Why matter is stable and diverse

Pauli explains the periodic table

The Pauli principle, combined with the energy levels, explains the structure of all atoms and the order of the periodic table. Electrons fill the shells from the lowest to the highest, two per orbital, until they run out.

   Remplissage des couches (schema simplifie) :

   couche 3 : _ _ _ _ ...    (se remplit apres la 2)
   couche 2 : ↑↓ ↑↓ ↑↓ ↑↓    (8 electrons max)
   couche 1 : ↑↓              (2 electrons max)
              ^
              on remplit du bas vers le haut, 2 par case

It is the electrons of the outer shell (the last one filled) that determine the chemical behavior of the atom. Two elements whose outer shells resemble each other have similar properties — this is exactly what arranges the elements into columns of the periodic table.

   Gaz nobles (helium, neon...) : couche externe PLEINE
        -> tres stables, ne reagissent presque pas

   Alcalins (lithium, sodium...) : UN electron seul en couche externe
        -> tres reactifs, cedent facilement cet electron

All of chemistry — why a given atom reacts, which bonds it forms — follows from this filling governed by Pauli. Without Pauli, there would be only one kind of atom, and no chemistry.

Pauli prevents matter from collapsing

The most profound consequence is the very stability of matter. Electrons, as fermions, refuse to pile into the same state. This creates a kind of "pressure" that keeps them apart from one another: the degeneracy pressure.

It is this pressure that gives atoms their volume, and that prevents ordinary matter from collapsing on itself. When you lay your hand on a table, what keeps you from passing through it is not, first and foremost, electrical repulsion: it is, in large part, the electrons' refusal to occupy the same quantum states.

   deux atomes qu'on rapproche :
        les electrons devraient partager les memes etats
        -> Pauli l'interdit -> forte "resistance" au rapprochement
        -> la matiere est "solide", elle occupe du volume

All the way to the stars

This degeneracy pressure acts even on the astronomical scale. In a white dwarf (the corpse of a star like the Sun), no nuclear reaction any longer supports the star against its own gravity. What keeps it from collapsing is solely the degeneracy pressure of the electrons — the Pauli principle, on the scale of a star.

Beyond a certain mass (the Chandrasekhar limit), even Pauli is no longer enough: the star collapses into a neutron star, or even a black hole. The same quantum principle that structures the atom decides the final fate of stars.

In summary

The Pauli principle, by forcing electrons to fill successive shells (two per orbital), explains the structure of atoms and the order of the periodic table: it is the electrons of the outer shell that do the chemistry. It also generates the degeneracy pressure, which gives matter its volume, prevents it from collapsing, and supports white dwarfs against gravity. From the atom to the star, the stability of matter rests on this simple prohibition.