Electron Clouds and Energy Levels Okay, so why is it that s only holds two electrons, but p has room for six? Well...remember that each electron is existing in one of those strange probability clouds, which, as you've seen, can have widely varying shapes and sizes. Another statement of the Pauli exclusion principle I mentioned is this: no two electrons in an atom can be in the same type of cloud with the same spin. So you're saying that p electrons have more cloud shapes available to them? Precisely. It happens that s clouds are always spherical; the spheres just get bigger as the primary energy level increases.

However, p and d states are more interesting: there can be several different-shaped clouds at the same energy. For example, here are two p states from the second primary level:

It turns out that there are three kinds of p clouds in each primary level.

...and each one can hold a spin up and a spin down, so that's why six electrons fit into the p column!

Very good. You can think of each electron's "quantum state," its full, unique description, as being the sum of a particular probability cloud plus a spin:

	+
A certain d state in the third primary level		spin down

Futhermore, there are five different shapes for d (hence room for ten electrons), and seven in the next sublevel, f...I hope you're noticing a pattern here. (If you want to know more about where these numbers come from, go ask Dr. Mahan about quantum numbers.)