Pauli Exclusion Principle

The Pauli exclusion principle is more than an intellectual game by which quantum number sets can be worked out for individual electrons. Beyond that, the principle allows one to determine the electronic configuration—the way the electrons are arranged—within any given atom. It shows that for an electron with 15 electrons, for example, two and only two can occupy the first shell, eight more (and only eight more) can occupy the second, leaving five electrons in a third shell.

The exclusion principle also demonstrates that, as with an onion, there are layers within layers. In the third shell of an atom, for example, there are three sub-divisions, known as orbitals. One orbital has the secondary quantum number of 0, one has the secondary quantum number of 1, and one, the secondary quantum number 2.