The big idea: An ionic compound is built from positive cations and negative anions. The whole compound must be electrically neutral, so the total positive charge has to exactly cancel the total negative charge.
To write its formula you only need two things: the charge of each ion and the rule that the charges must balance to zero.
The words you need: - Cation — a positively charged ion (a metal, or NH4⁺). - Anion — a negatively charged ion (a non-metal, or a polyatomic ion). - Polyatomic ion — a charged group of bonded atoms that stays together (e.g. SO4²⁻). - Formula unit — the simplest whole-number ratio of ions in the compound (ionic compounds are giant lattices, so we write the ratio, not a molecule).
| Charge | Common cations (positive) | Common anions (negative) |
|---|---|---|
| 1+ / 1− | Na⁺, K⁺, Li⁺, Ag⁺, NH4⁺ (ammonium) | Cl⁻, Br⁻, I⁻, NO3⁻ (nitrate), OH⁻ (hydroxide), HCO3⁻ (hydrogencarbonate) |
| 2+ / 2− | Mg²⁺, Ca²⁺, Zn²⁺, Cu²⁺, Fe²⁺ | O²⁻, S²⁻, SO4²⁻ (sulfate), CO3²⁻ (carbonate) |
| 3+ / 3− | Al³⁺, Fe³⁺ | N³⁻ (nitride), PO4³⁻ (phosphate) |
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A binary ionic compound contains just two elements — a metal and a non-metal (e.g. NaCl, MgO). Write the cation first, then the anion, and choose the numbers so the charges cancel.
The swap-and-balance (crossover) method
- Write the cation with its charge, then the anion with its charge.
- Cross over the size of each charge to become the subscript of the other ion.
- Cancel the subscripts down to the simplest whole-number ratio.
- A subscript of 1 is not written; drop the charges from the final formula.
Worked example — aluminium oxide
Deduce the formula of the compound formed between aluminium and oxygen.
Solution
- Ions and charges: aluminium is Al³⁺ (group 13) and oxygen is O²⁻ (group 16).
- Cross over the charge sizes: the 3 from Al becomes the subscript of O, and the 2 from O becomes the subscript of Al → Al2O3.
- Check it balances: 2 × (3+) = 6+ and 3 × (2−) = 6− → total charge is zero.
Final answer
Al2O3.
Always simplify: Crossover can give numbers that share a factor. For Mg²⁺ and O²⁻ the crossover gives Mg2O2 — but this must be simplified to MgO (a 1 : 1 ratio). Always cancel down to the simplest whole-number ratio.
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Many anions (and the ammonium cation) are polyatomic — a fixed group of bonded atoms carrying one overall charge. Treat the whole group as one unit: if you need more than one of it, put it in brackets.
| Name | Formula | Charge |
|---|---|---|
| ammonium | NH4⁺ | 1+ |
| hydroxide | OH⁻ | 1− |
| nitrate | NO3⁻ | 1− |
| hydrogencarbonate | HCO3⁻ | 1− |
| carbonate | CO3²⁻ | 2− |
| sulfate | SO4²⁻ | 2− |
| phosphate | PO4³⁻ | 3− |
When to use brackets: If a formula needs two or more of a polyatomic ion, enclose the group in brackets and put the subscript outside:
- calcium nitrate → Ca²⁺ + 2 NO3⁻ → Ca(NO3)2 - one nitrate only → NaNO3 (no brackets needed).
Naming the compound: Cation name first, then the anion.
- A simple non-metal anion ends in -ide: NaCl = sodium chloride, MgO = magnesium oxide, Mg3N2 = magnesium nitride. - A polyatomic anion keeps its own name: CaSO4 = calcium sulfate, Na2CO3 = sodium carbonate.
Worked example — ammonium sulfate
Deduce the formula of ammonium sulfate.
Solution
- Ions: ammonium is NH4⁺ (1+) and sulfate is SO4²⁻ (2−).
- Balance: the 2− needs two 1+ ammonium ions to cancel it.
- Brackets: two ammonium groups → (NH4)2, then one sulfate → (NH4)2SO4. Check: 2 × (1+) = 2+ balances 2−.
Final answer
(NH4)2SO4.
How this is tested: 'Deduce the formula of …' is a reliable Paper 1A MCQ and a quick Paper 2 mark. You will be given two ions (sometimes one polyatomic) and asked for the neutral formula.
The markers want the correct ratio with the charges fully balanced — and the brackets in the right place when a polyatomic ion appears more than once.
Easy marks: Write each ion with its charge first, then crossover and simplify. The most common slip is dropping the brackets — Ca(NO3)2 is right, CaNO32 is not.
IB-style question — calcium nitride (a)
(a) Deduce the formula of calcium nitride. [1]
How to score the mark
- Ions: calcium is Ca²⁺ (group 2) and nitride is N³⁻ (group 15).
- Crossover: the 3 from N becomes the subscript of Ca, and the 2 from Ca becomes the subscript of N → Ca3N2.
- Check: 3 × (2+) = 6+ balances 2 × (3−) = 6−. Already simplest ratio.
Final answer
Ca3N2.
IB-style question — aluminium sulfate (b)
(b) Deduce the formula of the compound formed from aluminium ions and sulfate ions, SO4²⁻. [1]
How to score the mark
- Ions: aluminium is Al³⁺ (3+) and sulfate is SO4²⁻ (2−).
- Crossover: 2 Al³⁺ (total 6+) balances 3 SO4²⁻ (total 6−).
- Brackets: sulfate appears three times, so it goes in brackets → Al2(SO4)3.
Final answer
Al2(SO4)3.