Kaplan Nursing Entrance Practice Exam

Which of the following would be classified as a Lewis acid?

1. Electron-pair donor

2. Electron-pair acceptor

3. Negative ion

4. A non-polar molecule

The correct answer is: Electron-pair acceptor

A Lewis acid is defined as an electron-pair acceptor. This concept originates from the Lewis theory of acids and bases, which expands upon the Brønsted-Lowry definitions. According to this view, a Lewis acid is capable of accepting an electron pair from a donor (the Lewis base), leading to the formation of a coordinate covalent bond. In the context of the options presented, the classification of a Lewis acid directly aligns with its ability to accept electrons. When a substance can attract and hold onto electrons, it fulfills the criteria of a Lewis acid. This characteristic is essential in various chemical reactions, particularly in those involving coordination complexes and certain catalytic processes. The other choices represent different concepts: an electron-pair donor directly corresponds to a Lewis base; a negative ion typically has excess electrons and could act as a base rather than an acid; and a non-polar molecule does not inherently fit the definition of an acid or base in this context, as its lack of polarity limits its ability to interact in an acid-base reaction involving electron transfer. Understanding these distinctions is vital for grasping Lewis acid-base theory in chemistry.