NEET 2015 Chemistry question

Because of lanthanoid contraction, which of the following pairs of elements have nearly same atomic radii ? (Nubmers in the parenthesis are atomic numbers).

(1)Zr (40) and Nb (41)
(2)Zr (40) and Hf (72)Correct
(3)Zr (40) and Ta (73)
(4)Ti (22) and Zr (40)

Answer: (2)

Solution

The poor shielding by 4f electrons causes a steady decrease in size across the lanthanoids (lanthanoid contraction). As a result the third-row (5d) elements have almost the same atomic radii as the corresponding second-row (4d) elements of the same group, despite having 32 more electrons. Zr (145

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Question ID VK-091685