Physicist: There’s no reason for electrons not to fill sub-shells past “f”, it’s just that they don’t need to. A good way to think of the way an electron hangs out around an atomic nucleus is as a “standing wave”. The lines on the periodic table loop around when the s and p sub-shells are filled. For comparison, uranium is element number 92. In order to fill up all of their shells and sub-shells, atoms need to be in a relatively cool environment.