Which of the following lanthanoid ions is diamagnetic?

Solution

1. The electronic configuration of $\text{Ce}$ is $[\text{Xe}]4f^{1}5d^{1}6s^2$.
2. The ${\text{Ce}}^{4+}$ ion is formed by losing four electrons, resulting in $[\text{Xe}]4f^0$.
3. Since there are no unpaired electrons in the $4f$ subshell, ${\text{Ce}}^{4+}$ is diamagnetic.
4. Hence the answer is (A).