In oxides of halogen, the bonds are mainly covalent due to small difference in electronegativity between the halogens and oxygen: the bond polarity, however, increases as we move from F to I. The stability of oxides of iodine is greater than those of chlorine while bromine oxides are the least stable. Iodine-oxygen bond is stable due to greater polarity of the bond while the stability of the chlorine-oxygen bond is due to multiple bond formation involving d-orbitals of the chlorine atom. Bromine being in between lacks both these characteristics. Thus, the stability of oxides of halogens decreases in the order : I > Cl > Br > F.