A nucleophile is a molecule or ion that donates a pair of electrons to form a new covalent bond. It is strongly attracted to a region of positive charge. Common nucleophiles include hydroxide ions; due to the negative charge on the ion, or ammonia; due to the electronegative nitrogen.

An electrophile is a molecule or ion that accepts a pair of electrons to make a new covalent bond. any electron deficient molecule can act as an electrophile. Common electrophiles include hydrogen ions in acid, or a carbocation (CH2+)