The Time Evolution of the Electronegativity - Modern Trends in Chemistry and Chemical Engineering - page 32

where μ is the dipole moment of molecules in debye unit, q is the dipole moment
charge on atomic site, e represents the electronic charge in esu unit and R e is the inter-
nuclear distance of diatomic molecules in centimeter unit.
As electronegativity is an abstractly defi ned property and also it is not an observ-
able, hence it cannot be directly measured. However, relative electronegativities can
be observed indirectly by measuring molecular dipole moments: in general, the greater
the dipole moment, the greater the separation of charges must be, and therefore, the
less equal the sharing of the bonding electrons must be.
It has long been recognized that the dipole moment, μ d , of the molecule AB can be
related to the difference of the two atomic electronegativities (χ B ~ χ A ). Indeed in 1932,
Pauling [1, 2] proposed the empirical relationship q = 1 - exp[-( χ B - χ A ) 2 ] for estimat-
ing the ionic character in the molecule. This form was chosen to agree with the then
available experimental values of the “dipole moment” charge. In the next year, Malone
[38] discovered that dipole moment in Debye (μ d ) of a hetero nuclear bond A-B and
the electronegativity difference, χ A - χ B , are proportional, that is,

μ d (31)
In 1935, Mulliken [29] in an attempt to correlate the relative electronegativities,
effective charges on atoms in partially polar molecule, dipole moments in terms of
LCAO-MO coeffi cients, proposed that the coeffi cients are affected by the polarities of
the bond and hence are important for the prediction of the electronegativity.
The bonding molecular orbital Φ AB , can be written as:
Φ AB = aΦ A + bΦ B (32)
If a = b, then the two atoms are same electronegativity, but if a > b then we can say
that electronegativity of A > B. Thus, Mulliken [29] pointed out that the difference of
electronegativity can be correlated with the difference of the LCAO coeffi cients, that
is, a - b or more precisely a 2 - b 2 .
For a diatomic molecule where the bonding electron pair only contributes to the
electrical moment the electronic distribution occurs as:
• 2ea 2 is the charge centered on the atom A,
• 2eb 2 is the charge centered on atom B.
• 4eabS is the charge centered the centers A and B and the distance of the electric
center from the midpoint of the bond A-B is considered as z. Then the electri-
cal net moment can be obtained by the contribution of the charges centered in
centers A, B, in between A-B and a charge + e on each atom.

χ A ~χ B μ

μ Net = er(a 2 - b 2 ) - 4ezabS (33)

where r is the distance between the atom A and B.
Mulliken [29] defi ned the term z as the measure of inequality of the polarity of
the two atoms A and B. If the molecule AB is homopolar then z = 0 its magnitude
increases with the inequality of the size of A and B. its sign is such that its positive
pole is directed towards the larger atom. Mulliken empirically correlated the charge