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Encyclopedia of Physical Science and Technology EN011H-551 July 25, 2001 18:33

688 Periodic Table (Chemistry)

SCHEME 1

of bonding and the strength of the bond. Our quest is
governed by four guidelines. SCHEME 2

1. Kinetic energy reduction and, thus, bonding strength be confused with artistic presentations in which color is of-
is related to the sum of the ionization energies of the
ten employed to indicate nonmetals, semimetals, and met-
overlapping AOs and, as a result, to the sum of atom
als aside from attaining a visually pleasing result. There
ionization energies and atom electronegativities. The
are three shades, each indicating the intrinsic affinity of
larger the sum is, the stronger the kinetic energy reduc-
each atom for one or more of the three bonding mecha-
tion and the stronger the bonding, everything else being
nisms. The affinities are blue atoms promote T-bonding,
equal.
green atoms promote I-bonding or I/T-bonding and are
2. The mechanism of delocalization, T or I, also de-
tolerant to T-bonding, and red atoms promote either I- or
pends on the sum of atom electronegativities, everything
E-bonding. The term I/T symbolizes bimodal bonding by
else being equal. In homonuclear molecules, strong bond-
which a single atom binds one set of ligands by I-bonds
ing implies T-bonding and weak bonding goes hand in
and another set by T-bonds. When a choice exists, exactly
hand with I-bonding. The origin of the crucial effect of
which type of bonding occurs depends on the precise com-
electronegativity on both the binding strength as well as
bination of atoms. The matrix of atom combination and
on the bonding mechanism is emphasized in Scheme 1.
the forecast mechanisms are shown in Scheme 2. Note the
3. The condition for I-bonding is that the atoms sup-
tolerance of green and the intolerance of red atoms for
port strong CT delocalization. For this to happen the sum T-bonding.
of atom electronegativities, x , should be small and the Examples of the different binding modes are given
+
difference of the atom electronegativities, x , should be below.
−
large. However, this dual condition cannot be met because One expectation is that green atoms can exhibit any one
the limits are as follows: x goes to zero as atom elec- of three possible bonding modes: T, I, or bimodal. What
+
tronegativity goes to zero at which limit x is also zero. is the electronic structure of the “hypervalent” trigonal
−
Thus, the “king of I-bonding” must involve some sort of
compromise.
4. E-bonding is classical Coulomb attraction comple-
mented by induction and dispersion. The conditions for
optimization of E-bonding are the following:
a. The interacting species are small and compact
counterions formed from precursor neutral atoms of
widely different electronegativities by transfer of one or
more electrons from the more electropositive to the more
electronegative atom. This maximizes classical Coulomb
attraction.
b. The interacting species are large polarizable neu-
tral atoms. This maximizes the dispersive interaction.
c. One species is a polar molecule or ion and the
other is a polarizable neutral molecule. This maximizes
the inductive interaction.

The proposed shading scheme for the periodic table has
been displayed in Fig. 3. Our shading scheme should not SCHEME 3