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Lesson 6 of 7 · Textbook 8.3

Chemical formulae

Drawing circles gets slow. A chemical formula says the same thing in a few characters — once you know how to read the small numbers.

What a formula tells you

Definition The chemical formula is another way to represent molecules. It identifies each constituent element by its chemical symbol, and indicates the number of atoms of that element.
Molecule Formula Type of atom Number of atoms
hydrogenH2hydrogen2
waterH2Ohydrogen2
oxygen1
Two rules people get wrong

A subscript — the small number after a symbol — counts the atoms of the symbol immediately before it. In H2O the 2 belongs to the hydrogen, not to the whole formula.

A symbol with no subscript means one atom. In H2O there is exactly one oxygen atom, even though no number is written.

Read a formula

Total atoms in one molecule—
Element or compound?—

S₈, CH₄ and C₆H₁₂O₆ are the three from the Learning Point on page 76 — work them out first, then check.

Telling element from compound at a glance

You do not need to know what the substance is. Just count the different chemical symbols in the formula.

One symbol → elementTwo or more symbols → compound
hydrogen H2 · oxygen O2 · nitrogen N2 · chlorine Cl2 · iodine I2 · sulfur S8 water H2O · ammonia NH3 · carbon monoxide CO · carbon dioxide CO2 · hydrogen chloride HCl

Notice that S8 is an element despite having eight atoms, and CO is a compound despite having only two. The number of atoms is irrelevant — only the number of different symbols counts.

Structural formulae — how far this can go A structural formula is another way to represent a molecule: it shows how the atoms in a molecule are chemically combined, not just how many there are.

Nobel Prize winner R. B. Woodward believed there are trends in how different molecules behave. With his colleagues he created molecules of cholesterol (C27H46O), chlorophyll (C55H72O5N4Mg) and vitamin B12 (C63H88CoN14O14P) in the laboratory — something that had once seemed impossible. With careful planning, experimentation and an understanding of existing molecules, it is possible to create new molecules.

Check yourself