In organic chemistry, a nucleophile is an ion or molecule that forms a covalent bond with another molecule by donating an electron pair. As it is electron-rich, a nucleophile looks for molecules that have electron-deficient spots in them. Losing the electrons makes the nucleophile chemically stable, which is determined by the number of electrons present in its valence shell (outermost electron shell). Nucleophiles, along with electrophiles, are involved in many different organic chemistry reactions.

Ideally, the valence shell should be at capacity or have the exact number of electrons required – not less or more. Otherwise, it will be chemically unstable, or constantly on the lookout to borrow or share electrons. The other atom receiving the two electrons is called electrophile. Excess electron presence means a nucleophile is negatively charged. And the electron deficiency makes electrophiles carry a positive charge. Hydroxide ions and ammonia are electrophiles, for example.