Bond energy of F2,Cl2,Br2 and I2 are as: F−F→150.6mol−1(kJ) Cl−Cl→242.7mol−1(kJ) Br−Br→188.3mol−1(kJ) I−I→150.6mol−1(kJ)
Bond energy of F2 is lower because of repulsion between non bonded electrons of small sized F atom.
So the order is F2<Cl2>Br2>I2