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Question #44176

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The Question

The bond parameters of gaseous transition metal dihalides $\ce{CuCl2}$ and $\ce{ZnCl2}$ at $298\text{ K}$ deviate from predictions based purely on electrostatic ionic models. The ionic radii of $\ce{Cu^{2+}}$ ( $0.73\text{ \AA}$ ) and $\ce{Zn^{2+}}$ ( $0.74\text{ \AA}$ ) are nearly identical. However, their polarizing powers differ significantly due to their electronic configurations. Let $BDE$ , $BL$ , and $CC$ represent the Bond Dissociation Energy, Bond Length, and Covalent Character of the metal-chlorine bonds, respectively. Based on Fajans' Rules and the shielding effects of their d-electrons, which of the following correctly represents the comparative trends between $\ce{CuCl2}$ and $\ce{ZnCl2}$ ?

Options

A.
$BDE_{\ce{Cu-Cl}} > BDE_{\ce{Zn-Cl}} \text{ ; } BL_{\ce{Cu-Cl}} < BL_{\ce{Zn-Cl}} \text{ ; } CC_{\ce{CuCl2}} > CC_{\ce{ZnCl2}}$
B.
$BDE_{\ce{Cu-Cl}} < BDE_{\ce{Zn-Cl}} \text{ ; } BL_{\ce{Cu-Cl}} > BL_{\ce{Zn-Cl}} \text{ ; } CC_{\ce{CuCl2}} < CC_{\ce{ZnCl2}}$
C.
$BDE_{\ce{Cu-Cl}} > BDE_{\ce{Zn-Cl}} \text{ ; } BL_{\ce{Cu-Cl}} > BL_{\ce{Zn-Cl}} \text{ ; } CC_{\ce{CuCl2}} > CC_{\ce{ZnCl2}}$
D.
$BDE_{\ce{Cu-Cl}} < BDE_{\ce{Zn-Cl}} \text{ ; } BL_{\ce{Cu-Cl}} < BL_{\ce{Zn-Cl}} \text{ ; } CC_{\ce{CuCl2}} < CC_{\ce{ZnCl2}}$

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Exception_Exposure

Learn to identify edge cases and boundary exceptions that JEE examiners hide in math problems.

Concepts Behind This Question

Bond Energy vs Bond Length|Fajans' Rule Polarization Power|E° (M²⁺/M) Anomalies

Why This Question Is Tricky

Common Mistakes

This question tests a critical exception in Fajans' Rules regarding pseudo-inert gas configurations.

Students often memorize that a $d^{10}$ configuration (like $\ce{Zn^{2+}}$ ) yields the maximum possible polarizing power compared to $s^2p^6$ noble gas configurations.

However, they overlook the 'hidden variable' of shielding efficiency within the d-block itself.

An incomplete d-shell ( $d^9$ in $\ce{Cu^{2+}}$ ) shields the nucleus even less effectively than a filled $d^{10}$ shell.

This forces the student to deduce that $\ce{Cu^{2+}}$ will distort the anion's electron cloud more severely, leading to higher covalent character.

The subsequent trap is linking this covalent character to physical bond parameters: increased covalency means more orbital overlap, which counterintuitively shortens the bond and increases bond dissociation energy relative to purely electrostatic ionic predictions.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Chemical Bonding and Molecular Structure - Class 11 Part 1
Concept	Bond Energy vs Bond Length
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free