Definition of a Lewis Acid & Base

lewis acid and base

Core Concepts

So what is a Lewis acid and a Lewis base? In this tutorial, you will learn how to identify Lewis acids and bases, as well as some examples of each of them. In addition, you will put the information together to form the Lewis theory for acid-base chemical reactions!

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Lewis Theory of Acid- Base Reactions

History of the Lewis acid-base theory

Gilbert N. Lewis, a physical chemist, from the University of California, proposed the Lewis acid-base theory in 1916. G.N. Lewis never won the Nobel Prize, and some people speculate he was blocked by someone on the prize committee for personal reasons. He was found dead in his lab after working with hydrogen cyanide, probably of natural causes, not from the cyanide.

What is a Lewis Acid?

Lewis acid definition: A Lewis acid is any molecule or compound that contains an empty orbital that can accept a pair of non-bonding electrons; it can also be known as an electron pair acceptor or electrophile.

A Lewis acid is a substance that can accept a pair of electrons to form a new chemical bond. This process is called electron-pair acceptance, and it results in the formation of a new species called a Lewis acid-base complex. The Lewis definition of an acid is based on the concept of electron-pair acceptance, and it is a more general and inclusive definition than the traditional Bronsted-Lowry definition, which only considers acids as substances that donate protons. For example, due to their positive charge, cations are all able to accept electrons, making them Lewis acids. The Lewis concept is widely used in chemistry to explain the behavior of acids and bases in a variety of chemical reactions.

For example, a Lewis acid can accept a pair of electrons from a Lewis base to form a new bond, or it can accept a pair of electrons from a molecule, such as a molecule of water, to form a hydrated complex. The Lewis definition of an acid is an important concept in chemistry, and it is used to explain many of the reactions and properties of acids and bases.

What is a Lewis Base?

Lewis base definition: A Lewis Base is any molecule or compound that has a filled orbital which donates a pair of non-bonding electrons; it can also be known as an electron pair donor or nucleophile.

The Lewis Theory

Putting together the two concepts above gives us the Lewis Theory of acid-base reactions. The base donates a pair of electrons and the acid accepts said electrons. This acid and base pair move electrons between themselves so that there are individual oxidation number changes but no net change. The electrons can travel from one molecule to the other or they can be shared between the molecules.

The Lewis theory is extremely similar in concept to oxidation-reduction (redox) reactions. This is because in redox reactions, there is movement of electrons from one atom to another, resulting in changes in oxidation numbers, which is essentially the same thing!

Is a Lewis Acid a proton donor?

Not exactly. A Bronsted-Lowry acid is a proton donor. A Lewis acid is defined as an electron acceptor, and a Lewis base is defined as a electron donor. All Bronsted-Lowry bases (proton acceptors) are Lewis bases, but not all Lewis bases are proton acceptors. The Lewis definition of an acid and base is the broadest of the three definitions.

Examples of Lewis Acids

  • H+
  • Cu+2
  • Fe+2 & Fe+3
  • BF3
  • AlF3, AlCl3
  • SiBr4
  • CO2
  • SO3

Examples of Lewis Bases

  • H2O
  • NH3
  • OH
  • CN
  • CH3COO
  • CO

Further Reading