[i love Pharmacology]

Please can some help me with this.

I'm  studying SN2 and SN1 reaction and the concept of nucleophile and leaving group confused me. in my text book(organic chemistry written by John Mcmurry),it says that the nucleophilicity of Iodide  is stronger than  Chloride  because Iodine's electronegativity is weaker than chlorine, makes Iodide easier  to react with others.
it also says that iodide is a better leaving group than chlorine because iodide is more polarizable than chlorine,makes Iodide stable.thus,iodide is a good leaving group.
But HOW CAN IODIDE BE A GOOD NUCLEOPHILE AND GOOD LEAVING GROUP  SIMULTANEOUSLY?Good nucleophile means it can easily ATTACH to substrate.Good leaving group means it can easily FALL from substrate.They are diametrically opposed

[fledarmus]

No, they aren't diametrically opposed, they are independent considerations. It can make it a little difficult to do direct comparisons though, when your nucleophiles can also function as leaving groups. It can also be a nightmare when you are trying to control chirality.

Frequently, when you are trying to do an substitution reactions on alkyl chlorides, with a rather weak nucleophile, you will find that either the reaction is very slow or the elimination reaction prevails. Adding a small amount of iodide ion (usually as sodium or potassium iodide) may allow you to drive the reaction to the substitution product or give you faster reaction rates. The iodide is a strong nucleophile and can substitute for the chloride, leaving the alkyl iodide which is a better leaving group than the chloride and can now be attacked by the weaker nucleophile you wanted there to begin with.