Time Evolution of the Electronegativity - Modern Trends in Chemistry and Chemical Engineering - page 9

atoms in the molecule and molecular properties like the partial charge of the atoms and
groups, dipole moment, bond distance, and so forth.

Allred and Rochow's Scale of Electronegativity [12]
Allred and Rochow [12] postulated that electronegativity of an atom is proportional to
the charge experienced by an electron on the outermost shell of an atom. The higher
the charge of atomic surfaces per unit area, the greater the tendency of that atom to
attract electrons.
Now, the charge experienced by an electron on the surface of an atom or on the
outermost shell of an atom can be described in terms of the effective nuclear charge,
Z eff experienced by valence electrons and the surface area of the atom. Now, as the
surface area of an atom is proportional to the covalent radius of the atom, the electro-
negativity can be represented as

χμ Z eff /r 2 cov (26)

Allred and Rochow [12] suggested a linear relationship between χ and Z eff /r 2 cov as,

χ = a(Z eff /r 2 cov ) + b (27)

where “a” and “b” are the correlation constants.
Scaling with Pauling electronegativity values [10, 11], Allred and Rochow [12]
proposed a new electronegativity(χ AR ) scale as:

χ AR = 0.359(Z eff /r 2 cov ) + 0.744 (28)

The concept of Allred and Rochow [12] was justifi ed by a good number of work-
ers. For example, Little and Jones [76] verifi ed and recalculated the electronegativity
of the atoms of the periodic table based on the force concept of Allred and Rochow
[12].
Mande, Deshmukh, and Deshmukh [77] on the basis of relativistic Dirac equation,
calculated screening constants using X-ray spectroscopic method and using the spec-
troscopic effective nuclear charge of the atoms for the valence states they evaluated
the electronegativity of the atoms by the following modifi ed ansatz:
χ = 0.778 (Z eff ) spectroscopic /r 2 cov ) + 0.5 (29)
The constants of the above ansatz (29) was evaluated by Mande et al. [77] by plot-
ting (Z eff ) spectroscopic /r 2 cov with the Pauling's electronegativity values [10, 11].
The electrostatic scale of Allred and Rochow [12] was further modifi ed by Boyd
and Markus [78]. Boyd and Markus [78] proposed a non-empirical electrostatic model
for calculating the attraction between the screened nucleus and an electron at a dis-
tance corresponding to the relative radius of the atom, that is, the electronegativity
χ BM, as:

r

kz

- #

χ BM = (30)

[ 1 Drdr ( ) ]

r

2

0

where Z is the atomic number, r is the relative radius of the atom, D(r) is the radial
density function and k is a constant so chosen(69.4793au) that the electronegativity