Question

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

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

Answer 1

Answer: (A)

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.