Why do molecules vibrate?

Hint: We know that vibrational motion is the motion which refers to the periodic motion of atoms and changes the shape of the molecule. In simple words, a molecular vibration is referred to as a periodic distortion of a molecule from its equilibrium geometry.

Complete Step By Step Answer:
The atoms in every molecule are always in vibrating motion and each bond between atoms vibrates at a certain frequency and in a certain direction. This can be explained on the basis of a fact that atoms are held together in the molecules because of the electrostatic force of attraction between negatively charged ions i.e., electrons of one atom towards the positively charged nucleus of another. But, at the same time, the electrons in one atom tend to repel other electrons and the protons in the nuclei of the two atoms repel as well because like charges repel each other.
This constant push and pull within a molecule is responsible for creating a vibration as if the atoms are connected by tiny springs in a constant motion. A bond in a molecule vibrates with both stretching and bending motions which is explained as follows:
1. A stretch is a vibration which occurs along the line of the bond and thus, is responsible for changing the bond length of the molecule.
2. A bend is a vibration which does not occur along the line of the bond and is responsible for changing the bond angle of the molecule.
Thus, we can conclude that since the atoms in the molecule are always in a constant periodic motion, the molecules show vibrations.

Note:
It is important to note that the energy required for a molecule to vibrate is not continuous, but is quantized i.e., absorbs energy in form of discrete packets or quanta of photons and in general, the energy lie in the infrared region of the electromagnetic spectrum.