The term isomer (meaning same member) refers to two molecules that have the same chemical formula, but which differ according to the spacial arrangement of their atoms. Molecules that are isomers have the same number and kind of atoms, but they differ in respect to the arrangement or configuration of their atoms. For example, dimethyl ether and ethyl alcohol have the formula C₂H₆O, but the two compounds have quite different properties. The term isomer was first used by the German chemist Jons Jakob Berzelius. Much of what is known about this class of compounds came as a result of work done by Friedrich Wohler and Justus Liebig in the early 1820s.

Isomerism can occur in one of several forms depending on the structure, position, and geometry of the chemical bonds. Chain isomerism exists among the alkanes, which consist of carbon atom chains. Because these chains may...