A stereoisomer is a molecule whose constituent atoms differ in their location on the molecule. They have the same chemical formula as each other, but differ in where the atoms are placed. An example of this is 1,2 dichloroethene (C2H2Cl2, the numbers before the molecular name indicate the chlorine atoms are located on both carbon atoms, one on each). There are 2 possibilities for the locations of the chlorine atoms: cis-1,2 dichloroethene (where the chlorine atoms are bonded on the same side of the molecule) and trans-1,2 dichloroethene (where the chlorine atoms are bonded on opposite sides).
- Solomon's Organic Chemistry, Fifth Edition