Bond dissociatio enthalpy should decreases as the bond distance increases from I2 to I1 due to this corresponding increase in the size of the atom as we move from F to I. However, the F-F bond dissociation enthalpy is smaller than that of Cl-Cl and even smaller than that of Br-Br. This is due to the reason that the F atom is very small and hence the three lone pairs of electrons on each F atom repel the bond pair holding the F - atoms F2 molecule.
∴ I2 < F2 < Br2 < Cl2
