isomer

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Chemical compound having the same molecular formula but with different molecular structure. For example, the organic compounds butane (CH₃(CH₂)₂CH₃) and methyl propane (CH₃CH(CH₃)CH₃) are isomers, each possessing four carbon atoms and ten hydrogen atoms but differing in the way that these are arranged with respect to each other.

Structural isomers have obviously different constructions, but geometrical and optical isomers must be drawn or modelled in order to appreciate the difference in their three-dimensional arrangement. Geometrical isomers have a plane of symmetry and arise because of the restricted rotation of atoms around a bond; optical isomers are mirror images of each other. For instance, 1,1-dichloroethene (CH₂CCl₂) and 1,2-dichloroethene (CHClCHCl) are structural isomers, but there are two possible geometric isomers of the latter (depending on whether the chlorine atoms are on the same side or on opposite sides of the plane of the carbon–carbon double bond).

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