Information Technology Reference
In-Depth Information
μ = [∂E⁄∂N]
v
(19)
Here,
E
is expressed as the function of the number of electrons,
N
i.e.,
functional of
N
,
E
(
N
).
Then, following Iczkowski and Margrave [45], Parr et al. [43] defi ned
the electronegativity as the additive inverse of the chemical potential.
χ = −μ
(20)
or, χ = −μ = − [∂E⁄∂N]
v
(21)
In 1983, the qualitative HSAB principle was placed on sound quantum
mechanical basis by the seminal work of Parr and Pearson [46].
The chemical hardness, η is one of the oldest concepts of chemistry.
The notion of hardness was fi rst introduced by Mulliken [47] when he
pointed out that the “hard” and “soft” behavior of various atoms, mol-
ecules, and ions can be conceived during acid-base chemical interaction.
Soon after Mulliken's classifi cation, the terms
hardness
and
softness
were
in the glossary of conceptual chemistry and implicitly signifying their de-
formability under small perturbation.
However, Pearson [9] and Klopman [24] tried to systematize and ratio-
nalize this intrinsic property of atoms and molecules; one serious objec-
tion to the HSAB concept was that no exact defi nition of hardness existed.
Nor was there an operational defi nition linking hardness to experiment. As
a result, there was no way to give a theoretical or experimental value to
the hardness for any chemical system. But after the seminal work of Parr
and Pearson [46], the defi nition of hardness and theoretical value to the
hardness for any chemical system could be assigned.
Parr and Pearson [46] defi ned the term
hardness
η as
2η = [∂μ⁄∂
N
]
v
= −[∂χ⁄∂
N
]
v
= [(∂
2
E
⁄∂
N
2
)]
v
(22)
The chemical hardness fundamentally signifi es the resistance toward
the deformation or polarization of the electron cloud of the atoms, ions,
or molecules under small perturbation of chemical reaction. Thus, the
hardness as conceived in chemistry signifi es the resistance toward the
deformation of charge cloud of chemical systems under small perturba-
tion encountered during chemical processes. Thus, the general operational
signifi cance of the hard-soft chemical species may be understood in the
Search WWH ::
Custom Search