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21.7: Lewis Acids and Bases

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  • 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 below.

    Figure 21.7.1: Reaction between ammonia and a proton. (CC BY-NC; CK-12)

    The lone pair on the nitrogen atom is transferred to the hydrogen ion, making the \(\ce{NH_3}\) a Lewis base while the \(\ce{H^+}\) 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 21.7.2: Ammonia and boron trifluoride. (CC BY-NC; CK-12)

    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.

    Table 21.7.1: Acid-Base Definitions
    Type Acid Base
    Arrhenius \(\ce{H^+}\) ions in solution \(\ce{OH^-}\) ions in solution
    Brønsted-Lowry \(\ce{H^+}\) donor \(\ce{H^+}\) acceptor
    Lewis electron-pair acceptor electron-pair donor


    • Lewis acids and bases are defined.
    • Examples of Lewis acids and bases are given.


    • CK-12 Foundation by Sharon Bewick, Richard Parsons, Therese Forsythe, Shonna Robinson, and Jean Dupon.