Ranking Nucleophiles: Understanding Reactivity and Polarizability

[AsuraSky]

One the practice problems i was doing in preparation for my exam was to rank the following nucleophiles in the order of decreasing reactivity, best nucleophile first.

Nucleophiles: CH₃OH, Br^-, NH₃, F^-, CH₃S^-

The answer was CH₃S^->Br^->NH₃>F^->CH₃OH

What I don't understand is why CH₃S^- is a stronger nucleophile than the bromide ion, shouldn't bromide be stronger because it's a bigger atom and thus have more polarizability?

 

[Trave11er]

You sure it is a good row? It seems that amonia should be a better nucleophile than Br-?