Page 30 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

the number of electrons. Each successive electron “feels” a larger nuclear charge. This 9
charge is partially screened by the additional electron density as the shell is filled.
However, the screening, on average, is less effective for each electron that is added. SECTION 1.1
As a result, an electron in fluorine is subject to a greater effective nuclear charge than Description of Molecular
Structure Using Valence
one in an atom on the left in the periodic table. As each successive shell is filled, the Bond Concepts
electrons in the valence shell “feel” the effective nuclear charge as screened by the
filled inner shells. This screening is more effective as successive shells are filled and
the outer valence shell electrons are held less tightly going down the periodic table. As
we discuss later, the “size” of an atom also changes in response to the nuclear charge.
Going across the periodic table in any row, the atoms become smaller as the shell
is filled because of the higher effective nuclear charge. Pauling devised a numerical
scale for electronegativity, based on empirical interpretation of energies of bonds and
relating specifically to electron sharing in covalent bonds, that has remained in use
for many years. Several approaches have been designed to define electronegativity
in terms of other atomic properties. Allred and Rochow defined electronegativity in
terms of the electrostatic attraction by the effective nuclear charge Z eff 6 :

0 3590Z eff
= +0 744 (1.1)
AR 2
r
where r is the covalent radius in Å. This definition is based on the concept of nuclear
screening described above. Another definition of electronegativity is based explicitly
on the relation between the number of valence shell electrons, n, and the effective
atomic radius r: 7
V = n/r (1.2)

As we will see shortly, covalent and atomic radii are not absolutely measurable
quantities and require definition.
Mulliken found that there is a relationship between ionization potential (IP) and
electron affinity (EA) and defined electronegativity as the average of those terms: 8
IP +EA
abs = (1.3)
2
This formulation, which turns out to have a theoretical justification in density functional
theory, is sometimes referred to as absolute electronegativity and is expressed in units
of eV.
A more recent formulation of electronegativity, derived from the basic principles
9
of atomic structure, has led to a spectroscopic scale for electronegativity. In this
formulation, the electronegativity is defined as the average energy of a valence electron
in an atom. The lower the average energy, the greater the electron-attracting power
(electronegativity) of the atom. The formulation is

aIP +bIP
s
p
spec = (1.4)
a+b
6 A. L. Allred and E. G. Rochow, J. Inorg. Nucl. Chem., 5, 264 (1958).
7
Y.-R. Luo and S. W. Benson, Acc. Chem. Res., 25, 375 (1992).
8 R. S. Mulliken, J. Chem. Phys., 2, 782 (1934); R. S. Mulliken, J. Chem. Phys., 3, 573 (1935).
9
L. C. Allen, J. Am. Chem. Soc., 111, 9003 (1989); L. C. Allen, Int. J. Quantum Chem., 49, 253 (1994);
J. B. Mann, T. L. Meek, and L. C. Allen, J. Am. Chem. Soc., 122, 2780 (2000).