What is a general characteristic of a Lewis acid? Of a Lewis base?

In the realm of chemistry, the term Lewis acid refers to a substance that, due to its ability to take on an additional pair of electrons, assumes the role of an electron pair acceptor. This concept broadens the traditional understanding of acids beyond just proton donors, as per the Bronsted-Lowry theory, accounting for a more extensive variety of chemical interactions.

A typical characteristic of Lewis acids is the presence of an incomplete electron shell, which creates the potential for an incoming electron pair from a Lewis base to be accommodated. Common examples involve metal ions such as Fe³⁺ or organic compounds with an electron-deficient carbon atom, like in the case of Boron trifluoride (BF₃). This adaptive definition advocates for the inclusion of a broader spectrum of chemical species in acid-base reactions, beyond the traditional H⁺ ion exchange.

On the flip side, we have the Lewis base, a key player in the landscape of chemistry defined by its innate capacity to bestow an electron pair onto other species. Acting as an electron pair donor, a Lewis base interacts with a Lewis acid to forge a new bond.

One must note that a Lewis base does not have the same constraints as a Bronsted-Lowry base, which is confined to the act of abstracting protons. Instead, any neutrally charged or negatively charged entity with a lone pair of electrons, like NH₃ (ammonia) or OH^- (hydroxide ion), can serve as a Lewis base. The centrality of the electron pair donation expands the horizons for chemical species to participate in acid-base reactions, offering a more inclusive and versatile definition.

Diving deeper into the interaction between Lewis acids and bases leads us to the concept of electron pair acceptors and donors. In any given Lewis acid-base reaction, the Lewis acid acts as an electron pair acceptor—ready to welcome a new pair of electrons—whilst the Lewis base assumes the complementary role of the electron pair donor—ready to deliver its electrons to form a bond.

Electron pair donation and acceptance are the cornerstone of countless chemical reactions, including coordination complexes, where central metal ions (Lewis acids) bond with ligands (Lewis bases). For example, the formation of AlCl₄^- from AlCl₃ and Cl^- is a classic illustration — AlCl₃ (electron pair acceptor) merges with Cl^- (electron pair donor) leading to a coordinated entity.

Contrasting Points of View

The Bronsted-Lowry theory offers a different perspective from Lewis's concept, focusing instead on protons, H⁺, as the main actors. A Bronsted-Lowry acid is described as a substance capable of donating a proton, whereas a Bronsted-Lowry base is a substance capable of accepting a proton.

Examples of Bronsted-Lowry acids include substances like hydrochloric acid (HCl), which readily parts with H⁺ in aqueous solutions. On the other end, ammonia (NH₃) is a conventional Bronsted-Lowry base as it readily accepts H⁺, forming NH₄⁺. This theory emphasizes the transfer of protons in chemical reactions rather than the transfer of electron pairs, as seen in Lewis theory, revealing the dynamism and diversity of acid-base chemistry.