Don't forget that to the name of the subshell and
the name of the orbital is the same.
So we're in the 1s subshell and it has one orbital.
I'm drawing that orbital with a box, and the name of that orbital is 1s.
So now we're ready for the Pauli exclusion principle.
This is the statement of that principle.
No two electrons in an atom can have the same four quantum numbers.
What are those quantum numbers?
n, l, m sub l, and m sub s.
You cannot have any two electrons with the exact same four quantum numbers.
Well, the net result of that
principle is that you can have no more than two electrons in any orbital.
Because the orbital is being defined by the first three numbers.
You can have no more than two,
because once you have one electron, it's going to be spinning in one direction.
And, then to have, not to have the same four,
the second one will have to spin in the opposite direction.
So that's the net result.
No two orbitals can have more than two electrons.
I mean, no orbital can have more than two electrons.
And those electrons must spin in the opposite direction.
So, let's look at helium.
Helium has two electrons.
If we were to do the orbital diagram of helium, we could put both of
those electrons in the same orbital that we had for the hydrogen, okay?
But it has to spin in the opposite direction, okay?
If we were to assign the quantum numbers for
those electrons, actually we'll do that here in a little bit.
Okay?
So each electron is going to have a set of four quantum numbers.
The first three give the location, what shell, subshell, and
orbital is the electron located in.
And the fourth gives the spin.
So if we look once again at the hydrogen atom, I mean, the helium atom,
and we look at those two electrons and we assign quantum numbers for it.
Okay?
The first one would have the quantum numbers 1, 0, 0,
plus one-half for the up spin.
And the other one would have 1, 0, 0, minus one-half.
So they do not have the same four quantum numbers.
Three are the same, but the fourth one is different, and that is acceptable.