Concept: Fajan's Rule

Fajan's rule describes the degree of covalent character in an ionic bond. It states that the covalent character increases with increasing polarization of the anion by the cation. Polarization is favored by:

• Small size of the cation (high charge density)
• Large size of the anion (electron cloud is easily distorted)
• High charge on both cation and anion
• Cations with pseudo-noble gas configuration (e.g., Cu+, Ag+, Au+)

Why (A) is correct:

According to Fajan's rule, a small cation has a high charge density, which allows it to pull the electron cloud of the anion more effectively. A large anion has a loosely held electron cloud, making it more easily deformable or polarizable. Both these conditions lead to a greater degree of electron sharing, thus increasing the covalent character of the bond.

Option Analysis:

• A) small cation, large anion: This combination maximizes the polarizing power of the cation and the polarizability of the anion, leading to the highest covalent character. This is correct.
• B) small anion, small cation: A small anion is less polarizable, and while a small cation is good for polarization, the overall covalent character will be less than with a large anion. This favors more ionic character.
• C) large anion, large cation: A large cation has a lower charge density and thus less polarizing power. While a large anion is polarizable, the weak polarizing power of the cation will result in less covalent character compared to a small cation. This favors more ionic character.
• D) small anion, large cation: A large cation has low polarizing power, and a small anion is less polarizable. This combination strongly favors ionic character.

Correct Answer: (A)