Nucleophilic strength increases down a column of the Periodic Table (in solvents

that can have hydrogen bonds, such as water, alcohols, thio alcohols).

Nucleophilic strength $\mathrm{R}{\mathrm{O}}^{\mathrm{\Theta }}<\mathrm{R}{\mathrm{S}}^{\mathrm{\Theta }}$ . Base strength $\mathrm{R}{\mathrm{O}}^{\mathrm{\Theta }}>\mathrm{R}{\mathrm{S}}^{\mathrm{\Theta }}$ . Thus, $\mathrm{R}{\mathrm{S}}^{\mathrm{\Theta }}$ is more nucleophilic but less basic than $\mathrm{R}{\mathrm{O}}^{\mathrm{\Theta }}$