Module 2: Foundations in Chemistry · Year 12

2.1.2 Compounds, formulae and equations

Predict ion charges, write ionic formulae, then balance full and ionic equations with the correct state symbols.

Revise the specification Practise formulae and equations

What you need to know

Open a line for a quick recap. If it feels obvious, move straight to the linked practice.

2.1.2 a(i) Predict simple ionic charges from an element's position in the periodic table. Quick revision

When you predict a simple ion, look at the group and ask how many electrons the atom needs to lose or gain to reach a stable outer shell. Group 1 atoms lose one electron, Group 2 lose two, Group 16 gain two and Group 17 gain one.

So magnesium forms Mg²⁺, while chlorine forms Cl⁻. You are changing the electron number, not the nucleus.

Group 1 → 1+
Group 2 → 2+
Group 16 → 2−
Group 17 → 1−

Watch forFor Groups 16 and 17, do not copy the group number into the charge: oxygen is O²⁻ and chlorine is Cl⁻.

Practise periodic position and ions →

2.1.2 a(ii) Recall and use the formulae and charges of nitrate, carbonate, sulfate, hydroxide, ammonium, zinc and silver ions. Quick revision

These are worth learning as complete formula-and-charge units. If you remember only “nitrate” but not NO₃⁻, you cannot reliably build the formula of an ionic compound from it.

I’d learn the polyatomic ions exactly as written and treat each one as a single unit when you use it in a formula.

nitrate: NO₃⁻
carbonate: CO₃²⁻
sulfate: SO₄²⁻
hydroxide: OH⁻
ammonium: NH₄⁺
zinc: Zn²⁺
silver: Ag⁺

Watch forKeep nitrate as NO₃⁻ and sulfate as SO₄²⁻. If you need several of a polyatomic ion, change the number of ions, not the ion itself.

Practise formulae and products →

2.1.2 a Write formulae for ionic compounds from ion charges. Quick revision

When you write an ionic formula, your job is to make the total charge zero using the smallest whole-number ratio of ions. I usually write the ion charges first; once the positive and negative totals match, the formula follows.

If you need more than one polyatomic ion, keep that ion together in brackets. Calcium nitrate is Ca(NO₃)₂, not CaNO₃₂.

Worked example

Write the formula of aluminium oxide from Al³⁺ and O²⁻.

2 × Al³⁺ gives +6
3 × O²⁻ gives −6
the charges cancel in a 2 : 3 ratio

Answer Al₂O₃.

Watch forDo not alter an ion's internal formula to make the charges work. Change how many ions you have.

Practise writing formulae →

2.1.2 b Construct balanced chemical equations including state symbols. Quick revision

Write the correct formula for each substance first, then balance the equation by changing coefficients in front of those formulae. Never repair an atom count by changing a subscript inside a correct formula.

When the question asks for state symbols, add them to every species: (s), (l), (g) or (aq). Before you move on, count each element on both sides.

2Mg(s) + O₂(g) → 2MgO(s)

Watch forState symbols are easy marks to throw away. OCR examiner reports repeatedly flag missing or incorrect ones.

Practise balancing equations →

2.1.2 b Construct ionic equations for familiar reactions and for unfamiliar reactions when given the necessary information. Quick revision

When you write an ionic equation, strip the reaction down to the particles that actually change. Split soluble ionic substances into ions, cancel spectator ions that appear unchanged on both sides, then check both atoms and total charge.

For AgNO₃(aq) reacting with NaCl(aq), Na⁺ and NO₃⁻ are spectators, so we remove them. What is left is the formation of solid silver chloride.

Ag⁺(aq) + Cl⁻(aq) → AgCl(s)

Watch forUse the actual reacting ions. OCR has specifically seen AgNO₃, Cl₂ and incorrect state symbols where Ag⁺(aq) + Cl⁻(aq) was required.

Practise ionic equations →