electric charge s.
Positively charged ions are cation s, negatively charged ones anion s. Ions are formed when electron s are added to or removed from neutral molecule s 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 3 ) combine to form ammonium cations (NH 4 + ); or when a covalent bond between two atoms is ruptured in such a way that the resulting particles are charged, as water (H 2 O) 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 electrolyte s. Ions are also formed in gases when heated to very high temperatures or when an electrical discharge passes through them (see plasma ).