Question

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

• A. Greater shielding of $5d$ electrons by $4f$ electrons
• B. Poorer shielding of $5d$ electrons by $4f$ electrons
• C. Effective shielding of one of $4f$ electrons by another in the subshell
• D. Poor shielding of one of $4f$ electron by another in the subshell

Answer 1

Answer: (B)

In lanthanides, there is poorer shielding of $5d$ electrons by $4f$ electrons resulting in greater attraction of the nucleus over $5d$ electrons and contraction of the atomic radii.