Atomic spectrum:

In case of atomic spectrum the transitions occur from one electronic energy level to the other and result in sharp lines.

Instead, in case of molecules the vibration and rotation motion is also quantised, and each electronic energy level is associated along with several vibrational levels and each of the vibrational level is associated with several of the rotational energy levels.

Therefore, the electronic transitions in the molecules have related vibrational and rotational transitions and radiations over a range of wavelengths are absorbed. Thus, the molecular absorption spectra have broad bands.