\(\mathrm{Ce}_{58}=[\mathrm{Xe}] 4 \mathrm{f}^{2} 5 \mathrm{d}^{0} 6 \mathrm{s}^{2}\)

\(\mathrm{Ce}^{2+}=[\mathrm{Xe}] 4 \mathrm{f}^{2}\) (two unpaired electrons)

\(\mathrm{Sm}_{62}=[\mathrm{Xe}] 4 \mathrm{f}^{6} 5 \mathrm{d}^{0} 6 \mathrm{s}^{2}\)

\(\mathrm{Sm}^{2+}=[\mathrm{Xe}] 4 \mathrm{f}^{6}(\mathrm{six} \text { unpaired electron })\)

\(\mathrm{Eu}_{63}=[\mathrm{Xe}] 4 \mathrm{f}^{7} 5 \mathrm{d}^{0} 6 \mathrm{s}^{2}\)

\(\mathrm{Eu}^{2+}=[\mathrm{Xel}] 4 \mathrm{f}^{7}\) (seven unpaired electron)

\(\mathrm{Yb}_{70}=[\mathrm{Xe}] 4 \mathrm{f}^{14} 5 \mathrm{d}^{0} 6 \mathrm{s}^{2}\)

\(\mathrm{Yb}^{2+}=[\mathrm{Xe}] 4 \mathrm{f}^{14}\) (No unpaired electron)

Because of the absence of unpaired electrons, \(Yb ^{2+}\) is diamagnetic.