The Pauli Exclusion Principle& the “Building up Principle”
Electrons in an atom are arranged in individual “orbitals” each having a set of quantum numbers (n, l, ml, ms)
No two occupied electron orbitals can have the same set of quantum numbers.
The electrons in an atoms occupy the set of lowest energy orbitals consistent with this exclusion principle.
This “building up principle” was a meaningful way to understand the periodic table of the elements, and the fact that it works shows that it contains a least some elements of truth and reality.