Definition

A anion is an atom or a cluster of atoms that has gained one or more electrons. With more electrons than protons, the species carries a negative electric charge. Such a charged particle is a type of ion.

Common examples

Typical anions include chloride (Cl−), bromide (Br−) and iodide (I−). These three are monovalent anions: each can pair with a single hydrogen atom, a concept described by valency and the combining capacity with hydrogen.

Relation to other ions and solids

An ion is any charged atom or group of atoms. Anions are the negatively charged half of ionic species; the positively charged counterparts are called cations. In many ionic solids the anions are the larger constituents and the smaller cations occupy the spaces between them in the crystal lattice (crystals).

Movement and acid–base role

Under an applied voltage, anions migrate toward the anode (the electrode where oxidation takes place). Because anions commonly accept a proton (H+), they are often classified as a base in acid–base descriptions.