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Which of the following factors may be regarded as the main cause of lanthanide contraction?

1. Greater shielding of 5d electrons by 4f electrons
2. Poorer shielding of 5d electrons by 4f electrons
3. Effective shielding of one of the 4f electrons by another in the subshell
4. Poor shielding of one of the 4f electrons by another in the subshell

Correct answer: Poor shielding of one of the 4f electrons by another in the subshell

Solution

The correct option highlights that the poor shielding of 4f electrons from each other leads to a reduced effective nuclear charge felt by the outer electrons, resulting in a greater contraction of the lanthanide series as the atomic number increases.

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