(i)    Size of the central atom
(ii)    electronegative of the central atom in the molecule
(iii)    inductive effects of substations on central atom.

Consider an oxo acid                 E-O-H

Where E is the central atom. A small and highly electronegative atom E will tend to attract the shared pair of electron more towards itself. This will create a high polarity in O-H bond thereby allowing the proton to dissociate and make the compound acidic. Thus higher the electronegative of E more easily the proton is lost and more is the acidity of oxo acid.

As we go down a group, the electronegative decreases as the size increases and acidic character decreases. For example the acidic character of oxo acids of halogens decreases in the given order as is care form pK_a values
                                                           HCIO > HBrO > HIO


For more help in Strength of Oxo acids click the button below to submit your homework assignment