Ion

An atom (or group of atoms) which has either gained or lost electrons; e.g., a chloride anion, ₁₇Cl^1-, contains 18 electrons (i.e., one more than neutral atom, ₁₇Cl⁰), and a copper(II) cation, ₂₉Cu²⁺, contains 27 electrons (i.e., two less than the neutral atom, ₂₉Cu⁰).

See also: Coulombic Force, Ionic Strength, Ionization, Monatomic Ion, Plasma, Polyatomic Ion, Recombination.