How did Gilbert N. Lewis define acids and bases in 1923?

In 1923, American chemist Gilbert N. Lewis introduced a generalized theory of acids and bases based on electronic structure.12

According to the Lewis theory:

• Acid: A species that acts as an electron-pair acceptor, capable of attaching itself to an unshared pair of electrons from another molecule.12
• Base: A species that possesses an available, unshared pair of electrons that it can donate to an acid.12

The reaction between a Lewis acid and a Lewis base results in the formation of an addition compound, where the chemical bond is formed by a shared electron pair (covalent bond).12

This definition is broader than the Brønsted–Lowry concept because it includes substances that do not contain protons, such as metal ions and compounds like boron trifluoride ($B{F}_3$) or sulfur trioxide ($S{O}_3$). It is important to distinguish this from oxidation-reduction reactions, where electrons are transferred completely, rather than shared to form a covalent link.12