Pauli exclusion principle

The rule that no two electrons in the same atom can be in exactly the same state. Set out by Wolfgang Pauli in 1925.

In 1925 Pauli announced that the state of each electron in an atom is specified by four numbers, and that no two electrons in one atom can share the same set of four^[1]. Of the fourth number Pauli would say only that it was a "two-valuedness" that could not be described classically; within the year it was interpreted as spin.

Because of this rule, electrons cannot all crowd into an atom's lowest-energy state; they fill up the available "seats" from the inside out. How the seats fill decides an element's properties, and so the rule explained why elements with similar properties recur at regular intervals in the periodic table.

In the chemical bond, too, two atoms attract only when their two electrons pair with opposite spins. The same rule applies to protons and neutrons as well as electrons, and it lies at the root of why matter cannot be squeezed without limit. Pauli received the 1945 Nobel Prize in Physics.