The Time Evolution of the Electronegativity - Modern Trends in Chemistry and Chemical Engineering - page 33

distribution of the molecule in terms of the electronegativity and proceeds to evalu-
ated the primary dipole moment of the molecule in terms of the LCAO coeffi cient as:

(μ d ) AB = Q B re - (4ezabS - μ s ) (34)
where Q is the net charge and Q A = -Q B = -e(a 2 - b 2 ). The term “4ezabS” is the
homopolar dipole and “Q B re” is the main dipole term.
In case of heteropolar system, this term is cancelled by μ s .
Thus for the heteropolar system, we can write

(μ d ) AB = Q B r e (36)

Thereafter, in 1954, Dailey and Townes (1954) in terms of LCAO coeffi cients (a
and b) proposed that the dipole moment for a heteropolar bond can be expressed as
sum of the primary moment (μ p ), overlap moment (μ o ) of the orbitals of atoms, hybrid-
ization moment (μ hy ) of the valence shell of the atoms and polarization moment (μ p ) of
the non bonding electron.

μ d = μ p + μ o + μ hy + μ p (37)

They defi ned the ionic character of a heteropolar bond as the difference between
polarizabilities for the electron to be found on atom A or B. They detonated it in terms
of the LCAO coeffi cient as (b 2 ~a 2 )

μ d = eR(b 2 ~a 2 ) + μ o + μ hy + μ p (38)
Dailey and Townes (1954) also pointed out that it is not possible to calculate the
contribution of the polarization of the non bonding electron to the total dipole moment
and considered the earlier assumption (Pauling) that the fi rst term of the equation is of
major importance.
that is, μ d = eR(b 2 ~a 2 ) (39)

COMPUTATION OF HETERO POLAR BOND LENGTH IN TERMS OF
ELECTRONEGATIVITY
Pauling [1, 2] first evaluated the bond length from the electronegativity. The Pauling
electronegativity was derived from heats of formation or essentially, bond energies,
the electronegativity difference between two atoms reflects the strength of two bonds
and moreover there exist a quantitative correlation between electronegativity and bond
polarity. Now, let us consider the formation of a diatomic molecule AB from its con-
stituent atoms A and B as follows,
A + B → AB (40)
Let the equilibrium bond length, the electronegativity of the molecule AB are r AB ,
χ AB, and the electronegativities of atoms A and B are χ A and χ B respectively. Now let us
imagine that, after the formation of the molecule, a point charge is located at a distance
r 1 from A and r 2 from B with the condition r 1 + r 2 = r AB . When atoms approach to form
the molecule, the electron density function over the whole space undergoes rearrange-
ment. Thus, there is a physical process of inter atomic charge transfer and rearrange-
ment during the chemical event of the formation of hetero nuclear molecules goaded