Question

c) How many vibrational modes are there in Ammonia
molecule ? Calculate.
1​

Answer 1

A linear three atomic molecule like CO2 has 4, a nonlinear three atomic molecule like H2O has 3, NH3,NH4 and N2O4 have 6,9 and 12 independent vibrational coordinates, respectively. Types of vibrations: All atoms of a molecule oscillate with the same frequency and in phase.

Answer 2

Vibrational modes in ammonia are 6.

• In a molecule transitional, rotational and vibration modes are present.
• A total number of modes is given by 3n. where n is the total no of atoms in a molecule.
• For a linear molecule, the vibrational mode is calculated by the formula 3N-5 whereas for a nonlinear molecule vibrational mode is calculated by the formula 3N-6.
• As ammonia (NH3) is non-linear so its Vibrational mode = 3×4-6 = 6