- 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
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!