10.1 Concepts of Acids and Bases (Lewis Concept)

10.1.3 Lewis Concept of Acids and Bases

The Arrhenius and Bronsted- Lowry concepts of acids and bases are limited to substances which contain protons. G.N. Lewis (1923) proposed a more general and broader concept of acids and bases. According to this concept:

An acid is a substance (molecule or ion) which can accept a pair of electrons, while a base is a substance (molecule or ion) which can donate a pair of electrons.

For example, a reaction between ammonia and boron trifluoride takes place by forming a coordinate covalent bond between ammonia and boron trifluoride by donating an electron pair of ammonia and accepting that electron pair by boron trifluoride.

The cations (proton itself or metal ions) act as Lewis acids. For example, a reaction between ${\mathrm{H}}^{-}$$\mathrm{H}^{- }$ and ${\mathrm{N}\mathrm{H}}_3$$\mathrm{NH}_3$ , where ${\mathrm{H}}^{-}$$\mathrm{H}^{- }$ acts as an acid and ammonia as a base.

The product of any Lewis acid- base reaction is a single specie, called an adduct. So, a neutralization reaction according to Lewis concept is donation and acceptance of an electron pair to form a coordinate covalent bond in an adduct.

Acids are electron pair acceptors while bases are electron pair donors. Thus, it is evident that any substance which has an unshared pair of electrons can act as a Lewis base while a substance which has an empty orbital that can accommodate a pair of electrons acts as Lewis acid. Examples of Lewis acids and bases are given below: