Diamagnetic ions have all their electrons paired, meaning they possess either an empty ($f^0$) or completely filled ($f^{14}$) subshell.

• $Ce$ ($Z = 58$) has the configuration $[Xe] 4f^1 5d^1 6s^2$. The $Ce^{4+}$ ion loses all valence electrons to achieve an empty $4f^0$ configuration, making it diamagnetic .
• $Yb$ ($Z = 70$) has the configuration $[Xe] 4f^{14} 6s^2$. The $Yb^{2+}$ ion loses its two $6s$ electrons to achieve a completely filled $4f^{14}$ configuration, making it diamagnetic . All other given ions, such as $Ce^{3+}$ ($4f^1$), $Eu^{2+}$ ($4f^7$), $Gd^{3+}$ ($4f^7$), $Eu^{3+}$ ($4f^6$), $Pm^{3+}$ ($4f^4$), and $Sm^{3+}$ ($4f^5$), have unpaired $f$ electrons and are therefore paramagnetic.