Question

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

• A. having similar chemical properties
• B. belonging to same group
• C. diagonal relationship
• D. lanthanoid contraction

Answer 1

The Correct Option is D

The question involves understanding why the elements Zirconium (Zr) and Hafnium (Hf) exhibit similar atomic and ionic radii. We are given four options to choose from: similar chemical properties, belonging to the same group, diagonal relationship, and lanthanoid contraction.

To answer this question accurately, we need to explore each of these terms and their effect on atomic and ionic radii.

1. Having similar chemical properties: While certain elements can show similar chemical properties, this does not directly lead to similar atomic and ionic radii.
2. Belonging to the same group: Zr and Hf do belong to the same group (Group 4) in the periodic table, which usually leads to elements having similar properties. However, this group relationship is not enough to explain their nearly identical radii.
3. Diagonal relationship: This concept usually applies to elements across periods and groups, such as lithium and magnesium, where differences in size do not negate the similarity in properties. This is unrelated to Zr and Hf having similar radii.
4. Lanthanoid contraction: This is the correct reason. Lanthanoid contraction refers to the phenomenon where the size of atoms shrinks across the lanthanide series due to poor shielding of the nuclear charge by f-electrons. This contraction affects elements following the lanthanides in the periodic table, such as Hafnium, causing its size to be almost the same as that of Zirconium.

Thus, the lanthanoid contraction results in Hafnium having an atomic and ionic radius almost identical to Zirconium, even though it follows the lanthanides in the periodic table.

Conclusion: The similar atomic and ionic radii of Zr and Hf are due to lanthanoid contraction.