I'm trying to understand the basic generalisations of how an atom works. I understand there are discrete energy levels or shells (in a simplified atomic model), in which only a specific number of electrons can occupy. The innermost shell (closest to the nucleus) is the lowest energy, and the outermost, the valence shell, has the highest energy.
I've read that, at absolute zero, only the valence shell is filled. I don't understand why this is the case and not the other way round. If you reduce the temperature of an atom down to 0K, then wouldn't that mean the electrons lose energy, instead of gaining it to hop away from the nucleus?
Also, in terms of energy bands, I'm confused as to why on band gap diagrams the valence band is shown as having the lower energy compared with the conduction band.
Finally, could someone please clarify in simple terms the difference between the quantum state of an electron and the energy level of a shell.
I know this is probably quite basic but any help would be appreciated!