Ion

Summary: A charged particle formed when an atom (or group of atoms) gains or loses electrons. Tags: igcse chemistry definition atomic-structure bonding Created: 2026-07-14 Last Updated: 2026-07-14

An ion is a charged particle formed when an atom or group of atoms gains or loses one or more electrons, resulting in an unequal number of protons and electrons. Positively charged ions, called cations (e.g., Na⁺, Ca²⁺, NH₄⁺), are produced when atoms lose electrons — metals typically form cations because they have few outer-shell electrons that are relatively easy to remove. Negatively charged ions, called anions (e.g., Cl⁻, O²⁻, SO₄²⁻), are produced when atoms gain electrons — non-metals typically form anions to achieve a full outer shell. Polyatomic ions such as sulfate (SO₄²⁻), nitrate (NO₃⁻), and carbonate (CO₃²⁻) consist of multiple atoms covalently bonded together that carry an overall charge, and IGCSE students must memorise their names, formulae, and charges. Ionic bonding arises from the electrostatic attraction between oppositely charged ions arranged in a giant ionic lattice, and the charges on ions determine the empirical formula of an ionic compound — for example, Mg²⁺ combines with Cl⁻ in a 1:2 ratio to give MgCl₂.

Key term — IGCSE Chemistry (0620/0971) glossary.