Chemistry has a precise naming system so that a formula tells you exactly what a substance contains.

• Metal + non-metal gives an ionic compound: the metal is named first, the non-metal ends in -ide (NaCl, sodium chloride).
• Polyatomic ions keep their names (SO₄²⁻ sulphate, NO₃⁻ nitrate, CO₃²⁻ carbonate).
• Balance the charges so the compound is neutral: Al³⁺ and O²⁻ give Al₂O₃.
• Prefixes name covalent compounds: CO₂ is carbon dioxide, N₂O₄ dinitrogen tetroxide.

• Ion — a charged atom or group of atoms.
• Valency — the combining power of an atom.