The Lewis theory of acids and bases, named after the chemist G.N. Lewis, seeks to clarify the way two classes of chemicals — acids and bases — interact with one another. A Lewis acid is defined as a substance that accepts a pair of electrons in a chemical reaction, while a Lewis base is a substance that donates a pair of electrons. Although not the most widely taught understanding of acids and bases, the Lewis definitions are broad and useful in studying many different types of reactions.
Chemical compounds are made up of atoms bonded together to form a molecule. Each of these atoms consists of a positively-charged nucleus and a certain number of electrons, which carry a negative charge. The electrons circle the nucleus in a series of orbitals, or different energy levels.
If an atom is deficient in electrons — or, in other words, if it has space in its orbitals to fill — it may attract electrons from another atom, connecting the two substances through a chemical bond in which the electrons are shared. A Lewis base is the compound that donates a pair of electrons in this process. The electrons in a bond are often unevenly shared between two atoms, so the base can effectively "lose" them to the other compound.
The Lewis theory is useful in explaining why certain chemical reactions occur. For example, carbon dioxide (CO2) reacts with water (H2O) to form carbonic acid (H2CO3). One carbon atom bonded to two oxygen atoms makes carbon dioxide, while two hydrogen atoms and one oxygen atom make water.
In the course of the reaction, the water’s oxygen atom donates a pair of electrons to the carbon atom of carbon dioxide, forming a bond between the two molecules. The water is a Lewis base in this situation, since it is the electron donor. Carbon dioxide accepts the electrons into its structure, so it is the Lewis acid.
The above reaction could not be described by the other popular theory of acid and bases, the Bronsted-Lowry model, because it does not involve a proton transfer. The Bronsted-Lowry theory, unlike the Lewis theory, is based on proton donation. In this system, the acid is the compound that donates a positively charged particle, called a proton, while the base is the proton acceptor. While this model is useful in describing many interactions, the Lewis theory is sometimes preferred due to its wider scope.
Lewis bases are classified into different types based on which electron orbital donates the pair of electrons. Often, a Lewis base will be an anion — a substance or compound that carries a negative charge due to an overabundance of electrons. Ammonia is considered a Lewis base, as are water and certain organic compounds.