Why BF3 is a Lewis acid but PCl3 is a Lewis base.
The Correct Answer and Explanation is:
Correct Answer:
BF₃ is a Lewis acid because it can accept a lone pair of electrons. PCl₃ is a Lewis base because it can donate a lone pair of electrons.
Explanation
The terms Lewis acid and Lewis base are defined based on the movement of electron pairs. A Lewis acid is a substance that can accept a pair of electrons, while a Lewis base is one that can donate a pair of electrons.
Boron trifluoride (BF₃) is a classic example of a Lewis acid. Boron, the central atom in BF₃, has only six electrons in its valence shell after forming three covalent bonds with fluorine atoms. This makes boron electron-deficient and eager to accept a pair of electrons to complete its octet. Therefore, BF₃ acts as an electron pair acceptor, which qualifies it as a Lewis acid. Despite the high electronegativity of the fluorine atoms, the molecule overall remains electron-deficient at the boron center. This deficiency is what drives BF₃ to behave as a Lewis acid when it interacts with electron-rich species.
Phosphorus trichloride (PCl₃), on the other hand, has phosphorus as the central atom, surrounded by three chlorine atoms. Phosphorus has five valence electrons. In PCl₃, three of these electrons are used to form bonds with chlorine, leaving a lone pair of electrons on the phosphorus atom. This lone pair can be donated to other molecules or ions that are electron-deficient. Therefore, PCl₃ acts as an electron pair donor, making it a Lewis base.
In summary, BF₃ lacks a complete octet and seeks to gain electrons, while PCl₃ has a lone pair available to share. Their behavior under the Lewis definition depends on whether they are looking to accept or donate electrons during a chemical reaction. This distinction classifies BF₃ as a Lewis acid and PCl₃ as a Lewis base.
