• Rotary reflection axis (S_n): Rotary reflection axis is an axis which rotates the object (molecule) around by 360°/n, followed by rotation in a plane perpendicular to the axis, such that the new position of an object is superimposable with the original one. All odd values of S_n are identical with C_n. Thus, in 1, there is a 4-fold rotary reflection axis.

If a molecule has only either centre of symmetry or plane of symmetry then it is achiral. However, in most cases, molecules have more than one element of symmetry. In such cases, it becomes important to know the point group to which the molecule belongs. A point group reflects the combination of symmetry elements present in the structure. The point group of a molecule can be determined by following the algorithm given below. The point groups of high symmetry are usually not important in simple organic molecules. The point groups C₁, C_n and D_n are chiral groups and they contain chiral molecules while all other groups are achiral (Figure 2).

Figure 2