Bond dissociation energy : Bond dissociation energies of chlorine, bromine and iodine decrease down the group as the size of the atom increases. The bond dissociation energy of fluorine, is however, lower than those of chlorine and bromine because of inter electronic repulsions present in the small atom of fluorine

Hence bond energy decreases in the order Cl₂ > Br₂ > F₂ > I₂

Bond length in X₂ molecule : As the size of the halogen atom increase, the bond length of X — X bond in X₂ molecule increases from F₂ to I₂

Thus, the bond length increases in the order F₂ < Cl₂ < Br₂ < I₂.