Which of the following pairs of elements has the most similar atomic radii?

Solution

1. The atomic radii of elements in the same group generally increase down the group.
2. However, $\text{Zr}$ ($4d$ series) and $\text{Hf}$ ($5d$ series) have nearly identical radii.
3. This phenomenon is caused by the lanthanide contraction, where the poor shielding of $4f$ electrons offsets the increase in size due to the addition of a new shell.
4. Hence the answer is (A).