A nucleophile is a chemical species that donates an electron pair to form a chemical bond. Nucleophiles are typically negatively charged or neutral molecules with lone pairs of electrons. They play a crucial role in various chemical reactions, particularly in nucleophilic substitution and addition reactions. In these reactions, nucleophiles attack positively charged or electron-deficient centers, such as carbon atoms in organic compounds. Common examples of nucleophiles include hydroxide ions (OH⁻), ammonia (NH₃), and halide ions (Cl⁻, Br⁻, I⁻). Their ability to donate electrons makes them essential in organic chemistry and biochemistry.