How does large atom size, its polarizability, and acid/base strength relate?

[chaser0]

There used to be a section in the BR where they noted how atomic size affect polarizability, and how that caused larger atoms to be more acidic (more diluted electron charge).
But they seemed to have taken this out.

Could anyone help me out with how polarizability related to acidity?

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[syoung]

Well a large atomic size affects polarizability by allowing the negative charge on the conjugate base to be distributed better, hence the acid is overall more acidic.
HI>HBr>HCl>HF is one. This is for the halo acids.

 

[chaser0]

So a larger atom is More polarizable?

 

[MedPR]

So a larger atom is LESS polarizable?

No, larger is more polarizable and more stable. More stable conjugate base = weaker conjugate base = stronger acid. Hence, HI is a stronger acid than HF because I- is more stable than F-.

In a non scientific way, you can remember that HI is more acidic than HF because I- and F- have the same number of electrons, but I- is much, much larger. So if a hydrogen comes along and tries to find an electron to share with I-, not only is I- pretty stable already (and thus less likely to look for someone to share with), I- has its electrons spread over a huge area, so the H has a tough time finding it and therefore is easy to pull away from the I- atom. F-, on the other hand, is small and definitely wants another electron to complete its octet, so its going to hang on to any H it finds.

 

[chaser0]

Ah thanks guys.

I understand how electron diffusion and "difficult target" make an atom more stable as a base and hence a stronger acid.
But how exactly does polarizability affect acidity?

 

[Buttafuoco]

I believe you can think about it in almost the same terms as to why it's polarizable. Fluctuations in electron density allow for sudden instances where there's less electron density to hold a proton. And of course a bigger molecule can spread charge around if it loses a proton, which makes it more stable as an anion.

What's probably much more important, at least in relation to a given atom on a molecule, though, is that valence electrons are held further from the nucleus and thus less tightly on a big atom. That's the main reason why acidity increases for a family as you move down the periods, like how H2S is more acidic than H2O.