The number of following factors which affect the percent covalent character of the ionic bond is _________
(A) Polarising power of cation
(B) Extent of distortion of anion
(C) Polarisability of the anion
(D) Polarising power of anion

Solution

Chemistry ma’am · 3 steps

In Fajans' rules the cation pulls and the anion gets pulled, so every factor must fit that picture.

1. Where covalency comes from. A small, highly charged cation pulls the electron cloud of the anion towards itself. This distortion of the anion puts electron density between the two nuclei, and that shared density is covalent character.

   Factor	Role in Fajans' rules	Affects covalent character?
   (A) Polarising power of cation	The cation is the one that distorts	Yes
   (B) Extent of distortion of anion	This distortion is the covalent character itself	Yes
   (C) Polarisability of the anion	A large, soft anion is distorted more easily	Yes
   (D) Polarising power of anion	The anion is large and diffuse, so it barely distorts the small cation	No

2. Why D is out. Fajans' rules use only the polarising power of the cation and the polarisability of the anion. The anion's own polarising power is not counted.
3. So. A, B and C affect the percent covalent character, so the answer is 3.

The common slip: 4. Treating the polarising power of the anion as a factor, by symmetry with the cation, gives 4, not 3.

The idea to keep. Polarising power belongs to the cation and polarisability belongs to the anion. If a factor swaps these roles, it does not count in Fajans' rules.

Answer from the NTA final answer key. Solution by PrepWiser.