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Nuclear Parity :
Parity is a fundamental nuclear property which is considered more important than spin. 
According to Quantum mechanics “every moving particle has a wave associated with it”
which corresponds to a wave function ψ which is function of x, y, z. 
If we replace (-x) with (x) , (-y) with (y)  and (-z)  with (z) then the wave function is said to possess an even parity, if 
                ψ (x, y, z) = ψ(-x, -y, -z) 
On the other hand, the wave function ψ will corresponds if :
               ψ(x, y, z) = –ψ( -x, -y, -z )
Thus parity is conserved. 
Pairing Energy :

If the nucleus contains even number of protons and even number of neutrons, it is stable. It either number is odd the stability changes. 
This pairing effect changes the binding energy.
where δ is a constant. 
Considering all the binding energy terms, the semi-emperical mass formula may be written as :