Atom or group of atoms with one or more positive or negative electric charges. Positively charged ions are cations, negatively charged ones anions. Ions are formed when electrons are added to or removed from neutral molecules or other ions, as sodium (Na) and chlorine (Cl) atoms react to form Na+ and Cl; when ions combine with other particles, as hydrogen cations (H+) and ammonia (NH) combine to form ammonium cations (NH+); or when a covalent bond between two atoms is ruptured in such a way that the resulting particles are charged, as water (HO) dissociates (see dissociation) into hydrogen and hydroxide ions (H+ and OH). Many crystalline substances (see crystal) are composed of ions held in regular geometric patterns by the attraction of oppositely charged particles for each other. Ions migrate to the electrode of opposite charge in an electric field and are the conductors of current in electrolytic cells (see electrolysis). Compounds that form ions are called electrolytes. Ions are also formed in gases when heated to very high temperatures or when an electrical discharge passes through them (see plasma).

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