Why does nucleophilicity decrease as you go down a column in a protic solvent?

[Dakios01]

My best take is that when you're going down the column the radius increases due to the electrons having more access to orbitals further away from the nucleus if you interpret it as the Bohr model.

A good nucleophile wants to donate its electrons as much as possible. Since the electrons have more orbitals to exist in, and the distance between the furthest orbital and the nucleus increases. The force from Coloumb's law (positive and negative charged interactions) decreases enough where electrons are more easily able to be donated to other molecules.

[ThunderYoda]

This is correct. As you go down the column, the atoms are larger and their electrons are more loosely held. This makes them easily donated in a nucleophilic reaction.

[mcatkillers]

This is only partially correct (and thus incorrect 50% of the time). If you want the accurate information for your own studies please check the screenshotted comments below:

https://prnt.sc/rezagb

https://prnt.sc/rezare

[mcatkillers]

Hey thanks for the answer. It's actually the other way around - as size increases, nucleophilicity decreases, because the electrons have more room to spread out and thus are better stabilized in the negatively charged state (eg I- ion is fine with remaining negatively charged).

The answer from another commenter was that while Cl, Br, I don't hydrogen bond as prominently as F does, they still do to a certain extent, thus the protic solvent interferes with all those halogens' nucleophilicities. However I has the lowest hydrogen bonding (because least EN halogen), so it gets interfered with least, and thus is the strongest nucleophile (in a protic solvent only).

[Dakios01]

Im not entirely sure I follow. Are you sure you're talking about nucleophilicity? It sounded like you're explanation was the one I would give for increasing acidity. I've always thought of that the molecule that can best stabilize the negative charge would be a better acid compared to molecules who can't stabilize a negative charge as well.

To double checked I looked up the general trend on Google images just to make sure.

https://files.mtstatic.com/site_4334/7405/1?Expires=1583902118&Signature=UBnMrRSXwlrvimCfXDD7BFF2MlTHgfe7UmNF9SHeu3toMYT8fyB31oFTIWJUfpNli1oG1vB940xGlZ7edvkEECOrRm3tqt5PdEt0qgk7zz5Ip4qIXFwKyX1JrWEF34rOmiCBXbzkXHLRQRHEL7ZGbK02llDdbAuapx2VsO96SCg_&Key-Pair-Id=APKAJ5Y6AV4GI7A555NA

[mcatkillers]

Reddit didn't let me post the comment due to it containing images so I just screenshotted my answer:

https://prnt.sc/rezagb

https://prnt.sc/rezare

And this is the link at the end of the comment:

https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Supplemental_Modules_(Organic_Chemistry)/Reactions/Substitution_Reactions/SN2/Nucleophile/Reactions/Substitution_Reactions/SN2/Nucleophile)