What is the Lewis definition of an acid? Of a base?

In the realm of chemistry, understanding the role of a Lewis acid is crucial for grasping how certain substances interact with electrons. A Lewis acid is a chemical entity that can accept a pair of nonbonding electrons. This definition is broad and encompasses a wide range of substances, as it doesn't necessitate the presence of hydrogen, which is a requirement in other acid definitions such as the Bronsted-Lowry approach.

For instance, metal ions like iron in the form of \(Fe^{3+}\) serve as a classic example of Lewis acids because they have the capacity to accept electron pairs, filling their incomplete valence shell. This characteristic enables them to engage in chemical reactions that involve electron sharing with a suitable partner – a Lewis base.

While a Lewis acid is an electron pair acceptor, a Lewis base is its complementary counterpart, known for its ability to donate a pair of electrons. This broad definition applies to various compounds, not just those containing hydroxide ions, as might be assumed from other acid-base theories.

A typical example of a Lewis base is ammonia (\(NH_3\)), which possesses a lone pair of electrons that is readily available for bonding. In a Lewis acid-base interaction, the base would donate this lone pair to an acid that lacks electrons, thereby facilitating a connection through electron sharing rather than a simple transfer of protons or hydrogen ions.

The interaction between a Lewis acid and a Lewis base results in the formation of a special type of bond known as a coordinate covalent bond. Essentially, this is a covalent bond where both of the shared electrons come from only one of the two atoms – the Lewis base.

The uniqueness of this bond lies in its origin: it is not the result of mutual sharing of electrons (as is the case with a regular covalent bond), but instead arises from the complete donation of an electron pair by the Lewis base to the Lewis acid. A well-known example of a molecule featuring a coordinate bond is the ammonium ion (\(NH_4^+\)), where an \(NH_3\) molecule (the base) donates its lone pair to an \(H^+\) ion (the acid), resulting in a bond that has shared ownership of the electron pair but a single source.