Mini Chemistry
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Formulae and Balanced Equations

Count atoms, construct formulae and conserve each element without changing the substances.

  • SEC G2 Science Chemistry component 2027
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Learning objectives

  • state the symbols of the elements and formulae of the compounds mentioned in the syllabus
  • deduce the formulae of simple compounds from the relative numbers of atoms present and vice versa
  • deduce the formulae of ionic compounds from the charges on the ions present and vice versa
  • interpret chemical equations with state symbols
  • construct chemical equations, with state symbols, including ionic equations.

A formula identifies the substance. A coefficient counts whole particles or formula units. Keeping that distinction lets you balance an equation without inventing a different product.

Read and construct a formula

Element symbols and formulae are case-sensitive: chlorine is Cl, not CL, while carbon monoxide is CO. A formula records relative atom numbers. Two nitrogen atoms for every five oxygen atoms gives N₂O₅; conversely, CO₂ represents one carbon atom for every two oxygen atoms.

Learn the exact symbols that recur across G2 Chemistry. A one-letter symbol is capitalised; only the first letter is capitalised in a two-letter symbol.

ElementsSymbols
hydrogen, carbon, nitrogen, oxygen, sulfurH, C, N, O, S
chlorine, bromine, iodine, argonCl, Br, I, Ar
lithium, sodium, magnesium, aluminium, potassium, calciumLi, Na, Mg, Al, K, Ca
zinc, iron, lead, copper, silverZn, Fe, Pb, Cu, Ag

Some elements that recur in this course exist as diatomic molecules in equations: H₂, N₂, O₂, Cl₂, Br₂ and I₂. Do not add a subscript 2 when the question asks for the element symbol rather than its molecular formula.

The total positive and negative charge must be zero. Aluminium ions are Al³⁺ and oxide ions are O²⁻, so the smallest neutral ratio is:

Al₂O₃

For a bracketed formula such as Al₂(SO₄)₃, the outside 3 multiplies both S and O inside the bracket. It does not multiply Al: there are 2 Al, 3 S and 12 O atoms per formula unit.

Modelled example 1

Write an Ionic Formula

Core

Problem

Write the formula of calcium chloride from Ca²⁺ and Cl⁻.
Study the worked solution

  1. Balance charge

    Method

    Pair one calcium ion with two chloride ions.

    Reason

    The total charge must be zero: + 2 + 2(-1) = 0.

    Working

    Ca²⁺: Cl⁻ = 1:2

  2. Write the smallest formula

    Method

    Use the ion ratio as subscripts.

    Reason

    A formula shows the smallest whole-number ratio of ions.

    Working

    CaCl₂

Balance whole formula units

Start with correct reactant and product formulae. Then change coefficients:

2Mg(s) + O₂(g) → 2MgO(s)

Changing MgO to MgO₂ would change the substance and is not balancing.

State symbols show physical state: (s) solid, (l) liquid, (g) gas and (aq) dissolved in water. In the equation above, magnesium and magnesium oxide are solids while oxygen is a gas. Coefficients give the relative numbers of particles or formula units: two Mg react with one O₂ to form two MgO. G2 questions do not require you to turn this ratio into reacting masses or gas volumes.

Common misconception 2

Balance with Coefficients

Find and correct the mistake

Learner attempt

A learner writes Mg + O₂ → MgO₂ to balance magnesium burning. Locate the error and correct the equation.

Diagnose first

Invalid change

View solution step by step

  1. Keep correct formulae

    Method

    Restore magnesium oxide as MgO.

    Reason

    Balancing must not change the identities of substances.

    Working

    Mg + O₂ → MgO

  2. Change coefficients

    Method

    Place 2 before Mg and MgO.

    Reason

    This gives two Mg atoms and two O atoms on each side.

    Working

    2Mg + O₂ → 2MgO

Apply the distinction

Write the smallest whole-number coefficients in Al + O₂ → Al₂O₃. Then count Al and O on each side. Explain why changing the product to AlO₂ is not an allowed balancing step.

Check the balance

4Al + 3O₂ → 2Al₂O₃: each side has four Al atoms and six O atoms. Subscripts fix a compound’s atom ratio; changing them changes the substance. A coefficient multiplies the entire formula, so 2Al₂O₃ contains four Al and six O atoms.

Existing ionic-equation work

An ionic equation shows only particles that change. For aqueous acid–alkali neutralisation, begin with the ions, remove spectator ions that appear unchanged on both sides, and check both atoms and total charge:

H + (aq) + OH-(aq) → H₂O(l)

For precipitation, the same idea applies. Mixing aqueous silver nitrate and sodium chloride forms solid silver chloride; sodium and nitrate ions remain in solution:

Ag + (aq) + Cl-(aq) → AgCl(s)

Practise formula construction and equation repair. Return here if you changed a subscript or missed a bracket multiplier.