Arrange the following in the decreasing order of their covalent character :
(A) LiCl
(B) NaCl
(C) KCl
(D) CsCl
Choose the most appropriate answer from the options given below :

Solution

Chemistry ma’am · 3 steps

Same anion in all four, so compare only the cations: the smallest one distorts Cl⁻ the most.

1. Fajans' rule. An ionic bond picks up covalent character when the cation pulls the anion's electron cloud towards itself. A small cation with a high charge does this most strongly, because its charge is packed into a tiny volume.
2. Fix the variables. All four compounds have the same anion, Cl⁻, and every cation carries a +1 charge. So cation size is the only thing that changes, and it grows down group 1.

   Cation	Ionic radius	Polarising power	Covalent character
   Li⁺ in LiCl	76 pm, smallest	highest	most
   Na⁺ in NaCl	102 pm	high	less
   K⁺ in KCl	138 pm	low	lesser
   Cs⁺ in CsCl	167 pm, largest	lowest	least

3. So. Covalent character falls steadily as the cation grows: LiCl > NaCl > KCl > CsCl, that is (A) > (B) > (C) > (D), option (c).

The idea to keep. When the anion and the cation charge are fixed, covalent character depends only on cation size. Smaller cation means stronger polarisation and more covalent character, so it decreases down a group.

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