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JEE Advanced levelSingle correct optionDifficulty 3 of 3df202

Hafnium (Z = 72) has nearly the same atomic radius as zirconium (Z = 40), although it has 32 more electrons and one more shell. The correct reasoning is

  1. Ahafnium has fewer valence electrons than zirconium
  2. Bthe electrons of hafnium shield the nucleus better than the electrons of zirconium
  3. Cthe 14 electrons added between them shield poorly, so the effective nuclear charge on Hf is high enough to cancel the size gain from the extra shell
  4. Dthe electrons of hafnium shield the nuclear charge completely

Answer: C

After the series the 14 lanthanoids add electrons with poor mutual shielding (lanthanoid contraction). The nuclear charge grows by 32 but shielding increases much less, so Zeff is large and the 6th-period atom is as small as its 5th-period congener. The other statements are wrong: both have four valence electrons, and shielding by is poor, not better.
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