The valence shell of an atom, is the set of orbitals which are energetically accessible for accepting electrons to form chemical bonds. For various atoms, the valance electrons are submerged beneath filled shells with a higher principal quantum number . This type of electronic configuration is called a submerged-shell .
Before I learnt about submerged shells, I was under the impression that valance electrons were always in the shell with the highest principal quantum number $n$, since it denotes the distance of a shell from the nucleus. However the literature I have read today indicates that this is not always the case, and I was wondering why. I expected that the outermost electrons would fill the lower shells first, and that if they did not, then they would get in the way of chemical bonds, shielding electrons in lower orbits in some way.
Why is it that the valance electrons of some atoms are not in the outermost shell, with the highest principle quantum number?