Lanthanoids

A diamagnetic ion has no unpaired electrons.
$Yb^{2+}$ (Z = 70): $[Xe]4f^{14}5d^06s^0$ — all electrons paired, so it is diamagnetic.

The general configuration of the lanthanoids is $[Xe]4f^{1-14}5d^{0-1}6s^2$.
All of them readily lose the two 6s electrons and one 5d or 4f electron to form $Ln^{3+}$ ions.
The +3 state is the most common and most stable oxidation state for every lanthanoid.
+2 and +4 states are shown only by a few elements where an empty, half-filled or completely filled 4f subshell results, e.g., $Ce^{4+}$ ($4f^0$), $Eu^{2+}$ ($4f^7$), $Yb^{2+}$ ($4f^{14}$), $Tb^{4+}$ ($4f^7$); +5 is not shown.
Hence the most common oxidation state is +3.