Reading chemical formulas

Mineral water bottles say CO₂, the news mentions CH₄, and everyone knows H₂O anyway. Chemical formulas are everywhere, and they only look mysterious at first glance. In truth they are a clever shorthand that you can learn to read in a few minutes.

Formulas are shorthand

A molecule of two hydrogen atoms and one oxygen atom: that is the long way to describe water every single time. Chemists simply write H₂O instead. Each element symbol stands for one type of atom, H for hydrogen, O for oxygen, C for carbon. The small subscript after it counts how often the atom right before it appears: H₂O means 2 H and 1 O. If there is no subscript, the atom appears exactly once, the 1 is never written.

Coefficients count whole molecules

Sometimes a big number stands in front of the formula, for example 3 H₂O. This number is called a coefficient and counts whole molecules: 3 H₂O means three complete water molecules. To find the total number of atoms, you multiply: 3 molecules times 2 H makes 6 hydrogen atoms, and 3 molecules times 1 O makes 3 oxygen atoms. Remember the difference: the subscript counts atoms inside one molecule, the coefficient counts the molecules themselves.

Counting atoms like a pro

With these two rules you can crack any simple formula. Go through it symbol by symbol: in CH₄ you read 1 C and 4 H, 5 atoms in total. In 2 CO₂ you read two molecules with 1 C and 2 O each, so 2 C and 4 O. That is exactly the counting you will practise now, until it runs on its own.