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Question #44298

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The Question

The covalent character of metal halides is governed by the polarizing power of the cation (Fajans' Rules). For cations with a pseudo-noble gas configuration ( $ns^2 np^6 nd^{10}$ ), the polarizing power is anomalously high due to poor shielding by $d$ -electrons. In Group 12, the sum of the first two ionization enthalpies ( $\Sigma IE_{1,2}$ ) reflects the effective nuclear charge ( $Z_{eff}$ ) experienced by the valence electrons, which directly correlates with the cation's ability to distort the anion's electron cloud. Given the interplay of $d$ -block and lanthanide contractions, determine the correct decreasing order of covalent character for the Group 12 dichlorides at $298\text{ K}$ .

Options

A.
$\ce{HgCl2 > ZnCl2 > CdCl2}$
B.
$\ce{ZnCl2 > CdCl2 > HgCl2}$
C.
$\ce{HgCl2 > CdCl2 > ZnCl2}$
D.
$\ce{ZnCl2 > HgCl2 > CdCl2}$

Check Your Answer

Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Mechanism_Trap

Don't get tricked by the process! Learn the mechanical traps used in competitive exam questions.

Concepts Behind This Question

Successive Ionization Energies|Fajans' Rule Polarization Power|Effective Nuclear Charge (Zeff)

Why This Question Is Tricky

Common Mistakes

The 'Chemical Exception' being tested is the non-linear trend in polarizing power and covalent character down Group 12.

Students typically apply the standard Fajans' rule (polarizing power $\propto 1/r^2$ ) and predict that covalent character strictly decreases down the group ( $\ce{ZnCl2 > CdCl2 > HgCl2}$ ).

The 'Hidden Variable' is the lanthanide contraction and the extremely poor shielding of $4f$ electrons, which drastically increases the $Z_{eff}$ of $\ce{Hg^{2+}}$ .

This anomaly is perfectly mirrored by the successive ionization energies ( $\Sigma IE_{1,2}$ ), making $\ce{HgCl2}$ the most covalent (existing as discrete molecules with the lowest melting/boiling points) despite having the largest cation radius in the group.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Classification of Elements and Periodicity in Properties - Class 11 Part 1
Concept	Successive Ionization Energies
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free