Main Characterstics or Lewis/Bronsted Definition

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Introduction to Acids and Bases in Organic Chemistry

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Key Questions

  • A Lewis acid is a molecule that accepts an electron pair, whereas a Lewis base is a molecule that donates an electron pair.

    The implications of this definition are much wider than more traditional definitions.

    For instance,

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    ammonia is a Lewis acid and boron trifluoride is a Lewis base in this example.

  • Both definitions are correct. Neither definition is wrong.

    The definitions are related, but they refer to different phenomena.

    A Brønsted acid is a proton donor. A Brønsted base is a proton acceptor.

    A Lewis acid is an electron pair acceptor. A Lewis base is an electron pair donor.

    If you want to discuss a reaction in terms of proton transfers, you use the Brønsted definitions.

    If you want to discuss a reaction in terms of electron pair transfers, you use the Lewis definitions.


    The fundamental concept of this theory is that when an acid and a base react with each other, the acid forms its conjugate base, and the base forms its conjugate acid by exchange of a proton (the hydrogen cation, or H+). This theory is a generalization of the Arrhenius theory.