[nonlinear]

Is it too simplistic to say that the stability/reactivity of the lone pairs on the nucleophile will determine its nucleophilicity?

I know that in terms of nucleophilicity, Cl- < Br - < I- ; this confuses me because I- stabilizes the negative charge better than Cl- does because it is a larger atom; doesn't that mean that the lone pairs on the I- are less reactive as well? Or does stabilization of negative charge by the entire atom NOT translate into lone pair reactivity? Is this trend so because the electronegativity of Cl- draws the electrons in or something?

Could somebody clarify this for me? Thank you!

[Polytriazole]

Nucleophilicity is a weird property, it's a combination of a couple different factors.  Stability of the negative charge is a big factor, which makes iodide a better leaving group during nucleophilic attack than chloride.  However, iodide is also a better nucleophile than chloride because it has such a large electron cloud.  Because the outer electrons are so far from the nucleus in iodide, they are not as strongly bonded to that particular atom and are more vulnerable to forming a new bond with another atom. 

I've also heard it explained in terms of the "squishiness" of the large electron cloud, allowing the iodide to squeeze in or deform to get proper HOMO and LUMO alignment for bonding.  It's weird.

[pfnm]

Is it too simplistic to say that the stability/reactivity of the lone pairs on the nucleophile will determine its nucleophilicity?

I know that in terms of nucleophilicity, Cl- < Br - < I- ; this confuses me because I- stabilizes the negative charge better than Cl- does because it is a larger atom; doesn't that mean that the lone pairs on the I- are less reactive as well? Or does stabilization of negative charge by the entire atom NOT translate into lone pair reactivity? Is this trend so because the electronegativity of Cl- draws the electrons in or something?

Could somebody clarify this for me? Thank you!

I believe that nucleophilicity is dependant on the solvent.

In a polar protic solvent, I- is a better nucleophile because it is large. It is incompletely solvated. A smaller atom like Cl- is more fully 'covered':

I think about it like this: the smaller the anion, the more easily protic solvents can crowd its electrons and prevent them from bonding. The larger the anion, the more difficult it is for the solvent to reduce the charge.

This site had a good explanation:
http://chemwiki.ucdavis.edu/Organic_Chemistry/Reactions/SN2_Reaction/Nucleophile#Increasing_Atomic_Size_Increases_Nucleophilicity

One of my books (Organic Chemistry Instant Notes by G. Patrick) says that, in polar aprotic solvents,  relative nucleophilicity changes to match their basicity: in DMSO for example
Cl- > Br- > I-

Would this be incorrect? What would explain it?  I thought the lines, that larger the anion, as Polytriazole said, the higher the affinity of the electrons for the nucleus of a carbon atom....(and also that perhaps the more electronegative atoms like F-, and Cl- hold their electrons more tightly than I-)

But that would be opposite of the trend for nucleophilicity in polar aprotic solvents given in the Patrick book.