Why is HClO stronger than HIO?

Question

also, why is H3PO4 a stronger acid than H3AsO4 since As is a larger atom than P??

Answer 1

In either HClO or HIO, the halogen atom has a +1 charge (-2 for the oxygen and +1 for the hydrogen leaves +1 for the halogen to make the compound neutral).  Halogens are electronegative, and so their compounds are more stable when the halogen atom has a more negative charge (these two acids are both unstable and strong oxidizers).  When we deprotonate HClO or HIO, the hydrogen goes off as H+, leaving behind ClO- or IO-.  The negative charge here is on the halogen atom, so its charge becomes less positive, and the anion becomes more stable.  However, chlorine is more electronegative than iodine, so the stability of ClO- increases more by this than does that of IO-.

The same logic can be used for H3PO4 and H3AsO4: As is less electronegative than P, so the conjugate base of H3AsO4 is less stable than that of H3PO4.

Of course, electronegativity values are dependent on size: As is larger than P, so its attractive nucleus is further from its electrons than the P nucleus is from its electrons, so the attraction is decreased compared to P (it has a larger nuclear charge, but I believe this is canceled out by the increased distance - look up Coulomb's law)