Pour vinegar on a spoon of baking soda and it fizzes. The liquid left behind contains sodium ethanoate. Its formula is CH₃COONa: one sodium ion, Na⁺, for every ethanoate ion, CH₃COO⁻, so the charges cancel. Every ionic formula you write follows that one rule.
Every element has a of one or two letters. The first letter is a capital; the second is small. Co is cobalt, but CO is carbon monoxide, a compound.
Seven elements exist as : H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂. Chlorine gas is Cl₂, not Cl.
An is an atom that has lost or gained electrons. Metals lose electrons and form positive ions; non-metals gain electrons and form negative ions.
The group gives the charge: I → +1, II → +2, III → +3, V → −3, VI → −2, VII → −1. For transition metals the Roman numeral gives it: iron(III) is Fe³⁺.
Some ions are a group of atoms that carries one charge: .
Learn them as whole units: OH⁻ hydroxide, NO₃⁻ nitrate, HCO₃⁻ hydrogencarbonate, CH₃COO⁻ ethanoate, NH₄⁺ ammonium, CO₃²⁻ carbonate, SO₄²⁻ sulphate, PO₄³⁻ phosphate.
Never change the small numbers inside them.
A compound has no overall charge. In its , take enough of each ion so that the charges add up to zero.
Al³⁺ and O²⁻: 2 × (+3) = +6 and 3 × (−2) = −6, so the formula is Al₂O₃.
Two or more of a polyatomic ion go in : Ca(OH)₂, (NH₄)₂SO₄.
In a model of a molecule, count each kind of atom: one nitrogen joined to three hydrogens is NH₃.
In an ionic diagram, find the simplest ratio of the ions: one Mg²⁺ for every two Cl⁻ gives MgCl₂.
Pick a positive ion and a negative ion. Add more of either until the charge total reads zero, then read off the formula — watch where the brackets appear for (NH₄)₂SO₄, Ca(OH)₂ and Al₂(SO₄)₃. Then try the name ↔ formula drill: only your first try counts, so check the charges before you tap.
Examiner report: 2023 Paper 3, Q9(d)(i)
Examiner report: Chemistry (0570) 2023 Paper 1, Q2
Examiner report: General Science (1430) 2025 Paper 1, Q8(c)
Examiner report: Chemistry (0570) 2023 Paper 3, Q6(a)(i)
Common error (teacher experience)
1What is the formula of potassium sulphate?
2What is the formula of calcium nitrate?
3How many atoms are in one formula unit of ammonium sulphate, (NH₄)₂SO₄?
4What is the formula of iron(III) oxide?
5Element Q is in Group II. What is the formula of its chloride?
Table 8.1 shows some ions.
| positive ions | negative ions |
|---|---|
| Na⁺ Ca²⁺ | Cl⁻ OH⁻ |
| Al³⁺ NH₄⁺ | SO₄²⁻ NO₃⁻ |
| Mark point | Mark |
|---|---|
| (a)AlCl₃ | B1 |
| (b)Ca(OH)₂ (brackets needed) | B1 |
| (c)(NH₄)₂SO₄ | B1 |
| (d)ratio 2 Al : 3 SO₄ | B1 |
| brackets correct: Al₂(SO₄)₃ | B1 |
| (e)+3 / Fe³⁺ | B1 |
| iron(III) oxide | B1 |
| (f)C₂H₄O₂ (accept CH₃COOH) | B1 |
| (g)(CH₃COO)₂Ca or Ca(CH₃COO)₂ | B1 |
Total: 9 marks
| positive | negative |
|---|---|
| Na⁺, K⁺, Ag⁺, NH₄⁺ | Cl⁻, Br⁻, I⁻, OH⁻, NO₃⁻, HCO₃⁻, CH₃COO⁻ |
| Mg²⁺, Ca²⁺, Cu²⁺, Zn²⁺, Fe²⁺ | O²⁻, S²⁻, CO₃²⁻, SO₄²⁻ |
| Al³⁺, Fe³⁺ | N³⁻, PO₄³⁻ |
Many learners struggle with formulae because they cannot see why Al₂(SO₄)₃ needs those numbers. Card tiles cut to size by charge — a +1 tile is one block long, a −2 tile two blocks — make it visible: the formula is right when the two rows are the same length.
Design the tiles on a phone or computer, print them at a copy shop, laminate and cut them, and sell kits to tutors, study groups and school science departments.
| Printing 10 A4 sheets in colour | P40 |
| Laminating 10 sheets | P50 |
| 10 zip bags and labels | P20 |
| Costs for 10 kits | P110 |
| 10 kits sold at P35 | P350 |
| Profit | P240 |
Check every tile against the Periodic Table and the ion table before printing — one wrong charge teaches a wrong formula to every learner who uses the kit. Use your own design; do not copy pictures from textbooks or websites without permission.
Example figures only — check printing prices at your local copy shop before you quote.
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