Question

Explain why absorption spectra for atomic species consist of discrete lines at specific wavelengths rather than broad bands for molecular species.

Answer 1

Absorption spectra of atomic species

Atoms have definite quantized energy levels

1s , 2s, 2p , 3s 3p etc.

The electron in the ground state absorbs photons of definite energy to get excited to a higher level

Since the energy levels are quantized the difference in energy levels also quantized of specific wavelengths and hence the absorption is not broad as seen in the molecular absorption.

Thus it gives discrete lines at specific wavelengths.

Absorption spectra of molecular species

But in case of molecules the energy of absorption is affected by bending, rotation, stretching vibration etc due to the presence of different atoms in a molecule and it varies depending on the electronegativity, dipole moment etc. of the various constituent atoms.

The bending and vibration alters the dipole moment of the molecule and the absorption varies within a range of wavelength and hence gives a broad spectrum.