Thorium compounds such as $\mathrm{ThO}{}_2$ are white and diamagnetic. The best explanation uses the configuration of Th ($[\mathrm{Rn}]6d^{2}7s^2$) and the fact that

1. A$\mathrm{Th}{}^{4+}$ has the radon core, with no $f$ or $d$ electrons to give unpaired spins or $f$-$f$ and $d$-$d$ transitions
2. B$\mathrm{Th}{}^{4+}$ has a half-filled $5f$ subshell
3. C$\mathrm{Th}{}^{3+}$ is the stable ion with one unpaired $6d$ electron
4. D$\mathrm{Th}{}^{4+}$ has $5f^2$ electrons that are paired