Polyatomic molecules with more number of atoms have many modes of vibrations depending upon their bond arrangements. For example, triatomic molecules may be linear like CO2 and HCN and may be angular like H2O and NO2. Accordingly, they have their modes of vibrations. The modes of vibrations can be classified as:
(a) Stretching vibration : In this type of vibration, the atoms move along the bond axis, so that the bond length increases and decreases periodically at regular intervals. The bond axis will not change.
(i) Symmetrical :
(ii) Unsymmetrical :
In symmetrical vibration both the atoms move away simultaneously, while in unsymmetrical vibration, one atom moves towards the central atom and the other moves away.
(b) Bending vibration :In this type of vibration there is continuous change of angle between bonds with a common atom. Sometimes it may be group of atoms. But the atoms to which the group is attached remains undisturbed.
(i) Rocking :
(ii) Scissoring :
(iii) Twisting :
(iv) Wagging :
These vibrations may involve twisting, rocking, wagging and scissoring. Because of this the bond angle in a molecule gets affected. Out of these rocking and scissoring occur in the same plane and twisting and wagging vibrations occur out of plane.