3.7 Pauli Exclusion principle
An electron is described by the four quantum numbers, the first three describe the orbital that the electron is located and the fourth describes the intrinsic electron property (its spin). Since each electron within an atom is described by the four quantum numbers, then no two electrons can have the same four quantum numbers. This is known as the Pauli Exclusion Principle. This means that only two electrons can be within the same orbital (share n, l, and ml values).
If we look at both electrons inside of a He atom, each electron will have the same values for the first three quantum numbers. Both electrons are found within the first shell (principal quantum number n = 1), both electrons are in an s orbital shape (l = 0), and finally both electrons are found within the only orientation possible for a spherical orbital shape (ml = 0). This should make sense, seeing as both electrons are within the same orbital. However, the last quantum number differentiates the difference between the two electrons, one is spinning up and the other is spinning down.
| Quantum Numbers | He (electron 1) | He (electron 2) |
|---|---|---|
n |
1 |
1 |
l |
0 |
0 |
ml |
0 |
0 |
ms |
+½ |
–½ |