Bond energy generally decreases on moving from top to bottom along a group. It happens due to fact that size increases on moving down the group and thus, the two nuclei are far apart and less capable of holding the two atoms together.

But in case of F₂ molecule due to more repulsion in between non-bonding electron pair (2p) of two fluorines (due to small size of F-atom) in comparison to non-bonding electron pair (3p) in chlorine, the bond energy of F₂ is less than Cl₂.

BE(F₂) = 158.5 kJ/mole and

BE(Cl₂)= 242.6 kJ/mole