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Lewis Acids and Bases

Defines acids and bases in terms of electron acceptors or donors.

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Why are there three definitions of acids and bases?

A Lewis acid is an electron-pair acceptor while a Lewis base is an electron-pair donor. However, there are other definitions for acids and bases such as the Arrhenius acids and bases and the Brønsted–Lowry acids and bases. But why are there three definitions?

Credit: Steven Depolo
Source: http://www.flickr.com/photos/stevendepolo/4078507307/lightbox/
License: CC BY-NC 3.0

Three [Figure1]

The variety of acid-base definitions developed overtime as scientists discovered the true actions of acids and bases. The earliest definition for acids and bases was the Arrhenius definition. Arrhenius acids increase the concentration of H+ ions while Arrhenius bases increase the concentration of OH- ions. The Brønsted–Lowry acids are proton-donors while Brønsted–Lowry bases are proton-acceptors. Finally, the Lewis acids and bases and their definitions were developed and are currently the most widely accepted. These three definitions can be pictured as concentric circles: The Arrhenius theory is the innermost circle, the larger circle is the Brønsted–Lowry theory which includes some of the Arrhenius concepts, and the largest, broadest circle is the Lewis theory, which includes the theories of the other two definitions.

Creative Applications

  1. Why are Lewis acids in lower energy levels while Lewis bases are in higher energy levels? (use the first resource link if necessary)
  2. What are HOMOs (highest occupied molecular orbitals) and LUMOs (lowest occupied molecular orbitals) in terms of Lewis acids and bases? (use the second resource link if necessary)
  3. Is there any atom or molecule that can act as either an acid or a base in chemical reactions? If so, what is it?


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  1. [1]^ Credit: Steven Depolo; Source: http://www.flickr.com/photos/stevendepolo/4078507307/lightbox/; License: CC BY-NC 3.0

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