Question

ASSERTION: Lithium fluoride is most covalent in nature.
REASON: Small anions can be easily distorted.
A. Both assertion and reason are correct and reason is the correct explanation for assertion.
B. Both assertion and reason are correct but reason is not the correct explanation for assertion.
C. Assertion is correct but reason is not correct.
D. Both assertion and reason are incorrect.

Answer 1

Ionic compounds are composed of one metal and one non-metal. The ionic compound has an ionic character. The ionic compound can have covalent character also. The extent of a covalent character depends upon the polarizing power of cation and polarizability of the anion.

Complete answer
The bond between metal and non-metal is known as an ionic bond and the compound is known as an ionic compound. Lithium is metal and fluoride is non-metal, so lithium fluoride is ionic compounds.
The covalent character in ionic compounds is determined by using Fajan’s rule. According to Fajan’s rule, the covalent character in ionic compounds is directly proportional to the polarizing power of the cation and inversely proportional to the polarizability of the anion.

The polarizing power of the cation is high if the size is small and the charge is maximum.
Polarizing power of the cation $\propto$ Charge on cation
Polarizing power of the cation $\propto$ $\dfrac{1}{size\; of\; cation}$
The polarizability of the anion will be high if the size is large and the charge is maximum.
Polarizability of the anion $\propto$ size of anion
Polarizability of the anion $\propto$ Charge on anion

Lithium is of the smallest size so, its polarizing power is very high but the fluoride is also of very small size, so its polarizability is very less, so lithium fluoride is not most covalent in nature. So, the assertion is incorrect.

In large anions, the uttermost shell electrons feel very less effective nuclear charge, so a cation can easily distort a large anion. So, the reason is incorrect. A small anion cannot be easily distorted. So, neither the assertion is correct nor the reason.

Therefore, option (D) Both assertion and reason are incorrect, is the correct answer.

Note: Ionic compounds have ionic bonds and covalent compounds have covalent bonds. The covalent compounds are made up of non-metals only. Fajan’s rule determines the covalent character in the ionic bond only, not the ionic character in the covalent bond. The covalent character is directly proportional to the solubility in non-polar solvents and inversely proportional to the solubility in the polar solvent.