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
Atomic radius generally increases down a group. However, due to the lanthanoid contraction, elements of the 5d series have nearly the same atomic radii as their counterparts in the 4d series. Lanthanoid contraction is the poor shielding effect of the 4f electrons, which increases the effective nuclear charge and pulls the valence electrons closer to the nucleus. Zr(4d) and Hf(5d) are in the same group, and size of Hf is reduced by the lanthanoid contraction, making their radii nearly identical. Mo (4d) and W(5d) are also a pair in the same group where the size of W is reduced by the lanthanoid contraction, resulting in almost identical radii. The other pairs, Y-La and Cr-Mo, do not exhibit this effect to the same extent, and their atomic radii differ significantly. Therefore, the pairs with nearly the same atomic radius are Zr−Hf and Mo−W.