What is the correct order of ionic radii for the following lanthanoid ions?

Solution

1. Lanthanoid contraction refers to the steady decrease in ionic radii from ${\text{La}}^{3+}$ to ${\text{Lu}}^{3+}$.
2. The atomic numbers are $\text{Ce}(58)$, $\text{Nd}(60)$, $\text{Eu}(63)$, and $\text{Gd}(64)$.
3. As atomic number increases, the effective nuclear charge increases, pulling electrons closer.
4. Hence the answer is (A).