The Foundations of Chemistry

VALENCE BOND (VB) THEORY

In Chapter 7 we described covalent bonding as electron pair sharing that results from the
overlap of orbitals from two atoms. This is the basic idea of the valence bond (VB) theory
—it describes howbonding occurs. In many examples throughout this chapter, we first
use the VSEPR theory to describe the orientationsof the regions of high electron density.
Then we use the VB theory to describe the atomic orbitals that overlap to produce the
bonding with that geometry. We also assume that each lone pair occupies a separate orbital.
Thus, the two theories work together to give a fuller description of the bonding.
We learned in Chapter 5 that an isolated atom has its electrons arranged in orbitals in
the way that leads to the lowest total energy for the atom. Usually, however, these “pure
atomic” orbitals do not have the correct energies or orientations to describe where the
electrons are when an atom is bonded to other atoms. When other atoms are nearby as
in a molecule or ion, an atom can combine its valence shell orbitals to form a new set of
orbitals that is at a lower total energy in the presence of the other atoms than the pure
atomic orbitals would be. This process is called hybridization,and the new orbitals that
are formed are called hybrid orbitals.These hybrid orbitals can overlap with orbitals on
other atoms to share electrons and form bonds. Such hybrid orbitals usuallygive an
improved description of the experimentally observed geometry of the molecule or ion.
The designation (label) given to a set of hybridized orbitals reflects the number and
kindof atomic orbitals that hybridize to produce the set (Table 8-2). Further details about
hybridization and hybrid orbitals appear in the following sections. Throughout the text,
hybrid orbitals are shaded in green.

8-4

Figure 8-1 A guide to determining

whether a polyatomic molecule is

polar or nonpolar. Study the more

detailed presentation in the text.

TABLE 8-2 Relation Between Electronic Geometries and Hybridization

Regions of Atomic Orbitals Mixed
High Electron Electronic from Valence Shell
Density Geometry of Central Atom Hybridization

2 linear one s, one psp

3 trigonal planar one s, two p’s sp^2

4 tetrahedral one s, three p’s sp^3

5 trigonal bipyramidal one s, three p’s, one dsp^3 d

6 octahedral one s, three p’s, two d’s sp^3 d^2

NO

Are polar

bonds

arranged so

that they

cancel?

Are there

lone pairs on

the central

atom?

Are lone pairs

on central

atom arranged

so that they

cancel?

Molecule is

POLAR

Are there

polar bonds

present?

Molecule is

POLAR

Molecule is

NONPOLAR

Molecule is

NONPOLAR

NO NO

YES

YES YES

NO

YES

See the Saunders Interactive

General Chemistry CD-ROM,

Screen 10.2, Models of Chemical

Bonding, and Screen 10.3, Valence

Bond Theory.

See the Saunders Interactive

General Chemistry CD-ROM,

Screen 10.6, Determining Hybrid

Orbitals.

We can describe hybridization as the

mathematical combination of the

waves that represent the orbitals of

the atom. This is analogous to the

formation of new waves on the

surface of water when different

waves interact.