Question

According to Fajan’s rule, covalent bond is favoured by:
A. Large cation and small anion
B. Large cation and large anion
C. Small cation and small anion
D. Small cation and large anion

Answer 1

Hint: Remember that Fajan's rule deals with the terms “polarization power” and “ polarisability”. So, now figure out accordingly what could be the possible answer to form a covalent bond.

Complete step by step answer:
When two oppositely charged ions approach each other the positive ion attracts electrons on the outermost shell of the anion and repels its positively charged nucleus. This results in the distortion, deformation or polarization of the anion. If the polarization is quite small, an ionic bond is formed, while if the degree of polarization is large, a covalent bond results.
Thus the power of an ion (cation) to distort the other ion is known as its polarization power and the tendency of the ion(anion) to get polarized by the other ion is known as its polarisability. Greater the polarization power or polarisability of an ion, greater will be its tendency to form a covalent bond.
The polarising power, or polarisability and hence formation of covalent bond is favoured by the following factors:
Small Positive Ion (Cation): Due to greater concentration of positive charge on a small area, the smaller cation has high polarising power. This explains why LiCl is more covalent than KCl.
Large Negative Ion (Anion): The larger the anion, the greater is its polarisability, i.e. susceptibility to get polarised. It is due to the fact that the outer electrons of a large anion are loosely held and hence can be more easily pulled out by the cation. This explains why iodides, among halides, are most covalent in nature.

Hence, we can conclude that the correct answer is option D.

Note: The polarising power, or polarisability and hence formation of covalent bond is also favoured by the following factors:
Large Charge on Either of the Two Ions: As the charge on the ion increases, the electrostatic attraction of the cation for the outer electrons of the anion also increases, with the result its ability for forming the covalent bond increases.
Electronic Configuration of the Cation: For the two ions of the same size and charge, one with a pseudo noble gas configuration (i.e., 18 electrons in outermost shell) than a cation with noble gas configuration (i.e. 8 electrons in outermost shell) will be more polarising.