Key Idea : Generally bond dissaciation energies decreases in a group. Bond dissaciation energy also decreases with repulsion. X−X Bond Bond length (A˚))Bond dissociation energy (kcal/mol)F−F1.4238Cl−Cl1.9957Br−Br2.2845.5I−I2.6735.6 In general the bond dissociation energy decreases as the bond length increases, but the bond dissociation energy of F2 is less than that of Cl2. It is due to greater interelectronic repulsions between the lone pair of electrons on the two bonded fluorine atoms. Hence, the order of bond dissociation energy is as : Cl2>Br2>F2>I2