lesson
A formula records types and numbers
1 · Learn
A chemical symbol identifies an element; capitalisation matters. In a molecular formula such as H₂O, the subscript 2 records two hydrogen atoms per molecule, while the missing subscript after O means one oxygen.
A coefficient before a formula counts whole units. Two water molecules contain four hydrogen atoms and two oxygen atoms. Changing a subscript changes the substance's formula, not just the number of molecules.
2 · Worked example
For 3 CO₂ molecules, I count 3 carbon atoms and 6 oxygen atoms. I do not read the subscript as a separate loose oxygen molecule.
3 · Your turn
How many hydrogen and oxygen atoms are represented by 4 H₂O molecules?
Check your answer
8 hydrogen atoms and 4 oxygen atoms.
4 · Apply your learning
Use labelled counters to build the represented molecules and compare changing a coefficient with changing a subscript.
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Check a calculation · Year 5 onward
Use this only when your teacher or adult allows a calculator for the task. Show your thinking first. It does not replace mental or written arithmetic and does not mark your lesson answer.
Powers, roots and trigonometry · Year 7 onward
Use only when the task allows a calculator. Choose one operation at a time and record your working. Trigonometry here always uses degrees (DEG), not radians. Inverse sine, cosine and tangent return an angle, not a reciprocal. Keep exact fractions and π in answers when requested.
Approximate numerical results use up to 12 significant digits. Inverse sine returns −90° to 90°, inverse cosine 0° to 180°, and inverse tangent −90° to 90°; these are principal values, not every possible solution of a trigonometric equation.
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