How many vibrational modes are there?
3N â€“ 6
It is important to understand that a molecular vibration occurs when the atoms in a molecule are obviously in periodic motion while the molecule as a whole has constant translational and rotational motion. Moreover, according to Wikipedia, the frequency of the periodic motion is known as a vibration frequency. Most of the cases, a molecule with N atoms has 3N â€“ 6 normal modes of vibration. On the contrary, a linear molecule has 3N-5 modes of vibration. When it comes to a diatomic molecule, it has one normal mode of vibration.