EDEXCEL IGCSE CHEMISTRY: DOUBLE SCIENCE 復習筆記： 1.6.3 Formulae for Ionic Compounds

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EDEXCEL IGCSE CHEMISTRY: DOUBLE SCIENCE 復習筆記： 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⁺?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

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!

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