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

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

Correct answer: Zr (40) and Hf (72)

Solution

Lanthanoid contraction causes the atomic radii of elements following the lanthanoids to be smaller than expected. As a result, Zr (40) and Hf (72) have nearly identical atomic radii despite being in different periods.

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