Read the small numbers
A subscript is a small number within a formula. H₂O contains 2 H and 1 O. If no number follows a symbol, the number is 1. CO₂ contains one carbon and two oxygens.
Lesson 3.2 · AQA Combined Science
Counting atoms and reading formulae
A big front number multiplies everything
3H₂O means three water molecules: 6 H and 3 O, so 9 atoms in total. A front number is called a coefficient. It multiplies the whole formula, not just the first element.
Brackets keep a group together
In Ca(OH)₂, the ₂ multiplies both O and H inside the brackets: 1 Ca, 2 O, 2 H. In 2Ca(OH)₂, the front 2 doubles all of those counts: 2 Ca, 4 O, 4 H.
Count each side before balancing
In H₂ + O₂ → H₂O, hydrogen is 2 on each side, but oxygen is 2 on the left and 1 on the right. This is not balanced. Keep the formulae unchanged and adjust the coefficients in the next lesson.
Remember
Subscripts belong to formulae. A front coefficient multiplies the whole formula; a bracket subscript multiplies the bracket contents.
20 quick questions
Try it
Build confidence with pictures, clues and instant feedback.
Question 1 of 20Spot · Match · Explain
Check your understanding
Look at the diagram.
Case closed!
One point for each answer you get right first time.
No timer. Take your time and use a clue whenever you need one.