Pauling Scale Of Electronegativity

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Pauling’s Scale of Electronegativity

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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