hello Zarikul hoque,

hope you are doing well now coming back to your question,

As we know that in general, a non-linear molecule with N atoms has 3N – 6 normal modes of vibration, but a linear molecule has 3N – 5 modes, because rotation about the molecular axis cannot be observed. A diatomic molecule has one normal mode of vibration, since it can only stretch or compress the single bond.

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Thank you.

Vibrations of polyatomic molecules are described in terms of normal modes,which are independent of each other,but each normal mode involves simultaneous vibrations of different parts of the molecule.In general,a non linear molecule with N atoms has 3N-6 normal modes of vibration,but a linear molecule has 3N-5 modes, because rotation about the molecular axis cannot be observed.A diatomic molecule has one normal mode of vibration,since it can only stretch or compress the single bond.