In Chapter 7 we used valence bond
terminology to discuss the bonding in
H 2 although we did not name the
theory there.8-1 A Preview of the Chapter 307ular structure. In this chapter we will study two theories of covalent bonding, which usually
allow us to predict correct structures and properties. Like any simplified theories, they
are not entirely satisfactory in describing everyknown structure; however, their successful
application to many thousands of structures justifies their continued use.
A PREVIEW OF THE CHAPTER
The electrons in the outer shell, or valence shell,of an atom are the electrons involved
in bonding. In most of our discussion of covalent bonding, we will focus attention on
these electrons. Valence shell electrons are those that were not present in the preceding
noble gas, ignoring filledsets of dand forbitals. Lewis formulas show the number of
valence shell electrons in a polyatomic molecule or ion (Sections 7-4 through 7-7). We
will write Lewis formulas for each molecule or polyatomic ion we discuss. The theories
introduced in this chapter apply equally well to polyatomic molecules and to ions.
Two theories go hand in hand in a discussion of covalent bonding. The valence shell
electron pair repulsion (VSEPR) theoryhelps us to understand and predict the spatial arrange-
ment of atoms in a polyatomic molecule or ion. It does not, however, explain howbonding
occurs, just whereit occurs and where unshared pairs of valence shell electrons are directed.
The valence bond (VB) theorydescribes howthe bonding takes place, in terms of overlap-
ping atomic orbitals.In this theory, the atomic orbitals discussed in Chapter 5 are often
“mixed,” or hybridized,to form new orbitals with different spatial orientations. Used
together, these two simple ideas enable us to understand the bonding, molecular shapes,
and properties of a wide variety of polyatomic molecules and ions.
We will first discuss the basic ideas and application of these two theories. Then we will
learn how an important molecular property, polarity,depends on molecular shape. Most
of this chapter will then be devoted to studying how these ideas are applied to various
types of polyatomic molecules and ions.
8-1
Different instructors prefer to cover these two theories in
different ways. Your instructor will tell you the order in
which you should study the material in this chapter. Regardless of how you study this
chapter, Tables 8-1, 8-2, 8-3, and 8-4 are important summaries, and you should refer to
them often.1.One approach is to discuss both the VSEPR theory and the VB theory together,
emphasizing how they complement each other. If your instructor prefers this
parallel approach, you should study the chapter in the order in which it is presented.
2.An alternative approach is to first master the VSEPR theory and the related topic
of molecular polarity for different structures, and then learn how the VB theory
describes the overlap of bonding orbitals in these structures. If your instructor
takes this approach, you should study this chapter in the following order:
a.Read the summary material under the main heading “Molecular Shapes and
Bonding” preceding Section 8-5.
b.VSEPR theory, molecular polarity.Study Sections 8-2 and 8-3; then in Sections
8-5 through 8-12, study only the subsections marked A and B.
c.VB theory.Study Section 8-4; then in Sections 8-5 through 8-12, study the
Valence Bond Theory subsections, marked C; then study Sections 8-13 and 8-14.IMPORTANT NOTE