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JEE Main levelSingle correct optionDifficulty 2 of 3df179

Assertion (A): The atomic radii of and are almost equal. Reason (R): The electrons shield the nuclear charge poorly across the lanthanoid series. Choose the correct option.

  1. ABoth A and R are true and R is the correct explanation of A
  2. BBoth A and R are true but R is not the correct explanation of A
  3. CA is true but R is false
  4. DA is false but R is true

Answer: A

Between Zr (Z = 40) and Hf (Z = 72) fourteen electrons are added. They shield poorly, so the effective nuclear charge rises and offsets the expected increase of size on adding a shell. Thus Zr and Hf have nearly the same radii, and R correctly explains A.
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