Chemistry - Complete Pblock- Question with Solution | TestHub

The bond dissociation enthalpy of halogens generally decreases down the group as the atomic size increases, which leads to a longer and weaker bond.

However, the bond dissociation enthalpy of ${\text{F}}_2$ is exceptionally lower than that of ${\text{Cl}}_2$ and ${\text{Br}}_2$. This is due to the very small size of the fluorine atom, which results in strong interelectronic repulsions between the non-bonding electrons (lone pairs) on the two closely spaced fluorine atoms.

The experimental values of bond dissociation enthalpies are:
$\text{Cl-Cl}=242.6{\text{ kJ mol}}^{-1}$
$\text{Br-Br}=192.8{\text{ kJ mol}}^{-1}$
$\text{F-F}=158.8{\text{ kJ mol}}^{-1}$
$\text{I-I}=151.1{\text{ kJ mol}}^{-1}$

Thus, the correct order of bond dissociation enthalpy is $\text{Cl-Cl}>\text{Br-Br}>\text{F-F}>\text{I-I}$.

Answer: $\text{Cl-Cl}>\text{Br-Br}>\text{F-F}>\text{I-I}$