Deducing formulae from common ions

You need to know the formulae and charges of some common ions. Positive ions: H⁺, Na⁺, K⁺, Ag⁺, Mg²⁺, Ca²⁺, Zn²⁺, Cu²⁺, Fe²⁺, Fe³⁺, Al³⁺ and ammonium NH₄⁺. Negative ions: Cl^-, O^2-, hydroxide OH^-, nitrate NO₃^-, carbonate CO₃^2- and sulfate SO₄^2-.

An ionic compound is electrically neutral overall, because the positive and negative charges balance. To deduce a formula, write the charge of each ion, then find how many of each are needed so the totals cancel. Aluminium ions are 3+ and oxide ions are 2-. The smallest total that both go into is 6, so the compound contains two aluminium ions and three oxide ions: Al₂O₃.

If more than one of a compound ion (an ion made of several atoms) is needed, its formula is put in brackets with the number after it. Calcium ions are 2+ and hydroxide ions are 1-, so two hydroxide ions are needed for each calcium ion: Ca(OH)₂. Aluminium sulfate needs two Al³⁺ (total 6+) and three SO₄^2- (total 6-), so it is Al₂(SO₄)₃. Do not use brackets when only one is needed, as in Na₂SO₄.