Fill in the blank. A species that can donate an electron pair is known as blank. (A) A Lewis base, (B) a Brønsted–Lowry base, (C) a Brønsted–Lowry acid, (D) a Lewis acid, or (E) an Arrhenius base.
In options (A) to (E), there is mention of three different types of definitions for acids and bases, which are Lewis, Brønsted–Lowry, and Arrhenius. Each of them defines acidity and basicity in different ways.
The Lewis definition defines acidity and basicity in terms of accepting or donating lone pairs, where a lone pair is a pair of valence or outer shell electrons not directly involved in bonding. The Brønsted–Lowry definition defines acidity and basicity in terms of accepting or donating hydrogen ions. Hydrogen ions can be represented by the symbol H+, and they are equivalent to protons. The Arrhenius definition defines acidity and basicity in terms of changing the concentration of H+, which we know to be hydrogen ions, or OH−, which are hydroxide ions, in aqueous solution, where aqueous solution means a solution in water.
The question asks us the definition in terms of electron pairs. As the Lewis definition defines acidity and basicity in terms of accepting or donating lone pairs, then we know that it has to be a Lewis definition. Of the five options given, two of them refer to Lewis. These are options (A) and (D). So we know that either option (A) or option (D) will be the answer to this question but not option (B), (C), or (E).
To figure out if the answer is (A) or (D), we need to know whether a species that can donate an electron pair is a Lewis base or a Lewis acid. We can use NH3, ammonia, and BH3, borane, as an example to help us work out the answer. Ammonia has a lone pair of electrons. This lone pair can be donated to borane, an electron-deficient species, forming a bond represented here with a coordinate covalent bond arrow. So ammonia has donated an electron pair. We can call it an electron pair donor. Borane has received a pair of electrons, so we can call it an electron pair acceptor. In this example, borane is acting as a Lewis acid and ammonia is acting as a Lewis base. Therefore, a Lewis base can be defined as an electron pair donor. Therefore, the species that can donate an electron pair is known as (A) a Lewis base.