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

The question is about identifying pairs of elements with nearly the same atomic radius. To solve this, we need to understand the concept of the atomic radius and the specific trends within the periodic table that affect atomic size. Specifically, we are looking for elements where the atomic radius remains nearly identical, often due to phenomena like the lanthanide contraction.

1. Understanding Atomic Radius: The atomic radius is the distance from the nucleus of an atom to the outermost stable electron orbital in a neutral atom. Atomic radii can be influenced by the number of electron shells and the effective nuclear charge experienced by the electrons.
2. Analyzing Given Pairs:
   • Pair I: Y and La - Yttrium (Y) is in the 4th period, and Lanthanum (La) is in the 6th period and the start of the lanthanide series. Because La has more electrons and a larger electron cloud, it generally has a larger atomic radius than Y.
   • Pair II: Zr and Hf - Zirconium (Zr) and Hafnium (Hf) are in the same group (Group 4). The presence of lanthanides between them causes what is known as the lanthanide contraction, leading their atomic radii to be almost identical despite the difference in periods.
   • Pair III: Mo and W - Molybdenum (Mo) and Tungsten (W) are in the same group (Group 6). Similarly, due to the lanthanide contraction effect, these elements also have nearly identical atomic radii.
   • Pair IV: Cr and Mo - Chromium (Cr) and Molybdenum (Mo) are different elements located in different periods with no lanthanide contraction intervening, so their atomic radii are not nearly the same.
3. Conclusion: Based on the analysis, pairs II (Zr and Hf) and III (Mo and W) have nearly the same atomic radius due to the lanthanide contraction effect. Therefore, the correct answer is: \(II \, \& \, III\).