphate is central to energy transfer and nucleic acids. In environmental science, nitrates and sulfates are common indicators of pollution. Analytical techniques such as ion chromatography and electrophoresis are used to detect and quantify anions in samples.