Chlorine, bromine and iodine form four series of oxyacids of the general formula HOX , HOXO, HOXO 2 and HOXO 3 in which the oxidation states of halogen (X = Cl, Br or I) is + 1, + 3 , and + 5 and + 7 respectively. However, due to high electronegativity, small size and absence of d-orbitals, F cannot act as central atom in higher oxacids such as HOFO. HOFO 2 and HOFO 3 In which the oxidation state of F would be + 3, + 5 and + 7. It just forms one oxoacid. i.e., HOF in which the oxidation state of F is –1 and oxygen is in +1.