Acids and Bases

The Brønsted-Lowry acid-base definition, while broader than the Arrhenius definition, is still limited to hydrogen-containing compounds, and is dependent on a hydrogen ion (that is, a proton) transferring from one molecule to another. Ultimately, a definition of acid and base that is completely independent of the presence of a hydrogen atom is necessary.

Such a definition was provided in 1923 by American chemist Gilbert N. Lewis (1875-1946). Instead of focusing on protons, Lewis's definition focuses on electron pairs. Since all compounds contain electron pairs, the Lewis definition is applicable to a wide range of chemical reactions.

A Lewis acid is defined as the reactant in a chemical reaction that accepts an electron pair from another reactant. A Lewis base is defined as the reactant in a chemical reaction that donates an electron pair to another reactant. Like the Brønsted-Lowry definition of acids and bases, the Lewis definition is reaction-dependent. A compound is not an acid or base in its own right; rather, how that compound reacts with another compound is what determines whether it is an acid or a base.

To show that the Lewis definition is not in conflict with previous definitions of acid and base, consider the fundamental acid-base reaction of H⁺ with OH^- to give H₂O. The oxygen atom in the hydroxide ion has three unbonded electron pairs around it, and during the course of the reaction one of those electron pairs is "donated" to the hydrogen ion, making a chemical bond. Thus, OH^-is the electron pair donor and the Lewis base, whereas H⁺ is the electron pair acceptor and, therefore, the Lewis acid. These assignments are consistent with both the Arrhenius definition and the Brønsted-Lowry definitions of acid and base.

However, the Lewis acid/base definition is much broader than the previous two definitions. Consider the reaction of BF₃ and NH₃ in the gas phase, in which NH₃ is donating an electron pair to the BF₃molecule:

Compounds like F₃BNH₃ are stable and can be purchased as solutions in organic solvents or even as pure compounds. In the above chemical reaction, BF₃ is accepting an electron pair and therefore is the Lewis acid; NH₃ is donating the electron pair and so is the Lewis base. However, in this case neither the Arrhenius definition nor the Brønsted-Lowry definition are applicable. Therefore, while the Lewis acid/base definition includes acids and bases from the other two definitions, it expands the definitions to include compounds that are not otherwise considered "classic" acids and bases.