Question

Pauling's electronegativity scale accounts for the difference in energy between the heteroatomic dissociation energy and the half the sum of the homoatomic dossiciation energies (This difference is converted to eV and the square root is taken to get the difference in electronegativities). Fluorine is arbitrarily given an electronegativity value of 4 and the rest are derived from there. This experimentally derived parameter tells us about the ionic contribution to a bonding interaction. Values are not given for the Noble Gases. There are, however, some experimental bond dissociation energies available. We can assume that the Xe-Xe bond dissociation energy in Xe₂ is 0 kJ/mol, and we know that the F-F bond dissociation energy in F₂ is 158 kJ/mol. If the Xe-F bond dissociation energy in XeF₂ (a known molecule) is simply half the sum of the homolytic bond dissociation energies then the expected value for the Xe-F bond dissociation energy D(Xe-F)_calculated would be _______ kJ/mol. The experimentally determined D(Xe-F) is 13 kJ/mol. Pauling took the difference between the dissociation energies (∆D = D(XeF)_experimental - D(Xe-F)_calculated = _______ kJ/mol). He then took the square root of this value to determine the difference between the electronegativity values of the two elements, Xe and F in this case. We cannot get a _______ number when we take the square root of a _______ number - therefore there is _______ Pauling electronegativity values for Xe (the same goes for the other Noble gases that do form compounds).

Answer 1

According to Pauling;

BE (Xe-F) = BE (Xe-Xe)+BE (F-F) / 2 + (Xe,F)

where BE is the bond enthalpy and (Xe,F) is the stabilization term attributed to ionic interactions.

If the Xe-F bond dissociation energy in XeF2 (a known molecule) is simply half the sum of the homolytic bond dissociation energies then the expected value for the Xe-F bond dissociation energy D(Xe-F)calculated would be (0+158)/2 = 79 kJ/mol. The experimentally determined D(Xe-F) is 13 kJ/mol. Pauling took the difference between the dissociation energies (∆D = D(XeF)experimental - D(Xe-F)calculated = 13-79 = -66 kJ/mol. He then took the square root of this value to determine the difference between the electronegativity values of the two elements, Xe and F in this case. We cannot get a real number when we take the square root of a negative number - therefore there is none Pauling electronegativity values for Xe (the same goes for the other Noble gases that do form compounds).