Use of Point Groups
Rotation with respect to a line (axis of rotation) which molecules rotate. Cn is a rotation of (360/n)°. Each rotation brings you to the indistinguishable state for original.
In this first example we do two improper rotations one after the other. Notice that 2 S4 = 1 C2.
H2O - A C2 axis There is a line(rotation of axis) passing through O. When you rotate H atoms 180° =(360/2)°, you get the same state.
CH3Cl - C3 axis In this example, line passes thru C and Cl. And in order to get the same state, we need to rotate 120°.To see do a mouse over or play with the applet.
BrF5 - C4 axis
Real life example-Snowflakes C2, C3, C6 axes