> 
> Which is the stronger acid in the given pairs?
> 
> 
> 1. $\ce{HClO3}$ and $\ce{HBrO3}$
> 2. $\ce{HClO2}$ and $\ce{HClO}$
> 3. $\ce{H2Se}$ and $\ce{H2S}$
> 
> 
> 

1. given that the number of oxygen atoms is the same, the most electronegative atom should result in a stronger acid, thus $\ce{HClO3}$ is stronger.
2. $\ce{HClO2}$ is stronger since it has more oxygen atoms.
3. Here's where my logic fails, since the structure is the same (both have $\ce{2H+}$) then it boils down to 2 things to account for: electronegativity and the size of the atom. Going with electronegativity, sulfur is more electronegative thus stronger acid which was my original answer. But, the correct answer is $\ce{H2Se}$ is the stronger acid which implies in this case atomic size ($\ce{Se}$ is larger than $\ce{S}$) outweighs electronegativity. I do not understand why in this case $\ce{H2Se}$ is stronger than $\ce{H2S}$ given sulfur is more electronegative, and most importantly why the same logic used in 1. can't be applied here and how do I discern where to account for atomic size specifically or when to account for electronegativity?

Any detailed-dumbed down explanations on the logic are highly appreciated.