A nucleophile is a chemical species that donates an electron pair to form a chemical bond in a reaction. Nucleophiles are typically negatively charged or neutral molecules with lone pairs of electrons. They are attracted to positively charged or electron-deficient areas in other molecules, making them essential in many chemical reactions, including nucleophilic substitution and addition reactions. Common examples of nucleophiles include hydroxide ions (OH⁻), ammonia (NH₃), and halide ions (Cl⁻, Br⁻, I⁻). Their reactivity is influenced by factors such as charge, electronegativity, and steric hindrance. Understanding nucleophiles is crucial in organic chemistry and reaction mechanisms.