Lewis Concept of Acids and Bases

The Brønsted-Lowry concept does not depend on the nature of the solvent (a shortcoming of the Arrhenius concept removed). However, like Arrhenius concept it does not explain the acidity of the substances which do not have a hydrogen. atom (AlCl₃) and the basicity of the substances devoid of a OH group (Na₂CO₃).

G.N.Lewis proposed (1923) a another concept of acids and bases that includes such substances also. An acid is defined as any atom, molecule or ion that can accept an electron pair from any other atom, molecule or ion. A Lewis base is defined as any atom, molecule or ion that can donate a pair of electrons.

For example,

AlCl₃ + NH₃ → Cl₃Al-NH₃

In the above example, AlCl₃ is an electron deficient species. It accepts an electron pair from a molecule of NH₃ which has a lone pair of electrons on N atom. Thus, AlCl₃ is a Lewis acid and NH₃ is a Lewis base.