The Time Evolution of the Electronegativity - Modern Trends in Chemistry and Chemical Engineering - page 29

2. the resonance integral, β rs = ∫Φ r Ĥ Φ s dτ. (13)

Coulson and Longuet-Higgnhs [27] point out that the Coulomb term can be related
to the ionization potential (I) of atom r. But after a detail examination on this topic,
Mulliken [28, 1949] concluded that in a bond XY the difference , α X - α Y , should be
proportional to the difference in electronegativity of the atoms X and Y in their proper
valence states.

THE DIPOLE CHARGE AND DIPOLE MOMENT IN TERMS OF
ELECTRONEGATIVITY
The most obvious application of electronegativities is the prediction of the polarity
of a chemical bond, for which the concept was originally introduced by Pauling. In
general, the greater is the difference in electronegativity between two atoms, the more
polar is the bond that will be formed between them, with the atom having the higher
electronegativity being at the negative end of the dipole. Dipole is an index or de-
scriptor of the asymmetry of charge distribution in molecules. The charge distribution
pattern in the hetero nuclear diatomic molecules always generates dipole. The bond
character is a common topic in chemistry for the determination of the physical and
chemical behavior of compounds. The simplest way to determine bond character is to
use the electronegativity difference between the bonded atoms. Again the dipole mo-
ment μ of a diatomic molecule AB has been related to the difference of the two atomic
electronegativities (χ B - χ A ).
According to Mulliken [29] and Dailey and Townes [30] the dipole moment in
hetero nuclear diatomic molecules has several components that is,

μ = μ atomic + μ overlap + μ hybridization + μ polarization (14)

However, the principal components are two. The fi rst one is bond moment which is
developed due to unequal sharing of charge between nuclei goaded by their difference
in electronegativities. The second component originates from the asymmetry of charge
distribution in lone pair electrons in hybrid orbitals.

μ = μ atomic + μ hybridization (15)

Mulliken [29] pointed out that, in diatomic molecules, the bond pair moment is the
main contributor to the electronic dipole moment of the molecules. However, the two
contributing components of dipole are deeply interlinked with the pattern of electron
distribution in molecules. Coulson (1961) and Dewar (1969) suggested the fact that
the lone pair of electrons accommodated in a hybrid orbital acquires asymmetry of
charge density distribution and generates atomic dipole. Ghosh and Bhattacharyya
[31] have derived quantum mechanical algorithm of dipole moment of molecules.
Between these two components of the dipole moment, we can calculate the bond
moment part semi-empirically but the lone pair contribution to the dipole moment of
molecules can only be calculated quantum mechanically and the possibility of empiri-
cal evaluation of lone pair component of dipole is ruled out.