It is difficult to separate the ores of Zr and Hf because:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Their radii are almost same
Zr (4d series) and Hf (5d series) are in the same group (Group 4). Normally the radius increases from the 4d to the 5d element, but the fourteen lanthanoids come just before Hf, and the poor shielding by their 4f electrons causes the lanthanoid contraction. This cancels the expected increase, so Zr (160 pm) and Hf (159 pm) have almost identical radii. With the same charge and nearly the same size, their chemical properties are so similar that they occur together in ores and are very difficult to separate.
Why the other options are incorrect
Option (2) They do not experience Lanthanoid contraction — Incorrect. The near equality of their radii is itself the result of the lanthanoid contraction acting on Hf.
Option (3) Zr has half filled orbital but Hf does not — Incorrect. Both have the configuration (n−1)d2ns2 (Zr 4d25s2, Hf 4f145d26s2); neither has a half-filled d subshell.
Option (4) They belong to different groups — Incorrect. Both are in Group 4; elements of different groups would be easier, not harder, to separate.
NCERT (2026-27 edition):
Atomic and Ionic Sizes – “… The almost identical radii of Zr (160 pm) and Hf (159 pm), a consequence of the lanthanoid contraction, account for their occurrence together in nature and for the difficulty faced in their separation.”