Concept: Fajan's Rules

Polarisation refers to the distortion of the electron cloud of an anion by a cation. This distortion leads to an increase in the covalent character of an ionic bond. Fajan's rules describe the factors that influence the extent of polarisation.

Why Option (A) is correct:

According to Fajan's rules, the polarising power of a cation increases with its charge. A cation with a higher positive charge will exert a stronger electrostatic attraction on the electron cloud of the anion, leading to greater distortion (polarisation). For example, $A{l}^{3+}$ has a higher polarising power than $N{a}^{+}$.

Option Analysis:

• A) Maximum polarisation is done by a cation of high charge. This statement is correct. Higher charge on the cation leads to greater electrostatic attraction and thus greater polarisation of the anion.
• B) A large cation is likely to bring large degree of polarisation. This statement is incorrect. A smaller cation has a higher charge density and thus a greater polarising power. Larger cations have lower charge density and cause less polarisation.
• C) A smaller anion is likely to undergo a high degree of polarisation. This statement is incorrect. A larger anion has a more diffuse electron cloud, which is more easily distorted (polarised) by a cation. Smaller anions hold their electrons more tightly and are less polarisable.
• D) Minimum polarisation is done by a cation of small size. This statement is incorrect. A cation of small size has a higher charge density and thus a greater polarising power, leading to maximum polarisation, not minimum.

Correct Answer: (A)