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Isomers.

Isomers are compounds which have the same molecular formula
but a different structural formula.
The structural formula (sometimes called the displayed formula)
shows how the atoms are joined together in the molecule.

For hydrocarbons, this means that isomers
 have the same number of carbon and hydrogen atoms,
but the carbon atoms are joined to each other in different ways.
Simply changing the shape of the molecule
does not make it a different isomer.

The isomers of butane are shown on the next page.
Different isomers are different molecular compounds.
They will have different physical properties (for example boiling point).

The more carbon atoms a compound has,
the greater the number of isomers there can be.
Propane (3 carbon atoms) has 0 isomers.
Butane (4 carbon atoms) has 2 isomers.
Pentane (5 carbon atoms) has 3 isomers.
Hexane (6 carbon atoms) has 5 isomers.
Heptane (7 carbon atoms) has 9 isomers.
Octane (8 carbon atoms) has 15 isomers.

There are also isomers of alkenes and alcohols.

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