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Filling 4f before 5d causes a steady fall in radii called:
ALanthanoid contraction
BActinoid contraction
CTransition expansion
DPeripheral extension
Answer & Solution
Correct answer: A. Lanthanoid contraction
1. The 4f orbitals fill before the 5d series begins.
2. That filling shrinks the atoms regularly.
3. The effect is called the lanthanoid contraction.
_Source: NCERT Class 12 Chemistry Part I, Chapter 8, The d- and f-Block Elements._
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