Question

Which of the following compound has maximum covalent character

• A. PbI₂
• B. Agl
• C. Hgl₂
• D. CsI

Answer 1

Answer: (C)

Hgl₂

Answer 2

To determine the compound with maximum covalent character, we use Fajan's rules. Fajan's rules state that covalent character increases with:

1. High charge on the cation.
2. Small size of the cation.
3. Large size of the anion.
4. Cations with pseudo noble gas configuration (like d¹⁰ or d¹⁰s²) have higher polarizing power than those with noble gas configuration (s²p⁶) of similar size and charge.

All the given compounds (PbI₂, AgI, HgI₂, CsI) have the same anion, I⁻. We need to compare the cations: Pb²⁺, Ag⁺, Hg²⁺, and Cs⁺.

Let's analyze the cations:

• Cs⁺: Charge +1. Electronic configuration [Xe]. Noble gas configuration. Ionic radius is large (~181 pm).
• Ag⁺: Charge +1. Electronic configuration [Kr] 4d¹⁰. Pseudo noble gas configuration (d¹⁰). Ionic radius is relatively small (~126 pm).
• Pb²⁺: Charge +2. Electronic configuration [Xe] 4f¹⁴ 5d¹⁰ 6s². Pb atom is [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p². Pb²⁺ is [Xe] 4f¹⁴ 5d¹⁰ 6s². This is sometimes considered a pseudo noble gas configuration (d¹⁰s²). Ionic radius is ~133 pm. Higher charge than Cs⁺ and Ag⁺.
• Hg²⁺: Charge +2. Electronic configuration [Xe] 4f¹⁴ 5d¹⁰. Hg atom is [Xe] 4f¹⁴ 5d¹⁰ 6s². Hg²⁺ is [Xe] 4f¹⁴ 5d¹⁰. Pseudo noble gas configuration (d¹⁰). Ionic radius is ~102 pm. Higher charge than Cs⁺ and Ag⁺ and smaller size than Pb²⁺.

Comparing covalent character based on Fajan's rules:

• CsI: Cs⁺ is large, has a low charge (+1), and noble gas configuration. Expected to be highly ionic.
• AgI: Ag⁺ has a low charge (+1), but is relatively small and has a d¹⁰ configuration. The d¹⁰ configuration leads to higher polarizing power than noble gas cations. AgI is more covalent than CsI.
• PbI₂: Pb²⁺ has a higher charge (+2) than Cs⁺ and Ag⁺. This increases covalent character. Pb²⁺ has a d¹⁰s² configuration and a radius of ~133 pm.
• HgI₂: Hg²⁺ has a higher charge (+2) than Cs⁺ and Ag⁺, is relatively small (~102 pm), and has a d¹⁰ configuration. The combination of high charge, small size, and d¹⁰ configuration gives Hg²⁺ very high polarizing power.

Comparing Hg²⁺ and Pb²⁺ (both +2 charge): Hg²⁺ is smaller (102 pm vs 133 pm) and has a d¹⁰ configuration, while Pb²⁺ has a d¹⁰s² configuration. The d¹⁰ configuration is generally more polarizing than the d¹⁰s² configuration for similar size and charge. Thus, Hg²⁺ is significantly more polarizing than Pb²⁺. So, HgI₂ is more covalent than PbI₂.

Comparing Hg²⁺ (+2, d¹⁰, 102 pm) and Ag⁺ (+1, d¹⁰, 126 pm): Hg²⁺ has a higher charge and smaller size. Both factors increase polarizing power. So, Hg²⁺ is much more polarizing than Ag⁺. HgI₂ is more covalent than AgI.

Comparing Ag⁺ (+1, d¹⁰, 126 pm) and Pb²⁺ (+2, d¹⁰s², 133 pm): Ag⁺ has a lower charge but d¹⁰ configuration and slightly smaller size. Pb²⁺ has a higher charge but d¹⁰s² configuration and slightly larger size. The higher charge of Pb²⁺ favors covalent character, as does the d¹⁰ configuration of Ag⁺. This comparison is less straightforward, but generally, the effect of charge is significant. However, the d¹⁰ configuration is also a strong factor. Compounds of Hg²⁺ are typically more covalent than those of Pb²⁺ and Ag⁺.

Based on the strong polarizing power of Hg²⁺ due to its high charge, small size, and d¹⁰ configuration, HgI₂ is expected to have the maximum covalent character among the given options.

The order of covalent character is expected to be CsI (most ionic) < AgI, PbI₂ < HgI₂ (most covalent).