The shell model we have been using is one representation of the atomic theory. We have been using to try to understand something about the influence electrons exert in sticking atoms together. But remember it is only a model, useful in describing one aspect of electrons under certain conditions. While it is, hopefully, useful, and may be correct as far as it goes, it is far short of the complete picture. Or else what would university chemistry lecturers talk about?

In the shell model some electrons are depicted as being in a closer orbit to the nucleus than others. This is a nice, clear picture, but has at least two flaws. The first is that electrons do not travel in neat, circular, or even elliptical paths around their nuclei. Do not imagine atoms as some sort of miniature solar system, with a sun-nucleus in the middle surrounded by lots of little planet-electrons. The path of an individual electron cannot be known. It isn't that we have no way of finding that out - rather in finding it out we necessarily alter it in ways we cannot predict. This discussion is a way into quantum mechanics, but we aren't going there just now.

The second flaw is that the shells occupied by the electrons are spatial. The shells do not represent physical distance from the nucleus. They represent amounts of energy. Electrons pictured as closer to the nucleus have less energy than more distant ones in the same way that a brick suspended one millimetre above your head has less energy (capacity to do damage) than a brick suspended one metre above your head. (And more on that next term.) Thus, the shells do not represent different distances from the nucleus, but different amounts of energy - energy levels, if you will.

Considering these two flaws, it follows that the amounts of energy in different electrons often coincide with their physical distance from the nucleus, but not always.

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