Order of Nucleophilicity I know this question has been asked many times but I need to confirm this doubt which has been pestering me for the past few days.

Suppose in an $\mathrm{S_N2}$ reaction (in polar aprotic solvent) we have two nucleophiles:


ethoxide $(\ce{EtO-})$ ion;
ethyl thiolate $(\ce{EtS-})$ ion.


Now, since we know fluoride $(\ce{F-})$ is stronger nucleophile than iodide $(\ce{I-})$ in polar aprotic solvent, i.e. nucleophilicity parallels basicity.
We should expect the alkoxide ion to be more nucleophilic than thiolate since alcohols are weaker acids than thiols.

But my book says the opposite. Is this a exceptional case? If so, could it be predicted in advance that thiolates are better than alkoxide even though fluoride is stronger than iodide?

Any help would be much appreciated.
Please remember that nucleophilic substitution at a saturated carbon atom does not directly correlate with pKa. 

The pKa of a substance (in the Bronsted-Lowry definition) relates to the substance's ability to have strong interactions with $\ce{H^+}$. Thus, this interaction is mainly electrostatic in nature; since $\ce{H^+}$ is a very small, weakly polarizable ion, according to hard-soft acid base theory, strong interactions with this ion are largely electrostatic in nature, i.e. proportional to charge density.

However, nucleophilic substitution at a saturated carbon atom occurs mainly through orbital interactions, not through electrostatic interactions. Namely, the highest occupied molecular orbital (HOMO) of the nucleophile interacts with the LUMO (sigma antibonding orbital) of the substrate (usually alkyl halide). 



Since higher energy HOMOs interact better with LUMOs, molecules or ions with higher energy HOMOs are more nucleophilic.

With the specific case that you gave ($\ce{EtO-}$ and $\ce{EtS-}$), consider that sulfur is a larger atom, and the electrons in HOMO in $\ce{EtS-}$ are going to be less tighly held by nuclear charge than electrons in the HOMO in $\ce{EtO-}$. Alternatively, you can consider that the highest energy level orbital in S is 3p while that of oxygen is 2p --> electrons are more tighly held and lower in energy in $\ce{EtO-}$ --> $\ce{EtO-}$ is less nucleophilic.



Thus, if you consider the relative energy levels HOMOs in related molecules and ions, you get the general trend below:



References

Clayden, J., Greeves, N., Warren, S. Organic chemistry, 2nd ed.; Oxford University Press: New York, 2012.