Formulae of ionic compounds

An ionic compound has no overall charge, because the total positive charge of its ions equals the total negative charge. To work out a formula, write down the formula and charge of each ion, then find the smallest numbers of each ion that make the charges add up to zero.

Some ions to know: Na⁺, K⁺, Ag⁺, Mg²⁺, Ca²⁺, Zn²⁺, Cu²⁺, Al³⁺, Cl^-, Br^-, O^2-, OH^-, NO₃^-, CO₃^2- and SO₄^2-. The last four are groups of atoms that stay together, so brackets are used when more than one is needed.

Examples: Ca²⁺ and Cl^- need two chloride ions for each calcium ion, so the formula is CaCl₂. Mg²⁺ and NO₃^- need two nitrate ions, so the formula is Mg(NO₃)₂. Al³⁺ and O^2- need two aluminium ions and three oxide ions, because 2 × 3+ equals 3 × 2-. The formula is Al₂O₃. Na⁺ and SO₄^2- give Na₂SO₄.