Normal Modes of Vibrations:

A molecule can vibrate only in certain modes, known as normal modes. In the course of general mode of vibration every atom is displaced within such a way in which all atoms move with the same frequency and in phase but with different amplitude. The amplitudes of vibrations of atoms are like that the middle of mass does not move i.e., there is no translational motion of the molecule as a result of vibration.

Each normal mode corresponds to a vibrational degree of freedom. Alternatively, we can visualise a normal mode refers to an independent simultaneous motion of atoms or group of atoms that may be excited without leading to the excitation of any other normal mode. There are two t ypes of the normal modes of vibration for a polyatomic molecule. These are stretching vibrations and bending vibrations. Let us try to understand about these.