Resource 4: Chemical formulas

You could use examples such as:

  • Sodium oxide Na2O
  • Calcium chloride CaCl2
  • Aluminium oxide Al2O3
  • Iron (III) chloride FeCl3
  • Magnesium hydroxide Mg(OH)2
  • Ammonium sulphate (NH4)2SO4
  • Calcium phosphate Ca3(PO4)2
  • Magnesium nitrate Mg(NO3)2

Common mistakes that students make include:

  • putting the number at the top: Na2O
  • using big numbers: Na2O
  • missing out a bracket: CaOH2.

It is also common for students to get the valancy wrong. They will need to learn the valencies of some atoms and groups. However, if you link the valancy to the position in the Periodic Table, then it will help them to understand why different atoms have different valencies and make it easier for students to remember the value. The rules are:

  • Group 1 metals: +1
  • Group 2 metals: +2
  • Group 3 metals: +3
  • Group 5 non-metals: –3
  • Group 6 non-metals: –2
  • Group 7 non-metals: –1

Resource 3: Elements, compounds and mixtures

Additional resources