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JEE Main levelSingle correct optionDifficulty 2 of 3df014

Zr (160 pm) and Hf (159 pm) have nearly identical atomic radii because

  1. AHf has fewer protons than expected from its position
  2. Bthe 4f electrons shield the nucleus very effectively
  3. Cthe 4f electrons shield the nuclear charge poorly, so effective nuclear charge rises across the lanthanides
  4. DHf lies in the same period as Zr

Answer: C

Between Zr and Hf the 14 lanthanides are filled, and the diffuse 4f subshell screens poorly. The resulting rise in effective nuclear charge (lanthanide contraction) offsets the extra shell of Hf, so the radii are almost equal.
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