Bond dissociation energy of halogen family decreases down the group as the size of atom increases. The decreasing order for the bond dissociation enthalpy of halogen molecules is Cl2(57kcal/mol)>Br2(45.5kcal/ mol)>F2(38kcal/mol)>I2(35.6 kcal/mol ). A Caution A halogen molecule having larger atoms should have low dissociation energy and vice versa. Fluorine is an exception because of interelectronic repulsion is present in small atom of fluorine