A nucleophile is a species which can donate a lone pair of electrons to an electron deficient species, known as an electrophile. By doing this, a chemical bond is formed.

An example of a reaction involving a nucleophile would by nucleophilic substitution. As the nucleophile attacks to form a bond, a leaving group departs, with the breaking of a bond.

Nucleophiles often have lone pairs or double bonds. Some good examples of nucleophiles are water, ammonia, the hydroxide ion and the thiocyanate ion.