Time Evolution of the Electronegativity - Modern Trends in Chemistry and Chemical Engineering - page 12

When N = 1, the electronegativity leads to an expression similar to that proposed
by Mulliken [55].

χ = (∂E/∂N) N=1 = a + 2b = ( IP + EA )/2 (39)
Finally Klopman [82] pointed out that however, that in order to represent the elec-
tronegativity of an atom in a molecule correctly, the atom must be considered in its
valency state and this requires the introduction of electron spin.

Hinze and Jaffe's Scale for Orbital Electronegativity of Neutral Atoms [62]
Hinze and Jaffe [62] opined that electronegativity is not a property of atoms in their
ground state, but of atoms in the same condition in which they are found in mol-
ecules, that is, in their valance state. They also noticed that the electronegativity can
be defined in terms of bonding orbital and the term “Orbital electronegativity” is then
suggested.
In the next year Hinze et al. [63] proposed that “ the power of an atom to attract
electrons in a given orbital to itself ” can be correlated with the orbital electronegativ-
ity. The orbital electronegativity is then defi ned as the derivative of the energy of the
atom respect to the charge in the orbital, that is, the number of electrons in the orbitals:
χ j = ∂E/∂n j (39)
where χ j is the orbital electronegativity of the jth orbital and n j is the occupation
number of the jth orbital.
This defi nition implies two assumptions—(a) that the occupation number n j may
have both integral and non-integral values and, (b) that once assumption (a) is made,
than the energy E is a continuous and differentiable function of n j .
Thus,

χ j = ∂E/∂n j = b + 2c n j (40)

where b and c are the constants.

Yuan's Scale of Electronegativity [83]
Yuan [83] defined electronegativity as the ratio of the number of valence electron to
the covalent radius. This scale was later modified by Luo and Benson [84-86] on the
basis of the number of valance electrons in the bonding atoms and covalent radius of
the atom.

Gyftopoulos and Hatsopoulos's Quantum Thermodynamic Scale of
Electronegativity [75]
Gyftopoulos and Hatsopoulos [75] identified electronegativity as the additive inverse
of the chemical potential, μ.
Gyftopoulos and Hatsopoulos [75] defi ned the electrochemical potential of a ther-
modynamic system as

μ = (∂E/∂N) entropy (41)