My teacher told me that to compare nucleophilicity, check the size of the attacking atom. Bigger the attacking atom more the nucleophilicity. But while reading Solomon, I found out that the order of nucleophilicity of halides in polar aprotic solvents is
$$\ce{F-}>\ce{Cl-}>\ce{Br-}>\ce{I-}$$
Which is opposite to the order expected from my teacher's explanation. How can we explain this order. Can I conclude that my teacher's explanation is valid only in protic solvents, and in aprotic solvents, more the basic strength implies more nucleophilicity, or this is just an exception?
