Which statement best describes a Lewis acid?

A Lewis acid is best described as an electron pair acceptor. This definition stems from the Lewis theory of acids and bases, which was proposed by Gilbert Lewis. According to this theory, a Lewis acid is any species that can accept a pair of electrons to form a covalent bond. This is a broader definition compared to traditional acid-base theories that focus on proton transfer.

For example, metal ions such as Fe³⁺ and Al³⁺ can act as Lewis acids because they have vacant orbitals and can accept electron pairs from electron-donating species, known as Lewis bases. This electron-accepting behavior is fundamental in many chemical reactions, particularly in organic and coordination chemistry.

The other choices describe different concepts: an electron pair donor characterizes Lewis bases, while proton donors relate to Brønsted acids, and substances increasing hydroxide ion concentration are connected to bases, not acids. Thus, the accurate understanding of a Lewis acid's role as an electron pair acceptor is essential in comprehending its behavior in various chemical contexts.