Kaplan Nursing Entrance Practice Exam

What is a Lewis base defined as?

1. Proton donor

2. Electron-pair donor

3. Cation acceptor

4. Neutral substance

The correct answer is: Electron-pair donor

A Lewis base is defined as an electron-pair donor. This definition is part of Lewis theory, which expands upon the Brønsted-Lowry acid-base theory. In Lewis terms, a Lewis base interacts with a Lewis acid by donating a pair of electrons to form a covalent bond. This characteristic distinguishes Lewis bases from other types of bases that may focus solely on proton transfer. Understanding this definition is crucial because it underlies many chemical interactions, particularly in coordination chemistry and organic reactions. An example of a Lewis base in practice is ammonia (NH3), which can donate a lone pair of electrons to form bonds with metals or other electrophiles. The other definitions do not capture the essence of what constitutes a Lewis base. For instance, the definition of a proton donor refers to Brønsted-Lowry acids, not Lewis bases. Similarly, a cation acceptor is not related to the role of a Lewis base; rather, it would pertain more to the behavior of Lewis acids, which accept electron pairs. Additionally, a neutral substance does not provide any insight into the electron-pair donating ability that is central to the definition of a Lewis base.