The ‘quantum’ principle that says why atoms are as they are

During a period when creative physicists were throwing wild guesses at quantum theory, Wolfgang Pauli came up with an idea that explained why matter behaves the way it does. His innovation: a new quantum number, spin. (Don’t try to picture it, another of Pauli’s innovations was not caring that the idea defies our classical intuition.) Pauli’s exclusion principle suggested, correctly it turns out, that some fundamental particles — such as electrons — can’t be in exactly the same state; if all their other quantum numbers are the same, they must have different spins.