Wolfgang Pauli · The exclusion principle
Why don't all the electrons fall to the lowest level?
That time, that place
Bohr's atom allowed electrons only on fixed shells. Something was odd about it.
If lower energy is more stable, every electron should drop to the innermost shell — and every element would then behave alike. In fact the shells fill up in order, and how they fill is what makes an element what it is.
Why this question
Pauli's rule was a prohibition: no two electrons may occupy the same state at once.
Like seats. However good the front row, if it is taken you sit further back. So electrons fill the shells in order.
Why this should be, he did not know. Only that assuming it made the observed spectra come out right.
What was found
This one prohibition sets the shape of the world.
It is why elements differ. The periods of the table running 2, 8, 8, 18 fall straight out of counting the available seats. All of chemistry sits on it.
It is also why matter takes up space. Your hand does not pass through the desk not only because of electrical repulsion but because electrons cannot share a state.
And whether a heavy star stops at a neutron star or collapses into a black hole is a question of whether this prohibition holds.
There was no reason why electrons fill shells in order
Only this prohibition explained the spectra and the periodicity
The periodic table acquired a reason, and matter a reason to occupy space