Question

What is meant by the highest occupied energy level in an atom?

Answer 1

It is the highest-energy atomic orbital in an atom that is filled with electrons. It is otherwise known as a valence orbital, or a frontier orbital (i.e. an orbital at the "frontier" of chemical reactions, performing the interesting legwork to move the reaction forward).

Ordering orbitals by energy is straightforward; energy is quantized, so the higher the principal quantum number `n`, usually the higher the energy of the orbital.

(Of course, that is a simplification that neglects the influence of the shape of the orbital on its energy, but for our purposes it is a good general rule.)

Suppose all the orbitals below are fully occupied. Can you identify the highest-occupied atomic orbitals here? (There are 3.)

Note that the `4s` orbital can be lower in energy than the `3d` sometimes, but it is actually the valence orbital for most first-row transition metals and is actually higher in energy in those cases, so this diagram is not entirely correct.