Chemical formulae — Chemistry, 11–13 years
A chemical formula is a compact recipe showing which elements are present and how many atoms of each are joined in a substance.
A recipe made of symbols
A formula uses element symbols and small numbers to describe one particle of a substance. In H₂O, the 2 belongs only to H, so each water particle has two hydrogen atoms and one oxygen atom. The formula does not tell you the colour or shape of the sample.
Why use formulae?
Writing “two hydrogen atoms joined to one oxygen atom” every time would be slow and easy to misread. Chemists needed a shared shorthand for labels, experiments and calculations. A formula solves that communication problem, but only if everyone reads the symbols and subscripts in the same way.
Reading CO₂
Take CO₂, the formula for carbon dioxide. First, C means one carbon atom because it has no small number. Next, O₂ means two oxygen atoms. So one particle contains 1 + 2 = 3 atoms altogether; the 2 does not mean two complete CO₂ particles.
The number in the wrong place
A common mistake is to read H₂O as two hydrogen particles and one oxygen particle. That feels reasonable because a number often counts whole objects in everyday life. In a formula, however, a small number written below an element counts its atoms inside one particle; a large number in front counts particles.
Labels and medicines
Formulae appear on mineral-water labels, cleaning products, medicines and laboratory bottles. They let someone identify a substance without relying on a brand name or a picture. A formula is useful only when the substance really has that fixed composition, not when a mixture contains changing amounts.
Keep exploring
Other languages
Loading MyLeoNes™…