The pair of lanthanoid ions which are diamagnetic is

• A

  $\mathrm{Ce}^{3+}$ and $\mathrm{Eu}^{2+}$
• B

  $\mathrm{Gd}^{3+}$ and $\mathrm{Eu}^{3+}$
• C

  $\mathrm{Pm}^{3+}$ and $\mathrm{Sm}^{3+}$
• Correct answer
  D

  $\mathrm{Ce}^{4+}$ and $\mathrm{Yb}^{2+}$

Solution

Magnetic moment $\mu=\sqrt{n(n+2)}$ $\mathrm{n} \rightarrow$ number of unpaired electron
Hence $\mathrm{Ce}^{4+}$ and $\mathrm{Yb}^{2+}$ are only diamagnetic.

Hence, the correct answer is option: 'D'.