Ionic formula — balance charges to zero
The formula of an ionic compound is the simplest ratio of cations to anions whose charges cancel to zero. Memorise each ion as a symbol-and-charge unit: Na⁺, K⁺, NH₄⁺, Ag⁺ (+1); Ca²⁺, Mg²⁺, Cu²⁺, Fe²⁺, Zn²⁺ (+2); Al³⁺, Fe³⁺ (+3); Cl⁻, OH⁻, NO₃⁻ (−1); O²⁻, S²⁻, SO₄²⁻, CO₃²⁻ (−2). Combine them for neutrality: calcium chloride is one Ca²⁺ with two Cl⁻, giving CaCl₂; aluminium oxide balances two Al³⁺ against three O²⁻, giving Al₂O₃.
Balanced symbol equations and state symbols
A word equation names reactants and products but gives no quantities: magnesium + oxygen → magnesium oxide. A balanced symbol equation uses formulae and adjusts only the large coefficients so each element has equal atoms on both sides — formulae are never changed: 2Mg + O₂ → 2MgO. State symbols follow each formula: (s), (l), (g), (aq). Balancing follows conservation of mass.
(Extended) Ionic equations show only what changes
An ionic equation shows only the ions that change. To write one: split every aqueous ionic substance (soluble salts, strong acids, alkalis) into separate ions; keep any solid, liquid or gas as a full formula; then cancel the spectator ions — those identical on both sides. Check that atoms and total charge balance. Example: for BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq), Na⁺ and Cl⁻ are spectators, leaving Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s).
Drawn from real examiner reports.
Ion charge omitted or wrong
Examiners repeatedly see the hydroxide ion written as OH with no charge, or given a 2− charge like oxide; it is OH⁻. The sulfate ion SO₄²⁻ is confused with sulfite SO₃²⁻ (three oxygens), and nitrate NO₃⁻ is sometimes given a 2− charge. An ion-formula error flows into the compound formula and costs later marks. Learn each ion as formula-plus-charge.
S23 Paper 32 Q4(e)(ii)
Balancing by changing subscripts
To balance, change only the large coefficients in front of a formula — never a subscript. Turning H₂O into H₂O₂ to "balance" oxygen makes hydrogen peroxide, a different substance. Routine: correct formulae, count each element, adjust coefficients (most complex first), recount, add state symbols. Check every element, not just one.
(Extended) Full equation when an ionic one is asked
When a question says "write an ionic equation", a full formula equation scores zero for that mark. Split the aqueous species into ions, keep solids, liquids and gases whole, and cancel spectator ions — a mark is lost for each left in. Identifying the insoluble product (the precipitate) is the usual sticking point. Check it balances for atoms and charge.
W22 Paper 41 Q2(e)(iii); W22 Paper 42 Q2(d)(iv); S23 Paper 41 Q6(d)(ii)
Missing brackets on polyatomic ions
When a compound needs more than one polyatomic ion, put the ion in brackets before the subscript. Calcium hydroxide is Ca(OH)₂, not CaOH₂ — the brackets show two whole OH⁻ groups (one Ca, two O, two H), whereas CaOH₂ reads as one O, two H. The same applies to (NH₄)₂SO₄ and Al₂(SO₄)₃. Bracket a polyatomic ion whenever its subscript exceeds one.
Not simplifying to the lowest ratio
An ionic formula must be the simplest whole-number ratio. Magnesium oxide comes out as Mg₂O₂ from cross-multiplying the equal +2 and −2 charges, but must simplify to MgO by dividing both subscripts by two. Leaving Mg₂O₂ (or Ca₂O₂) gains only partial credit. After cross-multiplying, check whether the subscripts share a common factor and divide through.
Giving a formula when a name is asked
Read whether the question wants a name, a word equation or a symbol equation, and answer in that form. Examiners often see a formula where a name is required (or the reverse), or a symbol equation where a word equation is asked. A word equation uses names only; a symbol equation uses formulae and can be balanced. The wrong form scores nothing.
Swap-and-drop for ionic formulae
Cross-multiply the charges: write both ions with their charges, swap each charge number (ignore the sign) to become the other ion's subscript, simplify to the lowest ratio, and bracket a polyatomic ion whose subscript exceeds one. Example: Al³⁺ and SO₄²⁻ give Al₂(SO₄)₃.
A reliable balancing routine
Balance in a fixed order: write correct formulae; count each element; adjust the large coefficients (most complex first), never subscripts; recount until every element balances; add state symbols last. Check all elements, not just the first.
(Extended) Turning a full equation into ionic
Split each aqueous ionic substance into its ions, but keep any solid, liquid or gas as a full formula. Delete the spectator ions unchanged on both sides, then check atoms and charge balance. Example: AgNO₃(aq) + NaCl(aq) reduces to Ag⁺(aq) + Cl⁻(aq) → AgCl(s).
Define chemical formula.
State the formula of magnesium oxide.
Ion charges: (charge +2) and (charge −2).
