In Lewis acid-base theory, which description correctly characterizes a Lewis acid?

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

Answer: (D)

A Lewis acid is defined by its ability to accept an electron pair. In Lewis chemistry, a base donates its lone pair, and the acid accepts that pair to form a bond, often a coordinate covalent bond where both electrons come from the base. That’s why the description “electron acceptor” fits a Lewis acid best. Classic examples include boron compounds like boron trifluoride, which have empty orbitals ready to accept an electron pair from a donor such as ammonia, and protons like H+, which accept electron density from bases.

Proton donor aligns with Brønsted–Lowry acid behavior, not the Lewis definition. Electron donor describes a Lewis base, and proton acceptor or similar terms can mix ideas from other theories, but they don’t capture the role of a Lewis acid.