As bond length increases, bond dissociation enthalpy decreases. Hence, bond dissociation enthalpy decreases from F2 to I2. But the bond dissociation enthalpy of F2 is lower than Cl2 and Br2. It is due to the presence of lone pair of e on fluorine atom which creates greater repulsion due to small size of fluorine. Hence the bond dissociation enthalpy of F, decreases.
I2 < F2 < Br2 < Cl2
