In case of diatomic molecules (X2) of halogens the bond dissociation energy decreases in the order : Cl2>Br2>F2>I2 The oxidising power, electronegativity and reactivity decrease in the order : F2>Cl2>Br2>I2 Electron gain enthalpy of halogens follows the given order : Cl2>F2>Br2>I2 The low value of electron gain enthalpy (electron enthalpy) of fluorine is probably due to small size of fluorine atom.