What exactly is the Pauli Principle?
If you have identical particles called ‘fermions’ which include electrons, protons and quarks for example, Pauli’s Exclusion Principle states that you may not have more than two of these particles in the same quantum state defined by its energy and angular momentum quantum numbers. All other elements of their quantum state being equal, you may put exactly two of these fermions in the same super-state, provided that one has its spin vector pointed ‘up’ and the other has its spin vector pointed ‘down’. This is because for a 1/2-integer spin particle such as a fermion, there are exactly two spin orientations that give the same magnitude of 1/2: +1/2 and -1/2. In chemistry we learn that there can be at most 2 electrons in each ‘orbital’ within the atom defined by N the energy quantum number, and ‘s, p, d, f …’ the angular momentum quantum number. When an orbital is filled, no more electrons can be accommodated, and in fact, this produces a degeneracy pressure that disallows other electro
