This is one of those question you won't find the answer in any book. From Wikipedia: it is impossible for two electrons of a poly-electron atom to have the same values of the four quantum numbers (n, ℓ, mℓ and ms). But how can an electron know the state (the quantum numbers) of the other electrons, that is, which states are already occupied and consequently occupy an available state? Or, vice versa, it's the atom that "tells" (how?) the electron the states that are free and that it can occupy? Thanks. EDIT: Sorry, I've just noticed the error in the title. It was too long and I had to cut it, but I made a mistake.