Which of the following specific pairs of transition elements definitively represent chemical "twins" — d and f Block Elements Chemistry Question

💡 Solution & Explanation

The lanthanoid contraction severely shrinks the atomic radii of the $5d$ series elements following the lanthanides, pulling them down to match the sizes of their corresponding $4d$ group members. This applies to Group 4 ($Zr/Hf$), Group 5 ($Nb/Ta$), and Group 6 ($Mo/W$). However, Yttrium ($Y$) and Lanthanum ($La$) belong to Group 3; $La$ is situated *before* the filling of the $4f$ block begins, so its size is not shrunken by the lanthanide contraction, and it remains noticeably larger than $Y$.