The Time Evolution of the Electronegativity - Modern Trends in Chemistry and Chemical Engineering - page 37

bond energy. The bond dissociation energy of a bond in a molecule A-B is the energy
needed to separate the radicals A and B to infi nity, each species being in its ground
state. The average bond energy of a bond A-B is defi ned as l/n th the energy needed to
separate each of the atoms in a symmetrical molecule AB, to infi nity, all species be-
ing in their ground states, that is, l/n th of the heat of atomization of the molecule. In
general, the bond dissociation energy plays a more important role in chemistry than
the average bond energy. Szwarc et al. [50] have demonstrated the importance of bond
dissociation energies in the interpretation of chemical kinetic data. Szwarc et al. [50]
have shown how the analysis of complex reactions into their elementary reactions can
be aided greatly by the knowledge of the bond dissociation energies. Because ther-
mal, photochemical, radiation, and discharge reactions are usually complex, tables of
bond dissociation energies should be very useful in interpreting kinetic data in these
fi elds. Bond dissociation energies also can be employed to calculate thermochemical
properties. Heats of formation of radicals can be obtained from the bond dissociation
energies, which, in turn, can be employed for calculating heats of reactions involving
free radicals. Furthermore, bond dissociation energies can be used to calculate heats of
reactions in which free radicals are not involved.
Finally, knowledge of bond dissociation energies is essential in interpreting most
results obtained by electron bombardment of molecules. In a mass spectrometer one
measures the appearance potential of an ion of known mass. In order to deduce what
process occurred in the mass spectrometer, one calculates the appearance potentials
of various possible processes and by comparing those to the observed value the most
likely process can be chosen. It is opined that the knowledge of the bond dissociation
energies is very essential to calculate the appearance potentials of various possible
processes (more on the topic please see [51]).
The bond dissociation energies or the bond energy (BE) for a bond A-B can be
defi ne as the standard-state enthalpy change for a reaction of the type AB → A + B at
a specifi ed temperature (T).

(BE) T = ∆H f 0 (A) + ∆H f 0 (A) - ∆H f 0 (AB) (62)

where ∆H f 0 is the standard state heat of formation.
The earliest method which correlates the BE with electronegativity was the land-
mark Pauling's equation [1, 2]. The scale correlates the extra ionic resonance energy to
the electronegativities of atoms. The energy difference ∆, was defi ned by:

∆ = D(A - B) - (1/2)[D(A - A) + D(B - B)]. (63)

Pauling was able to assign electronegativity of many elements which roughly sat-
isfy the equation

∆ = (χ A - χ B ) 2 (64)

Pauling took ∆ = -∆H f 0 (heat of formation).
Within the framework of SBC model [18-21], Pasternak [21] pointed out that in
the limit of infi nite internuclear separation, a diatomic molecule becomes two ions at
a infi nite separation and a bond charge at an infi nite distance from the ions and free