The Pauli Exclusion Principle
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
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.
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...