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I was working on a problem regarding the relative acidity of oxoacids in the form $\ce{HYO3}$ where $\ce{Y}$ represents a group 17 element. The question (which is from a worksheet created by a chemistry instructor at my institution) is as follows:

Which of the following is the most acidic: $\ce{HBrO3}$, $\ce{HFO3}$, $\ce{HIO3}$, and $\ce{HClO3}$?

The given answer is $\ce{HIO3}$. However, in my understanding, the acidity of oxoacids is driven by bond polarity, which is affected by the electronegativity of atom $\ce{Y}$, as this question suggests. For oxoacids of the same structure, only differing in the atom $\ce{Y}$, I believed that the acid strength increases as atom $\ce{Y}$ increases in electronegativity. Thus I believed that $\ce{HFO3}$ would have the greatest relative acid strength. As a matter of fact, I believed that $\ce{HIO3}$ would have the lowest relative acid strength as $\ce{I}$ is the least electronegative of the four $\ce{Y}$ atoms on this list. I would appreciate it if you could help me understand why this anomaly, that $\ce{HIO3}$ is stronger in acidity than $\ce{HFO3}$ contrary to the general trend regarding the electronegativity of $\ce{Y}$, applies in this case.

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    $\begingroup$ OK, first things first. How in the world is there HFO3? $\endgroup$ – Oscar Lanzi Feb 9 at 1:39
  • $\begingroup$ Oh, it did not occur to me that HFO3 might not exist. I still think, though, that if one was to go by the electronegativity-acid strength trend, then HIO3 would actually be the weakest out of all of the acids listed, as Br, Cl and F are all more electronegative than I. $\endgroup$ – naoya2020 Feb 9 at 1:47
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    $\begingroup$ There is some link between this acidity trend for oxoacids of the halogens and the acidity trend of the hydrohalides HX. The notion of charge dispersal over large atomic volumes being more important than electronegativity considerations remain relevant. $\endgroup$ – Tan Yong Boon Feb 9 at 5:23
  • $\begingroup$ You compare electronegativity when you have similar sized atoms. $\endgroup$ – vanshita rawat Feb 9 at 11:34

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