Symbols and formulas – a short recap

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Rule

Compound: A compound consists of atoms of different elements joined in a fixed ratio, for example water or carbon dioxide.

Molecule: A molecule is a small particle made of several joined atoms, for example a water molecule.

Formula: A formula uses element symbols to show which atoms a substance is made of. A small subscript number, the index, tells how many atoms there are.

The formula of water is $H_{2} O$ : one molecule has 2 hydrogen atoms (H) and 1 oxygen atom (O). If there is no index, it is 1 atom.

Some formulas and their atoms
Substance	Formula	Atoms in one particle
water	$H_{2} O$	2 H, 1 O
carbon dioxide	$CO_{2}$	1 C, 2 O
methane	$CH_{4}$	1 C, 4 H
ammonia	$NH_{3}$	1 N, 3 H
oxygen	$O_{2}$	2 O

Example

Reading the formula of methane

Methane has the formula $CH_{4}$ . C stands for carbon, H for hydrogen. The 4 says: for 1 carbon atom there are 4 hydrogen atoms. A methane molecule has 5 atoms in total.