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Lanthanoid contraction is caused due to

1. the same effective nuclear charge from Ce to Lu
2. the imperfect shielding on outer electrons by 4f electrons from the nuclear charge
3. the appreciable shielding on outer electrons by 4f electrons from the nuclear charge
4. the appreciable shielding on outer electrons by 5d electrons from the nuclear charge

Correct answer: the imperfect shielding on outer electrons by 4f electrons from the nuclear charge

Solution

Across the lanthanoids the 4f electrons shield the outer electrons poorly from the increasing nuclear charge. This imperfect (weak) shielding lets the effective nuclear charge rise steadily, pulling electron shells inward and causing the gradual decrease in size known as the lanthanoid contraction.

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