The big picture gets bigger

Ideas in science never stay static. One discovery builds on another. Our concepts of acids and bases have grown from the fundamental ideas of Arrhenius to Brønsted-Lowry to Lewis. Each step adds to our understanding of the world around us and makes the “big picture” even bigger.

Lewis Acids and Bases

Gilbert Lewis (1875–1946) proposed a third theory of acids and bases that is even more general than either the Arrhenius or Brønsted-Lowry theories. A Lewis acid is a substance that accepts a pair of electrons to form a covalent bond. A Lewis base is a substance that donates a pair of electrons to form a covalent bond. So, a Lewis acid-base reaction is represented by the transfer of a pair of electrons from a base to an acid. A hydrogen ion, which lacks any electrons, accepts a pair of electrons. It is an acid under both the Brønsted-Lowry and Lewis definitions. Ammonia consists of a nitrogen atom as the central atom with a lone pair of electrons. The reaction between ammonia and the hydrogen ion can be depicted as shown in the Figure 1.

Figure 1. Reaction between ammonia and proton.

The lone pair on the nitrogen atom is transferred to the hydrogen ion, making the NH₃ a Lewis base while the H ⁺ is a Lewis acid.

Some reactions that do not qualify as acid-base reactions under the other definitions do so under only the Lewis definition. An example is the reaction of ammonia with boron trifluoride.

Figure 2. Ammonia and boron trifluoride.

Boron trifluoride is the Lewis acid, while ammonia is again the Lewis base. As there is no hydrogen ion involved in this reaction, it qualifies as an acid-base reaction only under the Lewis definition. The Table below summarizes the three acid-base theories.

Glossary