What defines a Lewis base?

A Lewis base is defined specifically as a species that donates an electron pair to form a covalent bond. This is central to understanding acid-base reactions in the context of Lewis theory, which broadens the concept of acids and bases beyond simple proton transfer.

When a Lewis base donates a lone pair of electrons, it typically does so to a Lewis acid, which is defined as an electron pair acceptor. This interaction forms a coordinate covalent bond, resulting in the establishment of a new chemical species. The ability to donate an electron pair is a key characteristic of Lewis bases, allowing them to participate in various chemical reactions, particularly those involving complex formation or nucleophilicity.

In contrast, the other options describe different concepts. For instance, a proton donor relates to the Brønsted-Lowry acid-base theory rather than to Lewis acid-base concepts. Similarly, the notion of a hydronium donor is also specific to the Brønsted-Lowry framework and does not capture the broader role of a Lewis base. Thus, defining a Lewis base as a lone pair donor accurately reflects its role in chemical interactions and reactions.