Compounds having the same constitutions but the different spatial arrangement of their atoms are known as stereoisomers and this phenomenon is called stereoisomerism.
Tartaric acid can be taken as an example:
It has two asymmetric carbon atoms so using the 2n formula there should be a maximum of 22 = 4 stereoisomers. These can be represented as:
I and II i.e. the forms are mirror images of each other and are called enantiomers but in erythro form i.e., meso form, the plane of symmetry is present thus it does not show optical activity. Therefore, tartaric acid exists only in three isomeric forms of which one is meso (optically inactive) and the other two are optical active i.e., d (or) (+) and l or (-).
