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Anions are negatively charged ions. They form when atoms or groups gain one or more electrons, resulting in a net negative charge. In ionic compounds, anions are balanced by positively charged cations, and in solution they migrate as charged species. In electrochemical contexts, anions move toward the anode because they are negatively charged.

Common anions include monatomic examples such as chloride (Cl−), fluoride (F−), oxide (O2−), and nitride (N3−), as well as polyatomic anions such as hydroxide (OH−), nitrate (NO3−), nitrite (NO2−), carbonate (CO3^2−), sulfate (SO4^2−), and phosphate (PO4^3−). Acetate (C2H3O2− or CH3COO−) is another widely encountered polyatomic anion. Anions vary in size and charge; some are monovalent, others are divalent or trivalent. Many oxyanions are named with -ate or -ite endings (for example, sulfate vs sulfite; carbonate vs bicarbonate).

Naming and classification conventions help organize anions. Monatomic anions receive the -ide suffix in many cases (such as chloride, oxide). Oxoanions are often named by the balance of charge with oxygen, using prefixes like per-, hypo-, and suffixes -ate or -ite. The chemistry of anions intersects with acid-base behavior, solubility, and electrostatics, influencing the properties of salts, buffers, and biological systems.

Anions play crucial roles in chemistry, biology, environmental science, and industry, from maintaining pH balance and nutrient cycles to forming salts used in countless applications.