In 1932, Linus Pauling was the first to propose a scale of electronegativity. He based his scale on the difference ( ΔE ) between the measured energy of a bond A-B [E _{(A - B)} ] and the energy expected for the purely covalent bond A - B[ E _{(A - B)cov} ]  That is :
__________________      ΔE =  [E _{(A - B)} ] - [ E _{(A - B)cov} ]      
According to him if the tow atom A and B have the same electronegativity values then the molecule AB in bonded by purely covalent bonds as in homonuclear molecules A2 or B2. Then , [ E _{(A - B)cov} ] would be the mean of [E _{(A - A)} ] and [E _{(B - B)}] that is:                                            
____________________  [ E _{(A - B)cov} ]    = 1  [E _{(A - A)} ] + [E _{(B - B)} ] (Arithmetical mean)
                                                                              2
or __________________ [ E _{(A - B)cov} ] =   [E _{(A - A)} ] - [E _{(B - B)} ]^1/2 (Geometrical mean)
Pauling used geometric mean for his caluclatation. Substituting , we get
___________________   ΔE = E _{(A - B)} ] - [E _{(A - A)} ] - [E _{(B - B)} ]^1/2
When molecule AB is purely covalent then E _{(A - B)}] =  [ E _{(A - B)cov} ]  and ΔE = 0
____________________  ΔE = E _{(A - B)} ] _- [ E _{(A - B)cov} ] = 0
____________________  ΔE = E _{(A - B)} ] - [E _{(A - A)} ] . [E _{(B - B)} ]^1/2 = 0
However, if the two atoms A and B have different electronegativities, the bond A-B will no longer be purely covalent and the energy  E _{(A - B)}  would be greater than the mean of E _{A - A} and E _{B - B}
 That :  ______________   E _{(A - B)} ] > [E _{(A - A)} ] . [E _{(B - B)} ]^1/2
In other words, ΔE  would be greater than zero.

This excess bond energy ΔE, is called the ionic-covalent resonance energy and arises due to the resonance stabilization of sort:
___________________         A - B <==> A + B  _____________ ( B more electronegative than A )
                                              covalent         ionic



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