Basicity of Halogen Ions - Point Out My Error

[fuzzish]

This isn't technically a homework problem; I'm reading about Sn2 reactions and the basicity of halogen ions came up and for some reason I can't get my head around this Chem I concept.

Please point out the flaw in this thinking:
1) Fluorine (F) is a small, electronegative atom
2) F will hold onto it's electrons super tightly
3) Thus, it will less readily donate electrons.
4) Conclusion: F is a sucky Lewis base.
--PROBLEM: F- is a stronger base than I-.


I KNOW that I can think about F- vs I- in terms of conjugate acids, or in terms of Bronsted-Lowry bases. I just want to know what's wrong with this thinking in particular, and why this thought process is leading me to the wrong conclusion.

Much thanks! And no, I don't know why I'm making this simple concept harder for myself. But I know that on the test this will be my first thought progression and I WILL mess up if I don't figure out what's wrong with it.

 

[Nino MMP]

The flaw in your thinking is that you are assuming that a smaller, more electronegative atom will hold onto its electrons more tightly. While this is true for neutral atoms, it is not necessarily true for anions. In the case of F- and I-, the size difference between the two atoms is much more significant than the difference in electronegativity, so the larger atom (I-) is actually able to hold onto its electrons more tightly. This means that F- is actually a stronger base than I-.