How many of the following explicitly listed pairs of elements have nearly identical atomic radii str — d and f Block Elements Chemistry Question

💡 Solution & Explanation

Lanthanoid contraction heavily affects the elements of the $5d$ series immediately following the Lanthanides (from $Hf$ onwards). It causes the radii of $5d$ elements to shrink to the size of their $4d$ counterparts. The affected pairs here are $(Zr, Hf)$, $(Nb, Ta)$, and $(Mo, W)$. $(Y, La)$ are not affected because $La$ comes before the $4f$ filling. $(Ti, Zr)$ represents a $3d$ - $4d$ pair, where size naturally increases. Therefore, there are exactly $3$ such pairs.