Lewis acid-base theory

What is the Lewis acid-base theory?

In 1923, Lewis proposed a theory to explain the behavior of acidic and basic substances. This is because previous theories, such as Brønsted-Löwry, defined them in terms of proton acceptance/transfer (H).

However, substances were known which, lacking hydrogen in their molecule, such as SO3 and CO2, behave as acids. Thus these compounds, in the presence of basic oxides such as CaO and K2O and in the absence of solvent, so there is no proton transfer, react to form a salt.

SO3 + K2O -> SO4K2

In view of these facts, Lewis established a new concept of acid and base:

  • Acid: substance (atom, molecule or ion) capable of accepting the sharing of a pair of electrons (in a coordinated covalent bond) from a base.
  • Base: substance (atom, molecule or ion) capable of yielding a pair of electrons in a coordinated covalent bond.

Therefore a Lewis acid must have at least one free orbital and a base one unshared pair of electrons.

This electronic theory of Lewis is the most general, and naturally, it includes the two previous ones: Arrhenius acid-base theory and the Brønsted-Löwry acid-base theory.

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What does Lewis say about acids and bases?

Lewis proposed a generalized definition of acid-base behavior in which acids and bases are identified by their ability to accept or donate an electron pair and form a coordinating covalent bond.

What is a Lewis acid examples?

Since the proton, according to this definition, is a Lewis acid (it has an empty 1s orbital in which to house the electron pair), all Brønsted-Lowry acids are Lewis acids. Examples of Brønsted and Lowry acids: HCl, HNO3, H3PO4. Examples of Lewis acids: Ag+, AlCl3, CO2, SO3.