Spin

Spin? What's that?

Well, it's an additional property that electrons (and other particles) possess. Here's an analogy: think about the Earth orbiting the sun--

Haven't you just been pounding into my head that electrons don't orbit like planets?

It's true, they don't--and yet that picture remains helpful and illuminating in many contexts. So bear with me for a moment: think about the Earth. Not only does it orbit around the sun once a year, it's also spinning once a day on its own axis...

And that's what spin is! Although I suppose you're going to tell me that electrons don't really spin, any more than they really orbit.

You catch on quickly. They don't spin--but it's tremendously useful to think about them as if they did, and for most practical purposes, you can. In the present case, think of the two electrons in that lowest energy level as spinning in opposite directions. It's often said that one has "spin up" and the other "spin down."

So each level has room for a spin up and a spin down--that makes sense. But you haven't explained s, p, and d yet; there must be more complications to worry about.

Indeed...