Crossover Energy Levels What's happening here, roughly, is this: you can think of the series of primary levels as being based on the electrons' attraction to the protons; each successive row moves another "step" away from the nucleus. But once you start getting a lot of electrons around, they begin repelling one another like crazy and messing up this nice pattern. 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.

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