Match the correct bond dissociation in halogens    Halogens        Bond energy    1) F2            A) 158.8 KJmol-1    2) Cl2          B) 242.6KJmol-1    3) Br2         C) 151.1KJmol-1    4) I2            D) 192.8 KJmol-1

The decreasing order of bond energies of halogens is: Cl2 > Br2 > F2 > I2. The exceptionally lower bond energy of Fluorine is due to Fluorine atom is very small. The inter nuclear distance in fluorine is only 1.44 A°. Fluorine has notoriously weak bond. This was explained successfully by Coulson.The non-bonding electron pairs of adjacent fluorine atoms in F2 molecule are very close to each other.These lone pairs of electrons are under strong repulsive forces. Hence the bond strength in fluorine is greatly reduced.