Time Evolution of the Electronegativity - Modern Trends in Chemistry and Chemical Engineering - page 10

value of F becomes 4. Boyd and Markus [78] calculated the relative radius of atom,
r using the density contour approach of Boyd [79] on the basis of analytical Hartree-
Fock wave function of atoms proposed by Clementi and Roetti [80].
In a recent work, Ghosh et al. [52] have pointed out some inconsistency in the
Allred and Rochow's Electronegativity Scale [12] and also in the previous modifi ca-
tions of Allred and Rochow's Scale [12].
They found that:
1. Although, Allred and Rochow [12], Mande et al. [77], and Little and Jones
[76] used the force concept to evaluate the electronegativity of atoms but the
dimension of the electronegativity is not be mentioned by any of them.
2. Allred and Rochow [12], Mande et al. [77], Little and Jones [76] used the
covalent radii in atomic unit to calculate the electronegativity values in their
proposed electronegativity scale.
However, these considerations do not compute the electronegativity in force unit.
3. The absolute radius is the true descriptor of atomic electronegativity not the
covalent radius.
Ghosh et al. [52] replaced the covalent radii by absolute radii and solved the di-
mension problem of the Allred and Rochow Scale by proper dimension matching and
they reported electronegativity as force.

χ = Force = e 2 (Z eff )/r abs 2 (31)

Iczkowski and Margrave's Scale of Electronegativity [81]
Iczkowski and Margrave [81] considered electronegativity as a property of an isolated
gaseous atom or ion. By plotting (Fig. 1.1) the atomic energy change with degree of
ionization of a chemical system, Iczkowski and Margrave [81] discovered the energy
expression for the of an atom, X as:

E(N) X = aN + bN 2 + cN 3 + dN 4 + … (32)

where N is the number of electrons in the valence shell of nucleus X, and a, b, c
and d are the coeffi cients.
They identifi ed the electronegativity, for an atom or ion, as the slope at the origin
- (dE/dN), of the E versus N curve.

χ = -(dE/dN) (33)
Klopman [82] postulated that “the atomic terms for any atom can be defi ned as
the sum of those integrals in which the Hamiltonian represents the interaction of the
core of the atom with the electron around it” and critically justifi ed the Iczkowski and
Margrave's electronegativity concept [81] as under:
The E versus N relationship is usually linear so the higher terms of the equation
(32) can be neglected. This leads to the simplifi ed expression (34)

E = aN + bN 2 (34)