Question

According to Lewis concept, an acid is \(\mathbf{a}\) an (a) proton donor (b) electron pair donor (c) proton acceptor (d) \mathrm{\\{} d e c t r o n ~ p a i r ~ a c c e p t o r . ~

Short answer

A Lewis acid is an electron pair acceptor, so the correct answer is (d).

Step-by-step solution

Understanding the Lewis Concept of Acids

According to the Lewis concept, an acid is defined as a species that can accept an electron pair. This is different from the Bronsted-Lowry definition, which defines an acid as a proton donor. The Lewis definition is broader, including any species capable of accepting an electron pair, not necessarily just protons.

Identifying the Correct Answer

Review each given option and eliminate the incorrect ones. Option (a) 'proton donor' and option (b) 'electron pair donor' refer to the Bronsted-Lowry definition of an acid and a base respectively. Option (c) 'proton acceptor' is the Bronsted-Lowry definition of a base. Therefore, the correct answer according to the Lewis concept is option (d) 'electron pair acceptor'.

Key concepts

Acid-Base Chemistry

Acid-base chemistry is a fundamental aspect of chemical science, referring to reactions involving acid and base substances. These reactions are vital to understand as they are pervasive in both nature and industrial processes. An acid is traditionally defined as a substance that can donate protons (hydrogen ions, H+), whereas a base is a substance that can accept protons.

However, this traditional definition was expanded upon by the developments of the Lewis concept. In Lewis acid-base chemistry, the focus shifts from protons to electrons. A broader range of reactions are encompassed as acids and bases are classified by their ability to accept and donate pairs of electrons, respectively.

This shift in definition enables the inclusion of a greater variety of chemical species in acid-base reactions, including those that do not necessarily contain hydrogen. While the definition of acids and bases seems straightforward, it's nuanced by the different frameworks established to describe these chemical interactions, with the Lewis concept being one significant perspective.

Electron Pair Acceptor

In the Lewis concept of acid-base chemistry, an electron pair acceptor is a fundamental descriptor for acids. This ideation significantly broadens the spectrum of what can be considered an acid when compared to other acid-base theories. A Lewis acid doesn't need to possess a hydrogen atom to function as an acid; it simply needs to be able to accept an electron pair.

Why is the ability to accept an electron pair significant? Substances that can accept electron pairs usually have empty orbitals or positively polarized atoms, enabling them to accommodate electrons. This trait is pivotal in many chemical reactions, particularly those involving complex formation and catalysis. By looking beyond hydrogen ions, the Lewis concept accounts for the role of metal ions and other electron-deficient species in various chemical processes which act as Lewis acids.

Bronsted-Lowry Theory

Contrasting with the Lewis concept is the Bronsted-Lowry theory, which was the prevailing definition before G.N. Lewis introduced his theory in 1923. According to the Bronsted-Lowry theory, acids and bases are identified by their ability to donate and accept protons, respectively. This theory centers around the transfer of hydrogen ions, with acids being proton donors and bases being proton acceptors.

A clear distinction is made in the perspective of Bronsted-Lowry versus Lewis theories. The Bronsted-Lowry definition restricts acids and bases to those species involved in the transfer of protons. The theory is particularly useful in the study of aqueous solutions where water molecules play a pivotal role in accepting or donating protons. In contrast, the Lewis concept captures a broader range of chemical behaviors, including reactions in non-aqueous or gas-phase systems, by considering the broader criteria of electron pair acceptance or donation. Thus, while discussing acids and bases, it is crucial to understand the context and the specific theory being applied to the chemical scenario at hand.