Which of the following is not Lewis acid? (a) \(\mathrm{BF}_{3}\) (b) \(\mathrm{AlCl}_{3}\) (c) \(\mathrm{FeCl}_{3}\) (d) \(\mathrm{PH}_{3}\)

Understanding the nature of a Lewis adduct is essential for grasping the concept of Lewis acid-base chemistry. A Lewis adduct is a compound formed when a Lewis acid bonds with a Lewis base. The acid, by definition, is an electron pair acceptor, while the base is an electron pair donor.

Consider the reaction where a Lewis base, which has a lone pair of electrons, donates this pair to a Lewis acid, which has a vacant orbital prepared to accept electrons. The formation of a Lewis adduct is the result of this electron sharing, creating a more stable electronic arrangement for both species involved. For students to visualize the process, imagine two puzzle pieces fitting together; the base with its extra piece (electron pair) fills the gap (vacant orbital) in the acid, thereby completing the picture – the Lewis adduct.

In the realm of chemical reactions, the term 'electron pair acceptor' is synonymous with a Lewis acid. This species, by virtue of its inclination to accept a pair of electrons, actively seeks to interact with electron donors – Lewis bases.

The ability to be an electron pair acceptor often stems from an atom's electron configuration. Atoms that lack a full valence shell are prime candidates for acting as electron pair acceptors. For students seeking a practical example, consider how a metal ion with an incomplete d-shell can act as a Lewis acid, accepting electron pairs to achieve a more energetically favorable, lower energy state.

The concept of 'vacant orbitals' is critical to understanding Lewis acids. These orbitals are essentially empty spaces in an atom’s electron shells that can potentially house electron pairs.

During the assessment of compounds to ascertain their ability to function as Lewis acids, one must investigate if these vacant orbitals are present. For instance, in the case of \textbf{BF\(_3\)}, the boron atom has an empty p-orbital that is capable of accepting an electron pair from a Lewis base. Similarly, \textbf{AlCl\(_3\) and FeCl\(_3\)} have empty p- and d-orbitals respectively, making them competent Lewis acids. Without such vacant orbitals, a compound, like \textbf{PH\(_3\)}, can't act as a Lewis acid, as it lacks the necessary 'space' to accommodate additional electron pairs.