Nucleophilic strength depends on the electronegativity of the atom. Lower the

electronegativity of an atom, larger the nucleophilic strength of an atom and more readily it will donate electrons to the electrophile ( deficient) species. Since, ectronegativity decreases down the group, thus nucleophilic strength increases down the group. Hence, is more nucleophilic than due to less electronegativity of S than O . On the other hand, basic strength of an atom depends upon the tendency to accept a proton or to denote electrons more readily. Since, the size of is more than that of O hence the pair is more dispersed on it and hence less available for the donation. Hence, is a stronger base than .