Formulae for Ionic Compounds (Edexcel IGCSE Chemistry)

• The formulae of simple ionic compounds can be calculated if you know the charge on the ions
• Below are some common ions and their charges:

Common Ions & Their Charges Table

• For ionic compounds you have to balance the charge of each part by multiplying each ion until the sum of the charges = 0
• Example: what is the formula of aluminium sulfate?
  • Write out the formulae of each ion, including their charges
  • Al³⁺ SO₄^2-

• Balance the charges by multiplying them out:

  Al³⁺ x 2 = +6 and SO₄^2- x 3 = -6; so +6 – 6 = 0

• So the formula is Al₂(SO₄)₃