Question

Assertion (A) : Separation of Zr and Hf is difficult. Reason (R) : Zr and Hf have almost identical atomic and ionic radii due to lanthanoid contraction.

• A. Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
• B. Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
• C. Assertion (A) is true, but Reason (R) is false.
• D. Assertion (A) is false, but Reason (R) is true.

Hint:

Zr and Hf are one of the most important examples of lanthanoid contraction. Their almost identical sizes make their chemical properties nearly indistinguishable.

Answer 1

The Correct Option is A

Step 1: Verify the Assertion.
Zr and Hf (both Group 4) coexist in nature and share virtually identical chemical properties. Conventional separation relies on property differences; since almost none exist between these two elements, separation is extremely difficult. Assertion is TRUE.
Step 2: Verify the Reason.
Lanthanoid contraction (poor shielding by 4f electrons) offsets the expected size increase for Hf: Zr $\approx 160\,\text{pm}$, Hf $\approx 159\,\text{pm}$. Their atomic and ionic radii are nearly identical because of this contraction. Reason is TRUE.
Step 3: Connect Reason to Assertion.
Identical radii give Zr and Hf the same charge density, bond energies, and chemical reactivity. No standard separation technique can exploit enough of a difference to separate them, directly explaining why separation is difficult. \[ \boxed{\text{Option (A): Both A and R true; R correctly explains A}} \]