In Lewis acid-base theory, which statement correctly describes a Lewis acid?

• A. Proton donor
• B. Electron donor
• C. Proton acceptor
• D. Electron acceptor

Answer: (D)

In Lewis acid-base theory, a Lewis acid is an electron pair acceptor. Acids attract and take a pair of electrons from a Lewis base to form a bond, creating an adduct (for example, a molecule like BF3 or H+ accepts an electron pair from a base such as NH3 or H2O). This framework distinguishes acids by their role with electron pairs, not by protons.

Proton donors belong to Brønsted-Lowry acid behavior, since they donate H+. Electron donors are Lewis bases, since they provide the electron pair. Proton acceptors align with Brønsted-Lowry base behavior, as they accept protons. These descriptions describe other acid-base perspectives, but the defining feature for a Lewis acid is electron pair acceptance.