## Rules for Electron Configurations

I wish you'd stop being so mysterious and explain these "rules" you keep referring to.

I was just about to do that. I have to warn you, though, that the rules may seem arbitrary to you, and I won't be giving any satisfactory explanation of the reasons for them. Partly that's because I want to spare you a lot of complicated math, and partly it's because this is just the way nature is. I'll just ask you to have faith that all this numerology comes out of a sophisticated mathematical theory, and has been upheld time after time by experiment.

 I can, if you like, tell you about quantum numbers; they provide a more quantitative way of understanding these rules.

For now I think I'll be satisfied if you can tell me how to predict those electron arrangements you've been showing me.

I can do that. First of all, you were correct when you guessed that those colored rows in the chart correspond to the "main energy levels"; they're often called primary energy levels, incidentally. Usually, a higher row means a higher energy, and energy gaps between rows tend to be quite large, in comparison with the gap between, say, s and p.

Are you ever going to explain what s and p mean?

I'll do that right now. As you surmised, the s, p, and d columns represent smaller "sublevels" of the primary rows...

Then why not just call them A, B, and C, or something else at least vaguely logical?

 This is a bit of archaic notation left over from nineteenth century spectroscopy--rather silly, but everyone uses it, so we're stuck with it. If you must know, s stands for "sharp," p is for "principal," and d is for "diffuse"--supposedly they refer to the appearance of various spectral lines. The next one is called f, for "fundamental"; mercifully, the subsequent ones just go alphabetically: g, h, etc.

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