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General Chemistry Principles, Patterns, and Applications, 2011

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The properties described in Chapter 6 "The Structure of Atoms" <strong>and</strong> Chapter 7 "The Periodic Table <strong>and</strong> Periodic<br />

Trends" were properties of isolated atoms, yet most of the substances in our world consist of atoms held together in<br />

molecules, ionic compounds, or metallic solids. The properties of these substances depend on not only the<br />

characteristics of the component atoms but also how those atoms are bonded to one another.<br />

Carbon <strong>and</strong> silicon bonding. Both the group 14 elements carbon <strong>and</strong> silicon form bonds with oxygen, but<br />

how they form those bonds results in a vast difference in physical properties. Because of its simple molecular bond,<br />

carbon dioxide is a gas that exists as a volatile molecular solid, known as “dry ice,” at temperatures of −78°C <strong>and</strong><br />

below. Silicon dioxide is a giant covalent structure, whose strong bonds in three dimensions make it a hard, highmelting-point<br />

solid, such as quartz.<br />

What you learn in this chapter about chemical bonding <strong>and</strong> molecular structure will help you underst<strong>and</strong> how<br />

different substances with the same atoms can have vastly different physical <strong>and</strong> chemical properties. For example,<br />

oxygen gas (O2) is essential for life, yet ozone (O3) is toxic to cells, although as you learned in Chapter 3 "Chemical<br />

Reactions", ozone in the upper atmosphere shields us from harmful ultraviolet light. Moreover, you saw in Chapter 7<br />

"The Periodic Table <strong>and</strong> Periodic Trends" that diamond is a hard, transparent solid that is a gemstone; graphite is a<br />

soft, black solid that is a lubricant; <strong>and</strong> fullerenes are molecular species with carbon cage structures—yet all of these<br />

are composed of carbon. As you learn about bonding, you will also discover why, although carbon <strong>and</strong> silicon both<br />

have ns 2 np 2 valence electron configurations <strong>and</strong> form dioxides, CO2 is normally a gas that condenses into the volatile<br />

molecular solid known as dry ice, whereas SiO2 is a nonvolatile solid with a network structure that can take several<br />

forms, including beach s<strong>and</strong> <strong>and</strong> quartz crystals.<br />

8.1 An Overview of Chemical Bonding<br />

L E A R N I N G O B JE C T I V E<br />

1. To present three common features of chemical bonding.<br />

In Chapter 2 "Molecules, Ions, <strong>and</strong> Chemical Formulas", we defined a chemical bondas the force that holds atoms<br />

together in a chemical compound. We also introduced two idealized types of bonding: covalent bonding, in which<br />

electrons are shared between atoms in a molecule or polyatomic ion, <strong>and</strong> ionic bonding, in which positively <strong>and</strong><br />

negatively charged ions are held together by electrostatic forces. The concepts of covalent <strong>and</strong> ionic bonding were<br />

developed to explain the properties of different kinds of chemical substances. Ionic compounds, for example, typically<br />

dissolve in water to form aqueous solutions that conduct electricity. (For more information about solution<br />

conductivity, see Chapter 4 "Reactions in Aqueous Solution", Section 4.1 "Aqueous Solutions".) In contrast, most<br />

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