Chemical formulae and valency
Atoms and Molecules (School Level) · Chemistry
Study notes
Write the formula of aluminium sulphate. Step 1: Aluminium ion is Al³⁺ (valency 3); sulphate ion is SO₄²⁻ (valency 2). Step 2: Criss-cross the valencies: Al takes 2, SO₄ takes 3. Step 3: Formula: Al₂(SO₄)₃. Step 4: Check charges: 2×(+3) + 3×(−2) = +6 − 6 = 0. Balanced!