Explain why the bond dissociation enthalpy of `F_(2)` is less than that of `Cl_(2)`.

Explanation

F atom is much smaller in size than Cl atom. Therefore, the F - F bond is shorter than the Cl -Cl bond. Consequently, the lone pair of electrons on the two F atoms repel each other very strongly. The small F - F bond leads to a large internuclear repulsion. On the other hand, the distance between the two Cl atoms in `Cl_(2)` molecule is quite large and consequently, similar force of repulsion among the lone pair of electoms is less stronger. So, in spite of having shorter F - F bond length, the F-F bond dissociation enthalpy is less than the Cl-Cl bond dissociation enthalpy.