Zr and Hf (transition metals) occur together in nature and are difficult to separate. This can be attributed to
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Lanthanoid contraction
Zirconium (Z = 40) is in the second transition series (4d) and hafnium (Z = 72) in the third (5d), directly below it. Between lanthanum and hafnium the fourteen 4f electrons are filled. The 4f orbitals shield the outer electrons poorly, so the nuclear charge pulls the electrons in more and more: this steady decrease in size is the lanthanoid contraction.
It cancels the size increase expected from Zr to Hf. Their radii are almost the same (Zr 160 pm, Hf 159 pm), and so are their chemical properties. That is why the two metals occur together in nature and are very difficult to separate.
Why the other options are incorrect
Option (1) filling of same d orbital — Incorrect. Zr fills the 4d and Hf the 5d orbitals; they do not fill the same d orbital.
Option (2) Actinoid contraction — Incorrect. This is the contraction across the 5f series (Th to Lr); it affects the elements after the actinoids, not hafnium.
Option (4) both belong to same transition series of d-block elements — Incorrect. They are in the same group (group 4) but in different series, 4d and 5d.
NCERT (2026-27 edition):
Atomic and Ionic Sizes – “… The cumulative effect of the contraction of the lanthanoid series, known as lanthanoid contraction, causes the radii of the members of the third transition series to be very similar to those of the corresponding members of the second series. 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.”