Stereoisomers are species with the same structural formula but different arrangements of the atoms in space. There are two types: geometric and optical isomerism.Optical isomerism occurs when there are four different groups of atoms around a central or chiral carbon atom. Two non-superimposable mirror image arrangements or enantiomers are possible. Optical isomers have the same chemical properties but rotate the plane of polarised light in opposite directions - clockwise or anticlockwise. A 50-50 mixture of both enantiomers is known as a racemic mixture. All amino-acids except glycine exhibit optical isomerism.