Further information 9.3 The Pauli principle

The Pauh exclusion principle is a special case of a general statement called the Pauli principle  

When the labels of any two identical fermions are exchanged, the total wavefimction changes sign. When the labels of any two identical bosons are exchanged, the total wavefimction retains the same sign. 

The Pauli exclusion principle applies only to fermions. Total wavefimction means the entire wavefimction, including the spin of the particles. 

The Pauli principle imphes that it is a fact of nature that the wavefimction must change sign if we interchange the labels 1 and 2 wherever they occiu in the function v (2,l) = vr( 1,2). Suppose the two electrons in an atom occupy an orbital yr then in the orbital approximation the overall wavefimction is l)vr(2). To apply the Pauli principle, we must deal with 

There are several possibflities for two spins the state a( l)a(2) corresponds to parallel spins, whereas (for technical reasons related to the cancelation of each spin s angular momentum by the other) the combination a( 1)P(2) — P( l)a(2) corresponds to paired spins. The total wavefimction of the system is one of the following  

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