Edexcel IGCSE Chemistry

Revision Notes

1.6.3 Formulae for Ionic Compounds

Formulae for Ionic Compounds

  • 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+ SO42-
  • Balance the charges by multiplying them out:
    Al3+ x 2 = +6 and SO42- x 3 = -6; so +6 – 6 = 0
  • So the formula is Al2(SO4)3

Exam Tip

Another method that also works is to ‘swap the numbers’. In the example above the numbers in front of the charges of the ions( 3 and 2) are swapped over and become the multipliers in the formula( 2 and 3). Easy when you know how!

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.

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