The Pauli Exclusion Principle See what happens when you click on lithium (Li)--that's element number three in the periodic table. There are three electrons, all right--but why is the yellow one so much higher up on the chart? In the picture, it looks like that one is a lot farther away from the nucleus than the others...hey, does that mean it's in a higher energy level? Exactly. If you move the mouse over any electron on the chart, you'll see a little blue number appear above it. This tells you, in eV, how much energy it would take to free that electron from the clutches of the nucleus. In the case of the outermost electron, this is called the ionization energy. Hmm...the electrons in the lower row have higher numbers listed. I guess that makes sense--the closer they are to the nucleus, the more strongly the electric force would be pulling them in. But that means the electrons in higher energy levels have lower numbers on the chart... The terminology is a little confusing, I agree. Think of it this way: the more energy of its own an electron has, the less additional energy it needs in order to escape. Okay, so why is that third electron in a higher level than the first two? Why not just add it to the lowest one? Because the lowest level is "full"; it can't hold more than two electrons. Why? That sounds totally arbitrary to me. The rule that's operating here is called the Pauli exclusion principle, first proposed by Wolfgang Pauli. Pauli guessed that two electrons can't be in the same "quantum state"--I'll explain more fully what that means later. In this context, it means that two identical electrons can't be in the same energy level in the same atom. But you just told me that the lowest energy level can hold two electrons. Ah, yes--two electrons that are not identical. They differ in a characteristic called spin...

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