Diatomic gases, energy comparisons

Vapor pressure data, as reviewed by Brewer and Kane 37), can be represented by assuming the tetratomic molecule to be the gaseous species, with a heat of sublimation at 298 K. of 34,500 cal./mole, an entropy at 298 K. of 75.00 e. u., and a reasonable estimate of the heat capacity. According to this view, there is no appreciable concentration of the diatomic species in the vapor at saturation pressure below 1000 K., which sets a lower limit of about 48,000 cal./mole for the heat of sublimation at 298 K. for the diatomic gas. Comparison with the bond energies of P4 and Sb4 gives support to this value. 

It is this resonance energy that would be in the main responsible for the difference in energy of the crystal and the gas of diatomic molecules Li2. But the heat of formation of Li2 molecules from atoms is only 6-6 kcal./g.-atom, whereas that of the metal is 39kcal./g.-atom. It seems unlikely, by comparison for example with the analogous case of Kekule-like resonance in aromatic molecules, that the great difference, 32-4 kcal./g.-atom, could result from the synchronized resonance, of type f Li—Li Li Li)... 

The molecular orbital description of He 2 predicts two electrons in a bonding orbital and two electrons in an antibonding orbital, with a bond order of zero—in other words, no bond. This is what is observed experimentally. The noble gas He has no significant tendency to form diatomic molecules and, like the other noble gases, exists in the form of free atoms. He2 has been detected only in very low pressure and low temperature molecular beams. It has a very low binding energy, approximately 0.01 J/mol for comparison, H2 has a bond energy of 436 kJ/mol. 

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