4 Position isomers occur among substituted alkanes and other compounds. 5 Functional group isomers, on the other hand, have very different chemical properties because differences in their structure give rise to different functional groups. 6 Stereoisomers Stereoisomerism occurs when two or more molecules have the same basic arrangement of atoms in their molecules but differ in the way the atoms are arranged in space. There are two types of stereoisomerism. The first type, geometric isomerism, may occur when a compound contains a double bond or some other feature that gives the molecule a certain amount of structural rigidity.
Geometric isomers differ in physical properties such as melting point and boiling point.
The second type of stereoisomerism is optical isomerism. When plane-polarized light is passed through an optical isomer it is rotated into a different plane of polarization. Optical isomers exhibit this optical activity in varying degrees. Optical isomers of a given compound are often identical in all physical properties except the direction in which they rotate light. The molecules of optical isomers are asymmetrical.