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Pauli Exclusion Principle

Otto Stern and Walther first observed electron spin magnetism when they directed a beam of silver atoms into the field of a specially designed magnet.  The beam of atoms is split in half by the magnetic field.  The beam of atoms is split into two because the electron in each atom behaves as a tiny magnet with only two possible orientations.  Not all of the many arrangements of electrons among the orbitals of an atom are physically possible.  The Pauli exclusion principle states that no two electrons in the same atom can have have all four quantum numbers the same.  Because electrons in a given orbital must have the same values of n, l, and ml (for example n = 4, l = 0, ml = 0 in the 4s orbital) they must have different values of ms, Only two values of ms are possible: +1/2 and -1/2.  That means that an atomic orbital can accommodate only two electrons and these electrons must have opposing spins.