The atomic radii of elements in the transition series are noticeably larger than those in the series — d and f Block Elements Chemistry Question

💡 Solution & Explanation

This size similarity is due to the lanthanoid contraction. Because elements of the $4d$ and $5d$ series (e.g., $Zr$ and $Hf$, $Nb$ and $Ta$) possess almost identical sizes and similar valence electron configurations, their physical and chemical properties become extremely similar, making their separation in naturally occurring ores notoriously difficult.