Question

According to Lewis concept, acid is:
A. proton donor
B. electron pair donor
C. electron pair acceptor
D. proton acceptor

Answer 1

Hint: Lewis concept of acids and bases is based on electronic theory of valency. This concept eliminates the necessity of a proton.

Complete Solution :
According to the Lewis concept, the acid and bases may be defined as follows:
“Acid is a substance which can accept a pair of electrons while base is a substance which can donate a pair of electrons.”
Thus, according to this concept a Lewis base donates an electron pair, which is accepted by a Lewis acid. An acid-base reaction is the sharing of an electron pair between an acid and a base.
According to Lewis concept, the species that can act as Lewis acids are:
1. Neutral molecules in which the central atom has incomplete octet. For example, in \[\text{AlC}{{\text{l}}_{\text{3}}}\] and \[\text{B}{{\text{F}}_{3}}\], aluminium and boron have only six electrons in their valence shell.
2. Simple cations such as \[\text{F}{{\text{e}}^{3+}}\], \[\text{A}{{\text{g}}^{+}}\], since they are electron deficient.
3. Molecules in which the central atom of p-block has empty d-orbitals and may acquire more than 8 valence electrons. These molecules can accept electron pairs to be accommodated in their empty orbitals. For example, \[\text{Si}{{\text{F}}_{\text{4}}}\] and \[\text{SnC}{{\text{l}}_{\text{4}}}\].
4. Molecules like \[O=C=O\] in which atoms of dissimilar electronegativities are joined by multiple bonds.
Thus, in short, an acid is an electron pair acceptor and a base is an electron pair donor.
So, the correct answer is “Option C”.

Note: According to Bronsted-Lowery concept, an acid is a proton donor and a base is a proton acceptor.
But Bronsted-Lowery concept, fails to explain the acidic character of \[\text{AlC}{{\text{l}}_{\text{3}}}\], \[\text{B}{{\text{F}}_{3}}\], \[\text{SnC}{{\text{l}}_{\text{2}}}\], although they do not contain any hydrogen, hence, cannot donate a proton.
However, Lewis concept is more useful than Bronsted concept.