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Chemistry 06203.1

Formulae

Writing formulae of ionic and covalent compounds and balancing chemical equations.

Learning objectives

What you need to be able to do

Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.

  • 3.1.1Write and balance chemical equations, including state symbols.
  • 3.1.2Deduce the formula of an ionic compound from the charges on its ions.

7 minute read

Formulae and balancing equations

Deducing ionic formulae

An ionic compound is always electrically neutral overall, so the total positive charge must equal the total negative charge. To find the formula:

  1. Write the charge on each ion (e.g. Mg²⁺, Cl⁻).
  2. Find how many of each ion are needed so the charges balance to zero.
  3. Write the formula with these numbers as subscripts (1 is never written).

For magnesium chloride: Mg²⁺ needs two Cl⁻ to balance its +2 charge, giving MgCl₂.

Balancing equations

A chemical equation must have the same number of atoms of each element on both sides — atoms are neither created nor destroyed in a reaction. To balance an equation:

  1. Write the correct formulae of all reactants and products (never change a formula to balance an equation — only add numbers in front of it).
  2. Count the atoms of each element on both sides.
  3. Add whole-number multipliers in front of formulae until every element balances.

State symbols

Every substance in a balanced equation can be labelled with a state symbol: (s) solid, (l) liquid, (g) gas, (aq) aqueous (dissolved in water). These are often required for full marks, especially in questions about reactions in solution.

Think of it like this

Balancing an equation is like balancing accounts — every atom that goes in as a "reactant" has to be accounted for as a "product"; you are never allowed to invent or lose atoms, only to change how many complete molecules of each substance you have.

Worked examples

Method, step by step

Deduce the formula of aluminium sulfate, formed from Al³⁺ and SO₄²⁻ ions.

  1. 1Find the lowest common multiple of the charges: LCM(3, 2) = 6
  2. 2Need 2 × Al³⁺ (total +6) and 3 × SO₄²⁻ (total −6)

Al₂(SO₄)₃

Balance the equation: __Fe + __O₂ → __Fe₂O₃

  1. 1Balance oxygen: 3 O₂ gives 6 O atoms, matching 2 Fe₂O₃ (which has 6 O atoms)
  2. 2Balance iron: 2 Fe₂O₃ needs 4 Fe atoms

4Fe + 3O₂ → 2Fe₂O₃

Common misconceptions

  • Changing the subscripts inside a formula to try to balance an equation (e.g. writing H₂O₂ instead of H₂O). This changes what the substance actually is — only the large numbers placed in front of a formula may be changed.
  • Forgetting to balance charge as well as atoms when deducing an ionic formula — for a compound like aluminium oxide, both the number of atoms and the total charge must work out.
  • Leaving out state symbols when a question specifically asks for them, or when comparing a solid reactant with a dissolved product.

In the exam

  • Balance one element at a time, saving hydrogen and oxygen until last if they appear in several formulae — this avoids repeatedly having to re-balance elements you thought were already correct.
  • After balancing, always recount every element on both sides as a final check before moving on.