The Time Evolution of the Electronegativity - Modern Trends in Chemistry and Chemical Engineering - page 42

in their thermodynamically stable forms. This energy characterizes the atom core, the
“ion core” with valence electrons removed, in a state in which the atoms form metallic
bonding in metals or atomic bonding. Progresses in spectroscopic methods the above
periodic parameters have an important feature of being experimental values. It is not
derived from any models and due to advances in spectroscopic methods is determined
accurately.
Mulliken [59] Electronegativity Scale correlated the three periodic parameters to-
gether as:

χ = (I + A)/2 (83)

Thus, the any one among the three periodic parameters can be calculated by the
knowledge of other two periodic parameters.
The valence electron density is a parameter which can defi ne and measure the elec-
tronegativity of an atom. Nagle [66] found that the cube root of this ratio of the number
of valence electrons divided by the polarizability, (n/α) 1/3 , can be used as a measure of
electronegativity for all s- and p-block elements (except the noble gases). Ghosh and
Gupta [67] also suggested that both polarizability and electronegativity are periodic
properties and they are connected by an inverse relationship, χ ∞ (1/α) 1/3 .

ELECTRONEGATIVITY AND THE HSAB PRINCIPLE
One of the very purposes of the modern conceptual chemistry stands the capacity of
modeling and controlling the chemical reaction via theoretical methods. There is also
recognized that only with admission of the electronegativity and hardness concepts in
the chemical reactivity principle, like Hard and Soft Acid and Base principle [68] has
the benefit to describe the several fundamental phenomena on a sound mathematical
way. This principle was empirical till the publication of the landmark paper by Parr
and Pearson [69]. In this work, based on the assumption that the energy is quadratic
and utilizing the concept of absolute hardness, Parr and Pearson attempted to present
a theoretical deduction of the HSAB principle. They assumed the formation of A:
B from A and B: may be regarded as comprising two components: (i) shift of some
charge, ∆N from B to A and (ii) formation of the actual chemical bond.
Focus primarily on the fi rst effect, they wrote the energy expression for A and B in
the molecule as follows:

E A = E A 0 + μ A 0 (N A - N A 0 ) + η A (N A - N A 0 ) 2 (84)

and

E B = E B 0 + μ B 0 (N B - N B 0 ) + η B (N B - N B 0 ) 2 (85)

The electron numbers, N A and N B are

N A = N A o + ∆N (86)

and

N B = N B o - ∆N (87)