A nucleophile donates an electron pair to an electrophile to form a chemical bond during a reaction. All molecules or ions with a free pair of electrons or at least one pi bond can act as nucleophiles. As nucleophiles donate electrons, they are by definition Lewis bases. The most common features of a nucleophile are electrons that are available to be shared as a formal negative charge, a partial negative charge, usually in conjunction with a polar bond, a pi bond (double bond) or lone pairs. A Lewis base in the reaction shares an electron pair with a Lewis acid to make a new covalent bond. A Lewis base is an electrophile (a molecule that loves electrons). One of the important distinctions to make after saying that all nucleophiles are Lewis bases in that they donate a lone pair of electrons is that a “base” (or, “Brønsted base”) is a nucleophile when it’s forming a bond to a proton (H+). A nucleophile attacks any atom other than hydrogen. Reactions where nucleophiles attack carbon-based electrophiles are significantly more sensitive to steric effects, because empty orbitals on carbon are not as accessible. The medium (solvent) in which a reaction takes place can influence the rate of a reaction. Specifically, the solvent can greatly reduce the nucleophilicity of some Lewis bases through hydrogen bonding.