PuO 2 is solid under ambient conditions. It crystallizes in the flourite (CaF 2 ) structure and features eight-coordinate plutonium with cubic site symmetry and fourcoordinate oxygen sites. PuO 2 would appear to be a straightforward case of assigning Pu and O 2-formal ion charges, along with Pu(+IV) and O(-II) oxidation numbers (ONs) and an open-shell Pu 5f electron configuration. The number of open-shell electrons is therefore n open = 4. The magnetic susceptibility of PuO 2 as a function of temperature is fully consistent with the assigned 5f configuration and a non-magnetic ground state, although it took several decades of theoretical modeling to understand why the magnetic susceptibility is effectively constant in the temperature range 1-1000 K. A recent publication by Tobin et al. assigned the Pu ion in PuO 2 as having a 5f 5 configuration, however, and therefore a Pu(III) ion, based on X-ray spectroscopic measurements and accompanying model calculations.