Q.Briefly give the basis for Pauling's Scale of electronegativity.

Pauling based his electronegativity scale on the observation that a heteronuclear bond A-B is always stronger than the average (geometric mean) of the A-A and B-B homonuclear bond energies, and he attributed this 'extra' bond energy to the ionic-covalent resonance caused by the electronegativity difference between A and B.

Linus Pauling noticed experimentally that the bond dissociation energy of a polar covalent bond A-B is almost always GREATER than the geometric mean of the bond energies of the corresponding non-polar bonds A-A and B-B:

E(A-B) > sqrt[E(A-A) x E(B-B)]

He defined this excess bond energy as:

Δ = E(A-B) - sqrt[E(A-A) x E(B-B)]

Pauling reasoned that this extra stabilization arises because the A-B bond is not purely covalent — it has partial ionic character due to unequal sharing of the bonding electron pair, caused by a difference in the electronegativities of A and B (the more electronegative atom pulls the shared pair towards itself, adding an ionic-resonance contribution that strengthens the bond beyond a simple covalent bond). The greater the electronegativity difference, the greater this extra bond energy Δ.

He then related the electronegativity difference directly to this excess energy through the empirical relation: …