Lewis Base: Definition & Examples

Lesson Transcript
Instructor: Nissa Garcia

Nissa has a masters degree in chemistry and has taught high school science and college level chemistry.

Every day, we encounter solutions that can be classified as bases. What exactly is a base? A scientist named Gilbert N. Lewis came up with a definition of a basic solution, referred to as a Lewis base, which we will discuss in this lesson.

Lewis Base

What makes a solution a base? Bases are solutions that are typically described as bitter and slippery or slimy feeling. However, in an acid-base reaction, how can we tell which chemical compound is a base? Different scientists have come up with some definitions of a base, including Gilbert N. Lewis in 1923. His definition is now referred to as a Lewis base.

A Lewis base is an electron-pair donor. Because the Lewis base donates electrons, we can also say that it is a nucleophile, which is a substance that behaves as an electron donor. So, when we look at an acid-base reaction, the Lewis base donates a pair of nonbonding electrons to the acid. We can think of nonbonding electrons as loose change. We can give it away or donate it easily. In the following chemical reaction, the Lewis base (shown in red) donates its pair of nonbonding electrons to the acid. As a result, it forms a bond with the acid.

A Lewis base is an electron-pair donor
Chemical reaction

The bond formed between A and B indicates that A and B are now sharing a pair of electrons, because the single line between A and B is equivalent to two electrons.

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  • 0:00 Lewis Base
  • 1:15 Compounds
  • 2:22 Anions
  • 2:35 Acid-Base Reacions
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Let's go over some examples of compounds and ions that can act as Lewis bases, beginning with the compounds. When we look at a chemical compound, how can we recognize it as a Lewis base? Remember that Lewis bases donate electrons, which means that they have an excess of electrons or have nonbonding electrons. So, compounds that behave as Lewis bases have nonbonding electrons, as shown in the below image. Water (H2O) has two nonbonding pairs, and ammonia (NH3) has one nonbonding pair.

Compounds with nonbonding pairs can act as Lewis bases
Lewis Bases

Phosphine, or phosphorus containing organic compounds, and amines typically act as Lewis bases because they also have nonbonding pairs. Amines are organic compounds with a lone pair of electrons, so they contain carbon (C). We can think of them as relatives of ammonia (NH3), so they also contain nitrogen (N). Phosphine is a compound with the chemical formula PH3. Phosphorus-containing organic compounds, which are compounds with phosphorus (P) and carbon (C), also act as Lewis bases.

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