The concept of nucleophiles and electrophiles is closely related to bases and acids of the acid–base chemistry. Electrophiles are essentially same as Lewis acids while the nucleophiles are equivalent to Lewis bases. In practice, however, acid–base concept involves electron donation to H+ and the terms “electrophile” and “nucleophile” are normally used to indicate electron donation to a carbon atom. Nucleophiles contain a pair of electrons in a high-energy filled orbital that they can donate to an electrophile. As a complement, electrophiles are species with an empty atomic orbital of lower energy that can accept a lone pair (from a nucleophile). Nucleophiles can be neutral (like the amino group) or negatively charged (like –S−, the thiolate ion) while the electrophiles can be neutral or positively charged (like the carbocation or a divalent metal ion). If the electrophile is a proton, then the nucleophile in question is a base, by definition (see previous chapter).