How does electronegativity affect the strength of an acid?

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Nov 5, 2015


Two almost similar factors work behind this : 1. Across a row in periodic table, acidity increases with the increase in electronegativity. But as we go 2. down a group the periodic table, acidity increases with the decrease of electronegativity.


The first point makes pretty sense. But the second one is the confusing one. A reason behind this phenomena is the small size of elements in the top of a group of periodic table. As we go down a group, size of atom increases. So what's the problem with the smaller atoms? The problem is, their bond length is short too. So it is hard for them to dissociate in water, which doesn't seem to be a problem for the larger atoms down the group.

Another reason is the stability of the ion. For example, HF is an acid, it dissociates in water and gives a proton. In this process, #F^-# ion is also formed. But fluoride ion is a very small ion with relatively denser charge which makes it an unstable ion. That's why HF is weaker acid than HCl and HI though it has a greater electronegativity. Hope this picture will help.
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