Ions and Ionic Formulae
A concise revision guide to predicting the charge on a simple ion from its group, the formulae of the sulfate, hydroxide, nitrate, carbonate and ammonium ions, and constructing the formula of any ionic compound by balancing charges.
Predicting the Charge on a Simple Ion
A metal atom in Groups 1, 2 or 3 loses its outer electrons to reach the electron configuration of the previous noble gas; a non-metal atom in Groups 5, 6 or 7 gains electrons to reach the next. The number of electrons lost or gained is the charge on the ion, and it can be read straight from the group number.
| Group | Electrons lost or gained | Ion charge | Examples |
|---|---|---|---|
| 1 | Lose 1 | 1+ | Li+, Na+, K+ |
| 2 | Lose 2 | 2+ | Mg2+, Ca2+, Ba2+ |
| 3 | Lose 3 | 3+ | Al3+ |
| 5 | Gain 3 | 3- | N3-, P3- |
| 6 | Gain 2 | 2- | O2-, S2- |
| 7 | Gain 1 | 1- | F–, Cl–, Br–, I– |
| Transition metals | Variable | Given in the name | Fe2+ in iron(II), Fe3+ in iron(III) |
Key idea: Metals form positive ions with the charge equal to the group number; non-metals form negative ions with the charge equal to 8 minus the group number.
The Compound Ions You Must Know
Some ions are groups of atoms covalently bonded together that carry an overall charge. AQA names five that you must be able to write from memory.
| Ion | Formula | Charge | Typical compound |
|---|---|---|---|
| Sulfate | SO42- | 2- | MgSO4 |
| Hydroxide | OH– | 1- | Ca(OH)2 |
| Nitrate | NO3– | 1- | KNO3 |
| Carbonate | CO32- | 2- | Na2CO3 |
| Ammonium | NH4+ | 1+ | NH4Cl |
The five compound ions AQA expects you to recall.
Exam focus: Ammonium is the only common positive compound ion. Sulfate and carbonate are 2-; hydroxide and nitrate are 1-.
Constructing the Formula of an Ionic Compound
An ionic compound is neutral overall, so the positive and negative charges must cancel. Use the charge-balance method: write the ions, find the lowest number of each that makes the total charge zero, and write the formula with those numbers as subscripts. Brackets go round a compound ion when more than one is needed.
| Compound | Ions | Balance the charges | Formula |
|---|---|---|---|
| Magnesium chloride | Mg2+, Cl– | One 2+ needs two 1- | MgCl2 |
| Aluminium oxide | Al3+, O2- | Two 3+ (6+) balance three 2- (6-) | Al2O3 |
| Calcium nitrate | Ca2+, NO3– | One 2+ needs two nitrates; bracket the compound ion | Ca(NO3)2 |
| Ammonium sulfate | NH4+, SO42- | Two 1+ balance one 2- | (NH4)2SO4 |
| Iron(III) hydroxide | Fe3+, OH– | One 3+ needs three 1- | Fe(OH)3 |
A quick check: the total positive charge equals the total negative charge, and the subscripts are the smallest whole numbers that achieve that. The “swap the charges” shortcut gives the same answer but simplify afterwards: Mg2+ with O2- is MgO, not Mg2O2.
Exam sentence: The formula of an ionic compound shows the simplest whole-number ratio of ions that gives no overall charge.
Common Exam Mistakes
- Writing NaCl2 or MgCl. Check that the charges cancel: Na+ needs one Cl–, Mg2+ needs two.
- Leaving out brackets: CaNO32 instead of Ca(NO3)2.
- Giving sulfate as SO4– or carbonate as CO3–. Both carry a 2- charge.
- Writing a molecular formula for a giant lattice. NaCl is a ratio of ions, not a molecule of one sodium and one chlorine.
- Forgetting the Roman numeral: iron chloride is ambiguous; FeCl2 is iron(II) chloride and FeCl3 is iron(III) chloride.
Exam sentence: Charges cancel, smallest ratio, brackets round repeated compound ions.
Where This Sits in the AQA Specification
AQA 3.1.3.1 asks you to know the formulae of the sulfate, hydroxide, nitrate, carbonate and ammonium ions, to predict the charge on a simple ion from the position of the element in the Periodic Table, and to construct formulae for ionic compounds. These skills are used throughout the course, from writing equations in 3.1.2 to naming salts in 3.2.
AQA focus: Every formula question is a charge-balance question: identify the ions, balance the charges, simplify.
Check Your Understanding
Use these short activities to practise predicting ion charges and constructing formulae for compounds that are not used as examples above.
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Some ionic radii are shown.
| Ion | Ionic radius / nm |
|---|---|
| Na+ | 0.102 |
| K+ | 0.138 |
| F− | 0.133 |
| Cl− | 0.180 |
Which compound has the strongest ionic bonding?
Explain why the metallic bonding in magnesium is much stronger than that in sodium.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
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