Which description best fits a Lewis base?

A Lewis base is defined as a species that can donate an electron pair to another molecule or ion during a chemical reaction. This fundamental characteristic is based on Lewis's definition, which focuses on the role of electrons in chemical bonding and reactions rather than just protons, as seen in the Brønsted-Lowry definition.

The option indicating that a Lewis base has an extra pair of electrons to donate captures this definition perfectly. A Lewis base typically has lone pairs of electrons that are not involved in bonding and can be used to form covalent bonds with Lewis acids, which are electron pair acceptors. In this way, the donation of electron pairs is the hallmark of Lewis bases, allowing them to participate in various chemical reactions effectively.

Furthermore, the other options relate to different concepts within acid-base chemistry. The idea of a proton donor pertains to Brønsted-Lowry acids, while an electron acceptor aligns more with the definition of Lewis acids. Similarly, increasing the concentration of hydrogen ions in solution directly relates to the behavior of acids rather than bases. Thus, the option emphasizing the capability of a Lewis base to donate an extra pair of electrons offers the most accurate and complete depiction of this type of chemical species.