This study demonstrated a comprehensive theoretical & experimental approach to characterize the actinide 5f electrons, using plutonium as an example. Specifically, we emphasize that a distinction must be made between the formal open-shell 5f occupation n open and the combined effective closed-and open-shell orbital occupation n f . The latter reflects the donation and back-donation of electrons, that is, covalent bonding. The concept is illustrated by the example of PuO 2 , by using state-of-the-art high energy resolution synchrotron X-ray spectroscopy and quantum chemical calculations. Our conclusions impact the oxidation state problem in f-element chemistry more generally.