Finding the Formulae for Ionic Compounds (Edexcel IGCSE Science (Double Award) (Modular)): Revision Note

Formulae for ionic compounds

• Ionic compounds typically have no overall charge

  • This means that the size of any positively charged ion is cancelled by the size of any negatively charged ion

  • Careful: This should not be confused with an atom having no overall charge

• The formulae of simple ionic compounds can be determined if you know the charge on the ions

Direct comparison

• The formula of an ionic compound can be determined by directly comparing the charges of the ions:

  • For example, iron(II) sulfate

    • The iron(II) ion is Fe²⁺, which means that it has a 2+ or +2 charge

    • The sulfate ion is SO₄^2–, which means that it has a 2– or –2 charge

    • The charges cancel each other out

      • Mathematically, (+2) + (–2) = 0

    • This means that one SO₄^2– ion is needed to cancel the +2 charge on Fe²⁺ 

    • Therefore, the formula of iron(II) sulfate is FeSO₄ 

The swap-and-drop method

• When the ions in the ionic compound have different charges, it can be easier to use the swap-and-drop method

  • Careful: If you use this method with ions that have the same charge, then you must give the simplest whole number ratio to get the correct answer

• For example, copper(II) chloride:

  • The copper(II) ion is Cu²⁺, which means that it has a 2+ or +2 charge

  • The chloride ion is Cl^–, which means that it has a 1– or –1 charge

  • The size of the charge on the copper(II) ion indicates the number of chloride ions needed, and the size of the charge on the chloride ion indicates the number of copper(II) ions needed

Determining the formula of copper(II) chloride

The charges swap from element to element and drop down. The positive and negative signs are removed and there is no need for the number 1.

• This gives the overall formula of copper(II) chloride as CuCl₂