Question

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

• A. Poor shielding of $4f$- electrons as compared to other electrons
• B. Effective shielding of one of the $4f$ electrons as compared to other electrons
• C. Poorer shielding of $5d$ electron by $4f$ - electrons
• D. Greater shielding of $5d$ electron by $4f$ - electron

Answer 1

The correct option is A Poor shielding of $4f$- electrons as compared to other electrons

This phenomenon is associated with the intervention of the $4f$ orbitals, which must be filled before the $5d$ series of elements begin. The filling of $4f$ before $5d$ orbital results in a regular decrease in atomic radii called lanthanoid contraction. This is because of poor shielding of one of the $4f$ - electrons by another in the sub - shell.