Question

Which of the following pairs are more covalent and why?
(a) CuO and CuS
(b) AgCl and Agl
(c) PbCl₂ and PbCl₄
(d) NaCl and CuCl

Answer 1

Factors Governing Polarization and Polarisability (Fajan’s Rule):

Cation Size : Smaller is the cation, more is the value of charge density (φ) and hence more its polarising power.

Cationic Charge : More is the charge on the cation, the higher is the value of (φ) and higher is the polarising power.

Noble Gas vs Pseudo Noble Gas Cation : Greater departure from noble gas configuration, greater covalency.

Anion Size : Larger is the anion, more is the polarisability and hence more covalent character is expected.

Anionic Charge : Larger is the anionic charge, the more is the polarisability.

(a) CuO and CuS. Here cation is same and S anion is larger than Oxygen anion. So, there will be more covalent character in CuS than CuO.

(b) AgCl and Agl Since Cl is smaller in size, its polarisability is less and therefore it is having more ionic

(c) PbCl₂ and PbCl₄  Greater the charge on cation, more is the polarising power and hence more covalent character. As polarising power of Pb⁴⁺ > Pb²⁺. Thus PbC l₄ is more covalent than PbCl₂.

(d) NaCl and CuCl If two cations have the same charge and size, the one with pseudo noble gas configuration i.e. having 18 electrons in the outermost shell has greater polarizing power than a cation with noble gas configuration i.e. having 8 electrons.
Electronic configuration of Cu⁺ : 2, 8, 18
Electronic configuration of Na⁺ : 2,8
Therefore, CuCl is more covalent than NaCl although Cu⁺ ion (0.96Å) and Na⁺ ion (0.95Å) have same size and charge.