Finding the Formulae for Ionic Compounds (Edexcel IGCSE Chemistry (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 SO42–, which means that it has a 2– or –2 charge
The charges cancel each other out
Mathematically, (+2) + (–2) = 0
This means that one SO42– 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
Worked Example
The compound produced in the reaction between iron wool and chlorine contains the ions Fe3+ and Cl–.
a) Give the formula of this compound.
b) State the name of this compound.
Answers:
Part a)
Direct comparison method:
The iron ion is Fe3+, which means that it has a 3+ or +3 charge
The chloride ion is Cl–, which means that it has a 1– or –1 charge
The charges do not cancel each other out
Mathematically, (+3) + (–1) ≠ 0
Three Cl– ions are needed to cancel the +3 charge on Fe3+
Therefore, the formula is FeCl3
Swap-and-drop method
The formula is FeCl3
Part b)
The metal is iron and the chlorine will change to chloride
Therefore the name is iron chloride
Examiner Tip
If you are determining the formula of a compound containing a complex ion, such as carbonate, hydroxide or sulfate ions, don't forget to include brackets if needed.
For example, when determining the ionic formula of magnesium hydroxide:
Magnesium forms ions with a 2+ charge, and hydroxide ions have a 1+ charge.
The formula of magnesium hydroxide is therefore Mg(OH)2.
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