We have seen that a molecule or an ion having a lone pair of electrons can act as an electron pair donor in a reaction with either a proton or an electrophilic carbon atom. In both cases, a new bond is formed.

When the bond is to a proton, the electron pair donor has reacted as a base. When the bond is to a carbon atom, the electron pair donor has reacted as a nucleophile.

Chemists use the terms basicity and nucleophilicity when referring to the tendency of an electron pair donor to act as a base or a nucleophile in a reaction.

Basicity is a measure of the tendency of an electron pair donor to react with a proton. Nucleophilicity is a measure of how readily (how fast) a nucleophile attacks an electrophilic carbon atom.

We use equilibrium constants (K) when comparing molecules or ions in terms of their basicity, and thus basicity is an equilibrium (thermodynamic) property.

When comparing molecules or ions in terms of their nucleophilicity, we are comparing them in terms of the rates at which they attack an electrophilic carbon, and thus nucleophilicity is a kinetic property.

What can make things a little confusing is that a molecule or an ion with a lone pair can act as either a base or a nucleophile. For example, the methoxide ion, CH₃O‾, is both a strong base and a good nucleophile.

Thus, it is important to understand not only factors affecting basicity but also those affecting nucleophilicity.