Chemistry, Third edition

More about Bonding

Objectives

Discusses exceptions to the octet rule

Explains how we can predict the shapes of molecules

Distinguishes between polar and non-polar molecules

Describes the different types of intermolecular attractive forces

After reading this chapter you should realize how useful Lewis’s bonding theory is!

With small additions, the basic ideas can be used to explain a number of phenomena:

how molecules get their shape; why metals conduct electricity; what makes diamond

such a hard substance; and why water is a liquid under normal conditions. All these

questions are answered in the following pages. First, however, we need to discuss how

some exceptions to the ideas developed in the last chapter can be explained.

Exceptions to the octet rule

Compounds in which the central atom is surrounded by

fewer than eight electrons

The idea that there is a stability associated with the electron arrangements of the

atoms of noble gases is sometimes called the octet rule. This rule applies to many

compounds, but it is not always obeyed (unfortunately scientific theories are often

not as ‘neat’ as we would like them to be!). When does the octet rule break down?

Two important exceptions to remember are beryllium (Be, electronic structure

2.2) and boron (B, electronic structure 2.3). According to the reasoning in Chapter

4, they should react by losing two and three electrons, respectively. They do not react

in this way, however, because their ionization energies (the energies required to remove

their outer electrons) are very high. Because the atoms are small, the outer electrons in

their atoms are close to the positive nucleus and are strongly attracted to it. Other

atoms with two and three electrons in their outer shells, such as magnesium and alu-

minium, can lose these electrons when they form compounds because the electrons

are farther away from the nucleus and more shielded from its attractive charge by

full shells of electrons. Instead beryllium and boron choose to share electrons and

form covalent compounds. Those compounds are stable enough to exist at room

5.1

Contents

5.1Exceptions to the octet

rule 63

5.2Shapes of molecules 65

5.3Shapes of molecules

with multiple bonds 69

5.4Molecules with and

without dipoles 71

5.5Metallic bonding 72

5.6Giant molecules 73

5.7Forces between

covalent molecules 75

5.7Revision questions 81

5

UNIT