Question

Lanthanoid which has the smallest size in \( +3 \) state is:

• A. Tb
• B. Er
• C. Ce
• D. Lu

Hint:

In the lanthanide series, as the atomic number increases, the electrons are added to the \( 4f \)
subshell, which does not shield the nucleus effectively, leading to a decrease in atomic and ionic size. This is known as the lanthanide contraction.

Answer 1

The Correct Option is D

Within the lanthanide series, ionic size diminishes from \( Ce^{3+} \) to \( Lu^{3+} \) due to an increase in effective nuclear charge with rising atomic number, while electron shielding remains largely unchanged. Lanthanum and Lutetium exhibit the smallest ionic radii in their \( +3 \) oxidation state. Lutetium (\( Lu^{3+} \)) possesses the smallest ionic radius owing to f-block contraction, a phenomenon where atomic size decreases as atomic number increases from \( La^{3+} \) to \( Lu^{3+} \).Final Answer: Option (d) \( Lu \).