CBSE Class 11-science Answered

Dear student,

A molecule can vibrate in many ways and each of them we can call a vibrational mode.As a help to calculate the number of vibrational modes, it's convenient to determine the number of degrees of freedom available to vibration. As a generalization any molecule consisting of N atoms will have 3N freedoms for translational motion. 3 degrees are translational freedoms. There are also 3 degrees of rotational freedom for non-linear molecules and 2 degrees of rotational freedom for linear molecules. This leaves 3N-5 degrees of vibrational freedom for linear molecules and 3N-6 degrees of vibrational freedom for non-linear molecules.

As an example H₂O, a non-linear molecule, will have 3*3-6 = 3 degrees of vibrational freedom, or modes.

Another example is the linear carbon-dioxide molecule, CO₂, which has 3*3-5 = 4 degrees of vibrational freedom.

Hope this helps.

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