Why do zirconium (Zr, atomic number 40) and hafnium (Hf, atomic number 72) possess virtually identical atomic and ionic radii (approximately 160 pm and 159 pm, respectively)?

Answer and explanation

Correct answer: B. Due to the lanthanoid contraction caused by the poor shielding effect of intervening 4f electrons

  1. Step 1: Zirconium belongs to the second transition series (4d), and hafnium belongs to the third transition series (5d) directly beneath it in Group 4.
  2. Step 2: Normally, atomic radii increase down a group due to the addition of an extra principal electron shell.
  3. Step 3: However, before hafnium, the fourteen electrons are filled across the lanthanoids.
  4. Step 4: Because orbitals have diffused shapes and exhibit very poor shielding of nuclear charge, the effective nuclear charge increases steadily, pulling the valence shells inward (lanthanoid contraction).
  5. Step 5: This contraction precisely cancels the expected size increase from period 5 to period 6, making the radii of and virtually identical.
Answer
Due to the lanthanoid contraction caused by the poor shielding effect of intervening 4f electrons
Why not the others: 'Because both belong to the same period...' is false (Zr is in Period 5, Hf is in Period 6); 'Because hafnium has an inert s-electron pair...' confuses lanthanoid contraction with the inert pair effect; 'Because zirconium undergoes radioactive alpha decay...' is chemically and physically absurd.

Practise more Coordination Compounds and d- and f-Block Elements.

Free questions with an explanation for every one, and full timed NEET UG mock tests.

All Coordination Compounds and d- and f-Block Elements questionsTake a mock test

More Coordination Compounds and d- and f-Block Elements questions