Discussion
Which of the following is not correct (for number of fundamental modes of vibration) ?
Answer: B
The incorrect statement is that a non-linear molecule has 3N-3 modes.
A molecule of N atoms has 3N degrees of freedom. Three are translations. A non-linear molecule also has three rotations, leaving 3N - 6 vibrations; a linear molecule has only two rotations (rotation about its axis does not count), leaving 3N - 5.
3N - 3 subtracts only the translations and forgets the rotations. A spherical top such as CH4 is non-linear, so 3N - 6 applies to it, as option D says.
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