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Which of the following is considered the principal reason for lanthanoid contraction?

1. Better shielding of 5d electrons by 4f electrons
2. Inadequate shielding of 5d electrons by 4f electrons
3. Effective shielding of one 4f electron by another within the same subshell
4. Poor shielding of one 4f electron by another within the same subshell

Correct answer: Poor shielding of one 4f electron by another within the same subshell

Solution

Lanthanoid contraction arises because 4f electrons shield one another from the nuclear charge very poorly (imperfect/poor shielding of one 4f electron by another). The effective nuclear charge therefore increases across the series, steadily contracting the atomic/ionic radii.

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