Pauli Exclusion Principle: Why No Two Electrons Match

The Pauli Exclusion Principle was proposed by Wolfgang Pauli in 1925.
It states that no two electrons in an atom can have the exact same quantum state.
This rule arises from electrons being fermions—particles with half-integer spin.
It forces electrons to occupy different energy levels or spin orientations.
The principle explains why electrons build “shells” around atoms.
Without it, all electrons would collapse into the lowest energy state.
It gives matter its structure, stability, and resistance to compression.
The size of atoms, and thus all chemistry, depends on this principle.
White dwarf stars resist collapse because electrons obey the Pauli rule.
The Pauli Exclusion Principle is why the universe has volume instead of being a single dense clump.