Elements of Symmetry
Element of symmetry may be defined as the point, line or plane inside or passing through the molecular geometry about which some operations like inversion, rotation or reflection generate indistinguishable images. These operations are generally labeled as symmetry operations. Elements of symmetry are a simple tool to identify whether a molecule is chiral or not. The essential conditions for optically active molecule to be chiral is that, the molecule should not possess any kind of symmetry elements. The different elements of symmetry are given below:
a. Simple axis of symmetry (Cn)
b. Plane of symmetry (σ)
c. Centre of symmetry (Ci)
d. Alternating axis of symmetry (Sn)
Simple axis of Symmetry (Cn)
When a rotation of 360°/n (where n is any integer like 1,2,3...etc.) around the axis of a molecule is applied, and the rotated form thus obtained is indistinguishable from the original molecule, then the molecule is known to have a simple axis of symmetry. It is represented by Cn.
Plane of Symmetry (σ)
It is defined as when a plane that devide a molecule into two equal halves which are related to object and mirror image is known as plane of symmetry. It is represented by σ.
Centre of Symmetry (Ci)
It is also called centre of inversion. A molecule has a centre of symmetry when, for any atom in the molecule, an identical atom exists diametrically (diagonally) opposite to this centre and at equal distance from it.
Alternating Axis of Symmetry (Sn)
An alternate axis of symmetry is defined as, when a molecule is rotated by 360°/n degrees about its axis and then a reflection plane is placed exactly at perpendicular to the axis, and the reflection of the molecule thus obtained is identical to the original. It is represented by Sn.
