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20.6: Lewis Acids and Bases

Difficulty Level: At Grade Created by: CK-12

Lesson Objectives

The student will:

  • define a Lewis acid and a Lewis base.
  • identify Lewis acids and bases in equations.


  • Lewis acid
  • Lewis base


In the early 1930s, Gilbert Lewis saw the need for an even more general definition for acids and bases. Using the work of Brønsted and Lowry, he saw that in some cases, the acids may not have protons to donate but may still be able to donate an atom to the electron pair of a base. Most chemists today use the Brønsted-Lowry definitions of acids-bases, except for specialized situations that require the Lewis definitions.

Lewis Acids

Lewis defined an acid as a substance that accepts a pair of electrons from another substance. Therefore, Lewis acids must have room in their structure to accept a pair of electrons. Remember that each central atom can hold eight valence electrons. What this means is that if the atom has six or less valence electrons, then it can accept one more pair. Look at the structures below. Notice how, in each case, there is room to accept a pair of electrons.

The Lewis acid will accept the electron pair in order to form a bond. The bond that forms between the two atoms will be covalent. Remember that covalent bonds are formed when electrons are shared between two atoms.

Lewis Bases

A Lewis base is an electron-pair donor. These electrons are not transferred, but rather shared covalently to complete the acid/base reaction. If we look at the example of bases in the figure below, we can see that each of them has a lone pair of electrons available to donate.

This video contains textbook definitions of Arrhenius, Bronsted, and Lewis acids and bases (5b, 5e): http://www.youtube.com/watch?v=jLcoIRUlz20 (3:14).

Lesson Summary

  • Lewis defined an acid as a substance that accepts a pair of electrons from a substance to form a bond.
  • Lewis defined a base as a substance that donates a pair of electrons to a substance to form a bond.

Review Questions

  1. How do the Lewis definitions of acids and bases compare to the Brønsted-Lowry definitions of acids and bases?
  2. In the following reversible reaction, which of the reactants is acting as a Lewis base: \begin{align*}\mathrm{Cd}{^{2+}}_{(aq)} + 4 \ \mathrm{I}{^{-}}_{(aq)} \rightleftharpoons \mathrm{CdI}{_{4}}{^{2-}}_{(aq)}\end{align*}?
    1. \begin{align*}\mathrm{Cd}^{2+}\end{align*}
    2. \begin{align*}\mathrm{I}^-\end{align*}
    3. \begin{align*}\mathrm{CdI}{_{4}}{^{2-}}\end{align*}
    4. None of the above, this is not an acid-base reaction.
  3. Which of the following statements is false?
    1. \begin{align*}\mathrm{NH}_3\end{align*} is a Lewis base.
    2. \begin{align*}\mathrm{B(OH)}_3\end{align*} is a Lewis acid.
    3. \begin{align*}\mathrm{CO}_2\end{align*} is a Lewis base.
    4. \begin{align*}\mathrm{Ag}^+\end{align*} Is a Lewis acid.
  4. Which of the following statements is true?
    1. \begin{align*}\mathrm{NH}_3\end{align*} is a Lewis base.
    2. \begin{align*}\mathrm{B(OH)}_3\end{align*} is a Lewis acid.
    3. \begin{align*}\mathrm{CO}_2\end{align*} is a Lewis base.
    4. \begin{align*}\mathrm{Ag}^+\end{align*} Is a Lewis acid.
  5. Classify each of the following as a Lewis acid or base.
    1. \begin{align*}\mathrm{H}_2\mathrm{O}\end{align*}
    2. \begin{align*}\mathrm{BF}_3\end{align*}
    3. \begin{align*}\mathrm{S}^{2-}\end{align*}
    4. \begin{align*}\mathrm{Cu}^{2+}\end{align*}
    5. \begin{align*}\mathrm{O}^{2-}\end{align*}
  6. Write the balanced chemical equation between \begin{align*}\mathrm{SO}{_{3}}^{2-}\end{align*} and \begin{align*}\mathrm{H}_2\mathrm{O}\end{align*} and label the Lewis acids and bases.
  7. Identify the Lewis acid and Lewis base in the following reaction: \begin{align*}\mathrm{Cu}{^{2+}}_{(aq)} + 6 \ \mathrm{H}_2\mathrm{O}_{(l)} \rightarrow \mathrm{Cu(H}_2\mathrm{O})_6{^{2+}}_{(aq)}\end{align*}.

All images, unless otherwise stated, are created by the CK-12 Foundation and are under the Creative Commons license CC-BY-NC-SA.

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