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Q.
Which of the following factor may be regarded as the main cause of Lanthanide contraction ?
Solution:
This phenomenon is associated with the intervention of the $4 f$ orbitals which must be filled before the $5 d$ series of elements begin. The filling of $4 f$ before $5 d$ orbital results in a regular decrease in atomic radii called
Lanthanoid contraction. This is because of poor shielding of one of the $4 f$- electrons by another in the sub-shell.