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

$\mathrm{Ce}^{2+}=[\mathrm{Xe}] 4 \mathrm{f}^{2}$ (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.