In stereoisomerism, the molecules have the same molecular formula and the same structural formula (the atoms are connected in the same order in each molecule). However, in each molecule, the atoms have a different three dimensional arrangement in space which makes them non-superimposable. This means that no matter how you twist and turn the molecules, one isomer cannot fit exactly on top of the other.

There are two main types of stereoisomerism as shown in Figure 1.

Figure 1 - Isomerism

In this Topic, we will concentrate on stereoisomerism in organic compounds, although this phenomenon also occurs in other compounds such as transition metal complexes.