# Why is sulfide a better nucleophile than the structurally equivalent alkoxide?

Certainly, third period sulfide anion is LARGER than second period oxide anion....and on this basis its electron cloud is more diffuse, and more polarizable, and thus more likely to participate in a substitution reaction. And CLEARLY, sulfide is a WEAKER base than hydroxide....and clearly the $p {K}_{a}$ values of water and ${H}_{2} S$ reflect this.