Two molecules are isomers of each other if they have the same chemical formula but a different atomic configuration.

For example, propanol (CH3CH2CH2OH) has two isomers: n-propanol and iso-propanol. n-propanol is a straight chain (CH3CH2CH2OH), with its three carbons and its hydroxyl group in a line. Iso-propanol, on the other hand, is branched, with two of the carbons and the hydroxyl group all bonded to one carbon ((CH3)2CHOH). Both of these configurations have 3 carbons, 8 hydrogens, and 1 oxygen, but since they are isomers they behave differently in chemical reactions.

See also