Question

Although Zr belongs to 4d and Hf belongs to 5d transition series, but it is quite difficult to separate them. Why?

Answer 1

Hf (z=72) is the second atom belonging to the 5d transition series and comes after the Lanthanides.
Due to the lanthanoid contraction, the atomic size of Hf decreases and becomes nearly equal to that of Zr.

Thus, the size of Hf (159 pm) and Zr (160 pm) are almost the same. They have similar chemical properties as they belong to the same group, which makes the separation of these two elements quite difficult.

$AdditionalInformation$

Lanthanoid contraction is a phenomena by virtue of which the size of atoms decreases with increase in atomic number. As the atomic number increases in the Lanthanoid series the nuclear charge increases. Due to poor shielding effect of 𝑓−𝑜𝑟𝑏𝑖𝑡𝑎𝑙, outer shell electrons are attracted towards nucleus and size of atom decreases.