Fluorine has a very small size and high electronegativity. Therefore, it forms only one oxoacid, HOF(fluoric(I) acid or hypofluorous acid). The other elements of Halogens generally form four series of oxoacids namely hypohalous acids (+1 oxidation state), halous acids (+3 oxidation state), halic acids (+5 oxidation state) and perhalic acids (+7 oxidation state).
Chlorine forms four types of oxoacids. That is HOCl (hypochlorous acid), HOClO (chlorous acid), HOClO2(chloric acid) and lastly HOClO3 (perchloric acid). Bromine forms HOBr (hypobromous acid), HOBrO2(bromic acid) and HOBrO3 (perbromic acid). Iodine forms HOI (hypoiodous acid), HOIO2 (iodic acid) and HOIO3 (periodic acid).
Consider how the strength of oxoacids varies down the group. For example consider the series of hypohalous acids (+1): HClO, HBrO, HIO. The strength of the acid is determined by the central atom’s electronegativity relative to the surround atoms in the molecule. Because Cl is the most electronegative, it draws the bulk of the electrons in the HOCl molecule toward itself; because H and Cl are on opposite ends of the molecule, Cl pulls at the electrons in the H-O bond, thereby weakening it. The weaker the H-O bond, the more easily the H+ can ionize in water, and the stronger the acid. These acids can be arranged in order of their strength decrease: HClO>HBrO>HIO.
Consider how the strength of oxoacids varies with the change in the oxidation state of halogens. Consider the series of clorine oxoacids: HOCl, HOClO, HOClO2,HOClO3 . We can see that the number of oxygen atoms increases. Oxygen is a highly electronegative element, and the more oxygen atoms present, the more that the molecule’s electron density will be pulled off the O-H bond, weakening it and creating a stronger acid. These acids can be arranged in order of their strength decrease : HOClO3> HOClO2 >HOClO> HOClO.