Deducing formulae from ions

An ionic compound has no overall charge. The total positive charge of its ions must equal the total negative charge. You can use this to deduce its formula from the charges on the ions.

Metal ions are positive. Group 1 ions have a charge of 1+, such as Na⁺. Group 2 ions have a charge of 2+, such as Mg²⁺. Aluminium forms Al³⁺, zinc forms Zn²⁺ and silver forms Ag⁺. Non-metal ions are negative. Group 7 ions have a charge of 1-, such as Cl^-. Group 6 ions have a charge of 2-, such as O^2-. Common compound ions are hydroxide OH^-, nitrate NO₃^-, carbonate CO₃^2-, sulfate SO₄^2- and ammonium NH₄⁺.

Worked example: magnesium chloride. Mg²⁺ has a charge of 2+ and Cl^- has a charge of 1-, so each magnesium ion needs two chloride ions. The formula is MgCl₂. For aluminium oxide, Al³⁺ and O^2- need 2 aluminium ions (6+) and 3 oxide ions (6-), so the formula is Al₂O₃.

If more than one compound ion is needed, put it in brackets so the small number applies to the whole ion. Calcium hydroxide is Ca(OH)₂, because Ca²⁺ needs two OH^- ions.