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The 2nd and 3rd transition series (4d and 5d) have very similar atomic radii because of:
A**Lanthanoid contraction** — filling of 4f orbitals (poor shielding) before the 5d series → expected size increase is cancelled
BIdentical electron configurations
CCoincidence
DThe general trend of size increasing down a group
Answer & Solution
Correct answer: A. **Lanthanoid contraction** — filling of 4f orbitals (poor shielding) before the 5d series → expected size increase is cancelled
**Lanthanoid contraction**: 14 f-electrons added across the lanthanoid series (Ce → Lu); 4f electrons shield poorly → effective nuclear charge increases → atoms contract. Result: **5d series atoms are nearly the same size as 4d series**. Zr (160 pm) ≈ Hf (159 pm) — explains why they are hard to separate.
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