The small number in a formula and the big number before it
A subscript after a symbol counts atoms of that element in one particle. A number before the formula multiplies the whole particle, including every atom.
| Notation | Number of H atoms | Number of O atoms |
|---|---|---|
| H₂O | 2 | 1 |
| 2 H₂O | 4 | 2 |
| 3 H₂O | 6 | 3 |
One worked example
- H₂O: H = 2, O = 1O has no written subscript: that means one atom, not zero.
- 2 H₂O: H = 4, O = 2Two molecules: multiply both atom counts by two.
- CO₂: C = 1, O = 2The subscript after O applies to oxygen, not carbon. These expressions count atoms; they do not describe chemical reactions.
Watch out: Balance reactions by changing coefficients, not subscripts: H₂O and H₂O₂ are different substances. Ionic formulas express ion ratios rather than necessarily separate molecules.
Goal, prerequisites and card origin
What you will learn: Count atoms in a chemical formula with a coefficient.
What you need first: Multiplication of whole numbers; a chemical symbol identifies an element.
An original PEAZIT working card. Independent expert review and effectiveness testing have not yet taken place. · v1
Show connections
A subscript after a symbol counts atoms of that element in one particle. A number before the formula multiplies the whole particle, including every atom.