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JEE Main levelSingle correct optionDifficulty 2 of 3pt078

Which pair of elements has almost identical atomic radii as a direct result of the lanthanoid contraction?

A $Zr$ and $Hf$
B $Cr$ and $Mo$
C $Ti$ and $Zr$
D $Y$ and $La$

Answer: A

The 4f electrons added between La and Hf shield poorly, so

Z_{eff}

rises and cancels the size increase expected from an extra shell. Zr and Hf (and similarly Nb and Ta) end up with nearly equal radii.

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