Pauling electronegativity

Electronegativity (denoted χ) is not calculated from principles, the scales for electronegativity are empirical. The Pauling scale is the most commonly used one, in which he relates the electronegativity difference of two elements to the bond strength between two elements:

Where
B(A-B) = Disassociation energy of of A-B bond
B(A-A) = Disassociation energy of A-A bond
B(B-B) = Disassociation energy of B-B bond
χ_A = Electronegativity of A
χ_B = Electronegativity of B
C = Constant. Takes the value of 96.5 if disassociation energies are in kJ mol^-1

The strength of A-B bonds are higher than the average of A-A and B-B bonds, he attributed that extra strength to an ionic resonance structure:

The true structure therefore lying somewhere in between

The stability of this resonance structure increases with the electronegativity difference between the two atoms, therefore increasing the difference in energy. This ionic contribution is the C(X_A+X_B)² component.