Lewis acids are electron pair acceptors; Lewis bases are electron pair donors. Cations or electron deficient species are examples of Lewis acids; anions or molecules with non-bonding lone pair electrons are potential Lewis bases.

Carbon can be defined as a donor or acceptor of electrons. Cations of carbon (carbocations), or polar bonds with a partial positive charge, are electrophilic centers (left). Carbanions, heteroatoms (ones other than C and H; such as N, and O, halogens) and pi electrons are capable of donating electrons in a nucleophilic sense (right).