Zr (Z = 40) and Hf (Z = 72) have similar atomic and ionic radii because of :

Explanation:

Lanthanoid contraction :

→ The cumulative effect of the contraction of the lanthanoid series, known as lanthanoid contraction; 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 transition series.

The almost identical radii of Zr (160 pm) and Hf (159 pm) are a consequence of the lanthanoid contraction.

Additional Information

•  The whole periodic table is classified into 4 blocks namely s block, p block, d block, and f block.
• The elements of f blocks are also called inner transition elements.
• 14 elements following Lanthanum are knowns as lanthanoids series and are arranged in a row at the bottom of the periodic table.
• While moving from Lanthanum to Lutecium the atomic radii decreases, these cumulative effects of contraction of lanthanoids series are known as lanthanoids contraction.
• This effect causes the radii of the third transition series to be very much similar to those of corresponding members of the second transition series.
• Hence due to this consequence both these elements occur together thereby making their separation difficult.