Question

Consider the following pairs of elements and identify the pairs of elements which have nearly same atomic radius. I. Y, La II. Zr, Hf III. Mo, W IV. Cr, Mo

• A. I & II
• B. II & III
• C. III & IV
• D. I & III

Hint:

Due to the Lanthanide Contraction, pairs of elements in the same group from the second (4d) and third (5d) transition series have very similar atomic radii. Look for pairs like (Zr, Hf), (Nb, Ta), (Mo, W), etc.

Answer 1

The Correct Option is B

We are asked to identify pairs of elements having nearly the same atomic radius.
Step 1: Understand the concept
The phenomenon responsible is the Lanthanide Contraction: 
 

• Lanthanide Contraction is the steady decrease in atomic size of the lanthanide series elements due to poor shielding of 4f electrons.
• As a result, the effective nuclear charge increases across the series, pulling electron shells closer to the nucleus.
• This contraction causes elements in the 5th and 6th transition series (4d and 5d elements) to have nearly identical atomic radii.

Step 2: Analyze the given pairs
 

• Y and La: Y (Period 5, Group 3), La (Period 6, Group 3). Atomic radius increases down a group normally. La is larger than Y. Not nearly same.
• Zr and Hf: Zr (Period 5, Group 4), Hf (Period 6, Group 4). Hf comes after the lanthanides. Due to lanthanide contraction, Hf radius $\approx$ Zr radius. This pair has nearly same atomic radius.
• Mo and W: Mo (Period 5, Group 6), W (Period 6, Group 6). W comes after lanthanides. Lanthanide contraction makes W radius $\approx$ Mo radius. This pair has nearly same atomic radius.
• Cr and Mo: Cr (Period 4, Group 6), Mo (Period 5, Group 6). Normal increase down the group. Mo radius $>$ Cr radius. Not nearly same.

Step 3: Conclusion 
The pairs with nearly the same atomic radius are: 
 

\[\text{II (Zr, Hf) and III (Mo, W)}\]