For eg. in Finklestein reaction, $\ce{NaI}$ (reagent) taken in acetone(solvent) displaces $\ce{SH-}$ ion from alkyl group via SN2 mechanism to form alkyl iodide. But in a separate question we may be asked to write the product formed when $\ce{NaSH}$ reacts with alkyl iodide in the presence of acetone. Since I do not know which of the two is a stronger nucleophile, I will displace $\ce{I-}$ and form $\ce{alkyl-SH}$. But definitely only one of the reactions shall occur. So my question is that is there an order of strength of nucleophilicity (like a metal reactivity series)? I am preparing for JEE Advanced 2015 and am expecting answers which within this scope of knowledge.