Question

All halogens are coloured. The colour of halogens deepens on passing from $F_2$ to $C{l}_2$. $F_2$(yellow), $C{l}_2$(yellowish green), $B{r}_2$(red-brown) and $I_2$ (violet). This happens because:

• A. halogens with higher atomic number absorb light of longer wavelength since the difference in energy between ground state and excited state decreases as the atomic no. increases.
• B. energy levels of halogens differ widely with the increase in atomic number and hence electron excitation energy becomes larger and the wavelength of the light absorbed becomes shorter
• C. bond dissociation energy increases from fluorine to iodine
• D. fluorescence becomes more intense as the atomic number of halogens increases

Answer 1

The correct option is A halogens with higher atomic number absorb light of longer wavelength since the difference in energy between ground state and excited state decreases as the atomic no. increases.

$E_{n2}$-$E_{n1}$ = $\Delta E$(energy absorbed). (n₁ is ground state and $n_2$ is excited state). When atomic no. Increases, $\Delta E$ decreases, and as $\Delta E=hc/\lambda$, wavelength $\left(\lambda \right)$ of light increases. Thus, halogens of higher atomic no. absorb light of longer wavelength in visible region resulting in excitation of outer electrons to higher energy levels.