In atoms, why are the energy levels furthest from the nucleus considered to be higher in energy?
The reason why energy shells furthest from the nucleus are considered to be higher in energy than those closer to the nucleus is clear-cut. The further an energy shell is from the nucleus, the lesser the strength of its pull becomes. This is because the nucleus contains protons, which are attracted to electrons. The further electrons are away from protons, the less strength of the pull will become, and ultimately will result in electrons having more energy.