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

Answer
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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 AlCl3 and BF3, aluminium and boron have only six electrons in their valence shell.
2. Simple cations such as Fe3+, Ag+, 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, SiF4 and SnCl4.
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 AlCl3, BF3, SnCl2, although they do not contain any hydrogen, hence, cannot donate a proton.
However, Lewis concept is more useful than Bronsted concept.
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