Reading formulas and building a formula notebook
Count the atoms in a molecule or formula unit and connect familiar chemical names with their formulas.
Section 3.1 · Formulae
Exam scope and assessment notes
Core and Extended · Chemistry 0971, 2026–2028. Cambridge states that IGCSE 0971 has the same subject content as IGCSE 0620, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Recognise element symbols and distinguish an atom from a molecule.
- Know that an ionic formula describes a ratio of ions rather than a separate molecule.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How many oxygen atoms are in one CO₂ molecule?
What you will learn
- Read molecular formulas and formulas with brackets.
- Deduce a formula from a particle model and retrieve common named substances.
A subscript counts atoms; a coefficient counts particles
A molecular formula gives the actual number of each kind of atom in one molecule. CH₄ means one carbon and four hydrogen atoms; H₂O₂ means two hydrogens and two oxygens. An omitted subscript means one. A coefficient belongs to the entire formula: 3CO₂ represents three molecules, containing three carbon atoms and six oxygen atoms in total.
A bracket multiplier applies to everything inside its bracket. Ca(OH)₂ counts one calcium, two oxygens and two hydrogens. For an ionic solid this is a formula-unit ratio, rather than a claim that separate Ca(OH)₂ molecules exist. When a model shows an actual molecule, count all its atoms before writing the formula; do not simplify its molecular formula into a ratio.
Retrieve names in families
Start a formula notebook that you extend as each later topic names more substances. Common elemental gases are H₂, N₂, O₂ and Cl₂; helium and other noble gases are monatomic. Familiar compounds include water H₂O, ammonia NH₃, carbon monoxide CO, carbon dioxide CO₂, sulfur dioxide SO₂ and sulfur trioxide SO₃. Compare similar names carefully: monoxide and dioxide describe different oxygen counts.
Common acids are hydrochloric HCl, nitric HNO₃ and sulfuric H₂SO₄. Common alkalis include NaOH, KOH and Ca(OH)₂. Examples met in later reactions include NaCl, MgO, Al₂O₃, Fe₂O₃, CaO, CaCO₃, CuSO₄, PbBr₂, AgNO₃, Ba(NO₃)₂, NH₄NO₃ and KNO₃. Learn each formula alongside its name, use and reaction; this starter notebook is not the complete list for all later topics. Organic formulas and hydrated salts are introduced in their own sequences.
Pause and explain
Which formula matches a molecule with two hydrogen and two oxygen atoms?
Worked example
A model of one molecule contains two carbon atoms, six hydrogen atoms and one oxygen atom. Write its molecular formula and count atoms in four molecules.
Show the worked solution
- Keep the model's actual atom counts: C₂H₆O.
- Multiply every count by the coefficient four.
- The total is eight carbons, twenty-four hydrogens and four oxygens.
Answer C₂H₆O; four molecules contain 8 C, 24 H and 4 O atoms.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Count each element in Al₂(SO₄)₃ and then in 2Al₂(SO₄)₃.
Give me a hint
Apply the bracket multiplier before the coefficient.
Compare my reasoning
- One formula unit contains two aluminium and three sulfur atoms.
- Its oxygen count is 4×3=12.
- The coefficient two gives 4 Al, 6 S and 24 O overall.
One unit: 2 Al, 3 S, 12 O. Two units: 4 Al, 6 S, 24 O.
Look for these in your work
- The bracket affects sulfur and oxygen.
- The coefficient multiplies every atom count.
A molecular model has one nitrogen atom and three hydrogen atoms. Give the formula, name the familiar compound and count atoms in five molecules.
Give me a hint
Use the exact counts from the model.
Compare my reasoning
- One nitrogen and three hydrogens give NH₃.
- The familiar compound is ammonia.
- Five molecules contain five nitrogen and fifteen hydrogen atoms.
NH₃, ammonia; 5 N and 15 H atoms in five molecules.
Look for these in your work
- The molecule's formula and name agree.
- The coefficient changes number of molecules rather than their identity.
A label says sulfur dioxide but prints SO₃. Explain the mismatch and say why changing a coefficient cannot repair it.
Give me a hint
A name describes the formula within each particle.
Compare my reasoning
- Sulfur dioxide requires two oxygens per sulfur, giving SO₂.
- SO₃ is sulfur trioxide, a different compound.
- A coefficient multiplies whole particles and does not change the oxygen count within each one.
Use SO₂; changing a coefficient cannot turn sulfur trioxide into sulfur dioxide.
Look for these in your work
- The names and subscripts are distinguished.
- Coefficients and molecular identity are kept separate.
Common mistakes
- Simplifying a molecular formula when the task asks for actual atoms in one molecule.
- Applying a bracket multiplier only to the last element.
What can you explain now?
How many oxygen atoms are represented by 2Ca(OH)₂?
Compare with the explanation
4
Each formula unit includes two oxygens; the coefficient two multiplies that count, giving four.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Close your notebook and write the formulas of six named substances from different families. Draw one molecular model and count its atoms before checking.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.