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CBSE Class 12 · 2024 · d and f Block ElementsMultiple Choice

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Assertion (A): Zr and Hf have almost similar atomic radii.
Reason (R): This is due to lanthanoid contraction.
  1. (A)Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).✓ Correct
  2. (B)Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).
  3. (C)Assertion (A) is true, but Reason (R) is false.
  4. (D)Assertion (A) is false, but Reason (R) is true.

Answer: (A)

Solution

Hf follows the lanthanoids; the lanthanoid contraction (poor shielding by 4f electrons) exactly cancels the usual size increase from the 4d to the 5d series, so r(Zr) ≈ r(Hf) ≈ 160 pm.
Both statements are true and R is precisely the reason for A.
Correct option: (A).

CBSE Class 12 Board · 2024 · Single correct option

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