Who this is for: Cambridge IGCSE Chemistry (0620) students who want chemical formulae — writing formulas from valency, deducing composition and balancing simple equations — to become reliable marks instead of guesswork with subscripts.
What query it owns: how to understand and revise formulae in Cambridge IGCSE Chemistry.
Why this is safe: this page owns the formulae revision-guide angle, while Tutopiya’s Formulae subtopic page owns the learning resource and the free Formulae quiz owns the practice.

Chemical formulae tell you which elements are in a compound and how many atoms of each are present. In Cambridge IGCSE Chemistry (0620), examiners test your ability to write formulas from valencies, deduce formulae from diagrams, and interpret formulae in word and symbol equations. This guide covers the syllabus rules, the valency shortcuts that save time, and the question types that appear every year.

Key takeaways

• A chemical formula shows the symbols and numbers of atoms in one unit of a substance.
• Use valency (or ion charges) to cross-multiply when combining elements into compounds.
• Brackets multiply everything inside — e.g. Ca(OH)₂ has two O and two H per formula unit.
• State symbols (s), (l), (g), (aq) belong in equations, not in bare formulae unless asked.
• Common ions (NH₄⁺, OH⁻, SO₄²⁻, CO₃²⁻, NO₃⁻) must be recognised instantly.

What are formulae in Cambridge IGCSE Chemistry?

A chemical formula uses element symbols and subscript numbers to show the ratio of atoms in a compound. For ionic compounds, the formula represents the simplest whole-number ratio of ions in the lattice. For molecules, it shows the actual number of atoms bonded together. Cambridge IGCSE expects you to construct formulae from valency tables and deduce them from dot-and-cross or ball-and-stick diagrams.

You can read the full explanation, valency tables and worked examples on Tutopiya’s Formulae subtopic page before you attempt questions.

The core ideas you must master

Idea	What it means	How the exam uses it
Valency	Combining power of an element or ion	”Write the formula of aluminium oxide.”
Ionic formula	Balance positive and negative charges	”Write the formula of calcium nitrate.”
Molecular formula	Actual atoms in one molecule	”Deduce the formula from the diagram.”
Brackets	Group atoms that appear more than once	”How many oxygen atoms in CaCO₃?”
Empirical formula	Simplest whole-number ratio	”State the empirical formula of C₆H₁₂O₆.”

How to write formulae — step by step

1. Identify the elements or ions present and their charges/valencies.
2. Cross over valencies to get subscripts (or balance total charge to zero for ions).
3. Simplify subscripts to the smallest whole-number ratio if needed.
4. Use brackets when a polyatomic ion appears more than once — e.g. Mg(OH)₂.
5. Check the overall charge is zero (ionic) or that atom counts match the diagram (covalent).

Once you have worked through a few, test yourself with the free Formulae quiz — it tells you fast whether the method has actually stuck.

Ionic vs molecular: which approach does the question want?

Situation	What to do	Typical signal words
Metal + non-metal	Ionic formula from charges	”sodium chloride”, “magnesium oxide”
Polyatomic ions	Treat NH₄⁺, SO₄²⁻, etc. as single units	”ammonium sulfate”, “calcium carbonate”
Non-metal + non-metal	Count shared electrons in diagram	”deduce the formula of the molecule shown”
Empirical from composition	Divide by smallest mole ratio	”percentage by mass” data given

Formulae in past-paper wording: command words that matter

Command word / phrase	What the question wants	Typical formulae stem
Write	Construct the formula	”Write the formula of iron(III) chloride.”
Deduce	Work out from data or diagram	”Deduce the molecular formula from the structure.”
State	Give a known formula	”State the formula of sulfuric acid.”
Calculate	Often leads to empirical formula	”Calculate the empirical formula of the compound.”
How many atoms…	Interpret subscripts and brackets	”How many hydrogen atoms in one molecule of ethanoic acid?”

Worked exam-style stems (how to answer the wording)

1. “Write the formula of aluminium sulfate.” Ions: Al³⁺ and SO₄²⁻. Cross charges: Al₂(SO₄)₃. Mark-scheme reward: correct brackets and subscripts.
2. “A compound contains 40% calcium, 12% carbon and 48% oxygen by mass. Calculate the empirical formula.” Ca : C : O = 40/40 : 12/12 : 48/16 = 1 : 1 : 3 → CaCO₃. Reward: mole ratio working shown.
3. “How many atoms of hydrogen are in one formula unit of calcium hydroxide?” Ca(OH)₂ → 2 H atoms. Reward: brackets interpreted correctly.

When you can recognise the wording instantly, work the full set on the Stoichiometry topical past paper questions and the Formulae quiz to lock the method in.

How formulae connect to the rest of Stoichiometry

Formulae feed directly into Relative Masses of Atoms and Molecules for Mr calculations and into The Mole and the Avogadro Constant for mole conversions. Wrong formulae break every subsequent calculation. When you are ready to mix topics, the Cambridge IGCSE Chemistry resource hub lets you move straight from a weak subtopic into the next.

Common mistakes students make

• Writing Na₂Cl instead of NaCl — cross valencies then simplify.
• Forgetting brackets in compounds like Ca(OH)₂ or Al₂(SO₄)₃.
• Confusing empirical and molecular formulae when data is given.
• Using word equations when the question asks for symbol formulae.
• Miscounting atoms when polyatomic ions appear more than once.

When you need more support

If formulae questions keep tripping you up — especially polyatomic ions and empirical formula calculations — work through the Stoichiometry topical past paper questions and the Formulae quiz to pinpoint the exact gap, then get focused help from a Cambridge IGCSE Chemistry tutor to fix it quickly.

Frequently asked questions

Are chemical formulae hard in Cambridge IGCSE Chemistry? No — the rules are fixed. Marks are lost when students forget brackets, confuse empirical and molecular formulae, or misread valency tables.

What is the quickest way to write an ionic formula? Write the ions with charges, cross the numbers to become subscripts, simplify, and use brackets for polyatomic ions that appear more than once.

Do I need to memorise all common ions? Yes — NH₄⁺, OH⁻, NO₃⁻, CO₃²⁻, SO₄²⁻ and HCO₃⁻ appear constantly in formula and equation questions.

How do I revise formulae effectively? Read the subtopic notes, drill common ions daily, then take the Formulae quiz. Fix any empirical formula calculations before moving to mole work.

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