Among the following, the Lewis acid is

According to the Lewis acid-base theory, a Lewis acid is any substance that can accept a pair of electrons from a Lewis base (which donates a pair of electrons). This definition expands the concept of acids beyond the classical Arrhenius and Brønsted-Lowry theories.

• A Lewis acid has an incomplete valence shell or an atom with a vacant orbital that can accommodate a pair of electrons.
• Lewis acids include metal cations like Fe³⁺, Al³⁺, and molecules like BF₃ (boron trifluoride), where boron has an empty orbital to accept electron pairs.

Example of Lewis acids:

1. H⁺ (Hydrogen ion): It is a classic Lewis acid because it can accept a pair of electrons.
2. BF₃ (Boron trifluoride): In BF₃, boron has only six electrons in its valence shell, making it electron-deficient and capable of accepting an electron pair from a Lewis base.

By contrast, Lewis bases are species that donate a pair of electrons to form a covalent bond with the Lewis acid.

In summary, Lewis acids are electron-pair acceptors, and this concept is crucial for understanding a wide range of reactions, especially in coordination chemistry and organic chemistry.