[imperial]

The fact that I->Cl->Br- leads me to think that good leaving groups can hold extra e- because they have bigger outer shells and hence more room to hold e-. am I right? If not, can someone offer a valid explaination? Cheers

[renge ishyo]

That's not a bad explanation. The larger size allows for the charge to be stabilized, because the electron has more "room" to move around. In other words the electron is delocalized to a greater extent on a larger atom in comparison to a smaller one.