Number of vibrational modes

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How many vibrational modes are there?

3N – 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 – 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.

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