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Question #44326

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The Question

Consider the periodic trends and properties of the $d$ -block elements, specifically focusing on the $3d$ , $4d$ , and $5d$ series. Which of the following statements is INCORRECT?

Options

A.
Despite the increase in principal quantum number, the atomic radius of $Hf$ is nearly identical to $Zr$ , yet the first ionization enthalpy of $Hf$ is higher than that of $Zr$ due to the poor shielding by the $4f^{14}$ core.
B.
The thermodynamic stability of the $+6$ oxidation state increases in the sequence $Cr(VI) < Mo(VI) < W(VI)$ because the $5d$ electrons in Tungsten experience a lower effective nuclear charge ( $Z_{eff}$ ) than the $3d$ electrons in Chromium, lowering the required ionization enthalpies.
C.
The $4d$ and $5d$ transition metals predominantly form low-spin octahedral complexes because their spatially extended $d$ -orbitals lead to a crystal field splitting energy ( $\Delta_o$ ) that is significantly greater than that of analogous $3d$ complexes.
D.
The enthalpies of atomization ( $\Delta_a H^\circ$ ) for the $5d$ series are generally higher than those of the $3d$ series due to more effective orbital overlap and stronger metal-metal bonding facilitated by the extended $5d$ orbitals.

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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

d-block Variable Oxidation States|Lanthanoid Contraction cause|Post-Lanthanoid Radii Equality

Why This Question Is Tricky

Common Mistakes

This question targets the paradoxical relationship between Effective Nuclear Charge ( $Z_{eff}$ ) and oxidation state stability in the $d$ -block.

A common student trap is to assume that because higher oxidation states are more stable down a $d$ -block group, the valence electrons must be easier to remove (lower ionization energy/lower $Z_{eff}$ ), extrapolating from $s$ and $p$ -block logic.

The 'Chemical Exception' tested here is the Lanthanoid contraction: $5d$ elements actually have a higher $Z_{eff}$ and higher ionization enthalpies than their $3d$ counterparts.

The stability of states like $W(VI)$ is not due to ease of ionization, but rather the immense thermodynamic payoff from strong covalent bonding via diffuse $5d$ orbitals.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Classification of Elements and Periodicity in Properties - Class 11 Part 1
Concept	d-block Variable Oxidation States
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free