ânions

Anions are negatively charged ions that form when atoms gain electrons or when a molecule loses a proton to yield its conjugate base. They contrast with cations, which carry positive charges. In aqueous solutions, most anions are highly mobile and exist with hydration shells that affect their behavior in solution, transport, and reactivity.

Common inorganic anions include chloride (Cl−), hydroxide (OH−), nitrate (NO3−), sulfate (SO4^2−), carbonate (CO3^2−), and phosphate (PO4^3−). Many anions are polyatomic, bearing multiple negative charges, such as carbonate, sulfate, phosphate, chromate (CrO4^2−), dichromate (Cr2O7^2−), and permanganate (MnO4−). Anions form salts by pairing with positively charged cations (for example, NaCl, CaSO4).

Formation and roles: Anions arise from the deprotonation of acids (yielding conjugate bases) or by the gain of electrons by atoms. In biological systems, certain anions are essential: chloride supports fluid balance and nerve function; bicarbonate and phosphate buffers maintain pH and participate in metabolism and bone physiology.

Environmental and industrial relevance: Anions such as nitrate and phosphate are important nutrients but can cause environmental problems when present at high concentrations, including eutrophication of water bodies and health concerns in drinking water (notably nitrate contamination). Anions are analyzed and removed using techniques such as ion chromatography, ion-exchange, reverse osmosis, and electrodialysis, depending on context.

Terminology: In English, the term is anion (plural anions). In some languages, including Portuguese, the plural may be written as ânions. Anions play a central role across chemistry, biology, environmental science, and industry.