Crossover Energy Levels
That sounds kind of vague to me. How does the repulsion of other electrons
mess up the energy levels?
Well...the fundamental difference between the various sublevels is that higher
sublevels have more angular momentum. If you don't know what that means,
don't worry; all you need to understand is that more angular momentum tends to
fling an electron farther out from the nucleus. When there are many other
electrons around, a screening effect occurs.
So that's why the different sublevels have different energies.
Yes. Furthermore, when the number of electrons becomes large, this screening
effect becomes so strong that it actually begins to overlap the next primary
level. An added electron will then prefer to enter that next level rather
then go to the orbital where it "should" be.