SamF
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Measuring Electronegativity
The general chemistry textbook I'm reading explains how Linus Pauling came up with his electronegativty scale. Given a
pair of atoms A and B, if we let ΔE<sub>AA</sub> and ΔE<sub>BB</sub> be the dissociation energies of an A-A and a
B-B bond respectively, then an estimate of the covalent component of the bond energy of an A-B bond is
(ΔE<sub>AA</sub>*ΔE<sub>BB</sub> <sup>1/2</sup>. So that if ΔE<sub>AB</sub> is the dissociation energy of an A-B bond, then
Δ= ΔE<sub>AB</sub> - (ΔE<sub>AA</sub>*ΔE<sub>BB</sub><sup>1/2</sup>
is a measure of the ionic component of the bond. More specifically, Pauling defined
X<sub>A</sub> - X<sub>B</sub> = 0.102Δ<sup>1/2</sup>
Where X<sub>A</sub> and X<sub>B</sub> are the electronegativities of atoms A and B and Δ is the expression in the prior
equation.
I have some questions. If I can directly observe in the lab the dissociation energies of an A-A bond, a B-B bond and a A-B bond, then I can compute
the difference between the electronegativities of A and B, but how would I arrive at the exact number listed on the periodic table if all I can
observe are differences? Would I have to set one element to 1 and then set all electronegativities relative to that one? Also, wouldn't the subtle
influences of the rest of the molecule cause these bonds to have slightly varying dissociation energies depending on the molecule they're in? Which
one is the "real" dissociation energy? And finally, where did the 0.102 come out of? It seem kind of ad hoc to me and my book doesn't explain it
further.
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kmno4
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As far as I remember, Pauling originally used arithmetic mean value, not gemetrical one (as in your post). However, results are almost the same  .
Google will tell you the rest ( if you bother yourself using it).
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