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Isomers in chemistry are compounds with the same empirical formula but different arrangement of atoms.

A simple example of structural isomerism (position) is the alcohol with the formula, C3H8O, (known as propyl alcohol) which has the isomers:

    n-propyl alcohol            isopropyl alcohol
        H H H                        H H H
        | | |                        | | |
      H-C-C-C-O-H                  H-C-C-C-H
        | | |                        | | |
        H H H                        H O H

Note that the critical difference is the position of the oxygen atom - it is attached to an end carbon in the one isomer, and to the centre carbon in the other isomer. It can be readily shown that the number of possible isomers rapidly increases as the number of atoms increase; for example the next largest alcohol, named butanol (C4H10O), has four different isomers.

Different forms of isomerism

There are two main groups of isomers: structural isomerism and stereoisomerism. Structural isomerism deals with the different ways atoms and functional groups are joined together in a molecule. This group includes chain isomerism whereby hydrocarbon chains have variable amounts of branching; posistion isomerism which deals with the position of a functional group on a chain; and functional group isomerism which is when one functional group can be split up in to different ones.

The second group, stereoisomerism, is the different positioning of atoms in space. This class includes geometric isomerism - where functional groups at the end of a chain can be twisted in different ways; and optical isomerism where different isomers are mirror-images of each other.