Lewis’s theory of acids and bases explains how they interact with each other. A Lewis acid accepts an electron pair, while a Lewis base donates an electron pair. This theory is useful in explaining chemical reactions, such as the formation of carbonic acid. Unlike the Bronsted-Lowry model, Lewis’s theory is based on electron transfer. Lewis bases are classified based on which electron orbital donates the electron pair. Common Lewis bases include anions, substances, and compounds with an overabundance of electrons.
Lewis’ theory of acids and bases, named after the chemist G. N. Lewis, seeks to clarify how two classes of chemicals — acids and bases — interact with each other. A Lewis acid is defined as a substance that accepts an electron pair in a chemical reaction, while a Lewis base is a substance that donates an electron pair. While not the most taught understanding of acids and bases, Lewis’s definitions are broad and useful for 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. Electrons surround 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 can attract electrons from another atom, linking the two substances through a chemical bond in which the electrons are shared. A Lewis base is the compound that donates an electron pair in this process. The electrons in a bond are often shared unevenly between two atoms, so the base can actually “lose” them to the other compound.
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 produces carbon dioxide, while two hydrogen atoms and one oxygen atom produce water.
During the reaction, the oxygen atom in water donates a pair of electrons to the carbon atom in carbon dioxide, forming a bond between the two molecules. Water is a Lewis base in this situation, as it is the electron donor. Carbon dioxide accepts electrons in its structure, so it is Lewis acid.
The above reaction could not be described by the other popular acid-base theory, the Bronsted-Lowry model, because it does not involve a proton transfer. The Bronsted-Lowry theory, unlike the Lewis theory, is based on the donation of protons. In this system, the acid is the donor compound of a positively charged particle, called a proton, while the base is the proton acceptor. While this model is useful for describing many interactions, Lewis’ theory is sometimes preferred due to its broader scope.
Lewis bases are classified into different types based on which electron orbital donates the electron pair. Often, a Lewis base will be an anion, substance, or compound that carries a negative charge due to an overabundance of electrons. Ammonia is considered a Lewis base, as are water and some organic compounds.
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