Bond dissociation energy among halogens
The strength of the bond is due to 2 things : Difference in electronegativity and atomic radius of the elements .Halogens except fluorine , have a very big atomic radius so less bond energy.And they have 0 difference in electronegativity so even less bond energy.They both "want" to get the bonding pair of electrons closer to them , so because they cant both atoms are "unhappy".