Chemical Formulae

A chemical formula shows the symbols of the elements in a substance and the number of atoms or ions of each. The numbers are written as subscripts after the symbol they refer to. For example, H₂O contains two hydrogen atoms and one oxygen atom.For an ionic compound the formula gives the ratio of ions. The compound is neutral overall, so the total positive charge must equal the total negative charge.The charges on simple ions can be worked out from the Periodic Table: Group 1 metals form 1+ ions, Group 2 metals form 2+ ions, aluminium forms Al³⁺, Group 6 elements form 2- ions and Group 7 elements form 1- ions. For transition metals the charge is shown by a Roman numeral, for example iron(III) is Fe³⁺.Some ions contain more than one atom and must be learnt: hydroxide OH^-, nitrate NO₃^-, hydrogencarbonate HCO₃^-, carbonate CO₃^2-, sulfate SO₄^2- and ammonium NH₄⁺. If more than one of these ions is needed, it is written in brackets.Worked example: aluminium sulfate contains Al³⁺ and SO₄^2- ions. The lowest common multiple of 3 and 2 is 6, so two Al³⁺ ions (6+) balance three SO₄^2- ions (6-).Al₂(SO₄)₃In the same way, calcium hydroxide is Ca(OH)₂, iron(III) oxide is Fe₂O₃ and ammonium carbonate is (NH₄)₂CO₃.