An ionic compound’s formula records the simplest whole-number ratio of its ions. Balance total positive and negative charge; check the charges before putting the ion symbols together.
A polyatomic ion contains more than one atom. When you need more than one of these ions, put the group in brackets before adding its subscript. A nitrate ion remains NO₃⁻; it does not become a different ion when it occurs twice.
A coefficient counts complete formula units in a reaction. A formula subscript specifies the composition of one unit. Changing a subscript to balance an equation changes the substance.
Use the given ion charges to write the formula. A formula alone does not tell you how to make a substance, whether it dissolves or what state it is in. These questions exclude aluminium carbonate and ammonium hydroxide.
Worked example
Mg²⁺ needs two NO₃⁻ ions: Mg(NO₃)₂. Total charge is +2 + 2(−1) = 0.
Al³⁺ and SO₄²⁻ combine in a 2:3 ratio: Al₂(SO₄)₃. Six positive charges balance six negative charges.
Specification references
AQA 4.2.1.2–4.2.1.3 and 4.4.2.2; Edexcel 1.26. You should also revise ionic bonding and structure.
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Explain why magnesium nitrate is Mg(NO₃)₂ rather than MgNO₃ or MgNO₆.
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Balance the charges.
Keep nitrate as a group.
Explain the brackets.
Example answer
One magnesium ion carries +2 charge and each nitrate ion carries −1. Two nitrate ions are needed, so the intact NO₃ group is repeated with brackets: Mg(NO₃)₂. The result contains one magnesium, two nitrogen and six oxygen atoms.
An answer to improve
Cross the numbers over because that is the formula rule.
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