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OVERVIEW

In chapter 8, you learned how the modern atomic theory is used to describe the structures of atoms of different
elements. The electron structures of different atoms successfully account for the position of elements in the periodic
table as well as for groups of elements with similar properties. On a large scale, all metals were found to have a
similarity in electron structure, as were nonmetals. On a smaller scale, chemical families such as the alkali metals
were found to have the same outer electron configurations. Thus, the modern atomic theory accounts for observed
similarities between elements in terms of atomic structure.
So far, only individual, isolated atoms have been discussed; we have not considered how atoms of elements join
together to produce chemical compounds. There is a relationship between the electron structure of atoms and the reactions
they undergo to produce specific compounds. Understanding this relationship will explain the changes that matter itself
undergoes. For example, hydrogen is a highly flammable, gaseous element that burns with an explosive reaction. Oxygen,
on the other hand, is a gaseous element that supports burning. As you know, hydrogen and oxygen combine to form water.
Water is a liquid that neither burns nor supports burning. What happens when atoms of elements such as hydrogen and
oxygen join to form molecules such as water? Why do such atoms join and why do they stay together? Why does water have
different properties from the elements that combine to produce it? And finally, why is water H 2 O and not H 3 O or H 4 O?
Answers to questions about why and how atoms join together in certain numbers are provided by considering
the electronic structures of the atoms. Chemical substances are formed from the interactions of electrons as their
structures merge, forming new patterns that result in molecules with new properties. It is the new electron pattern of
the water molecule that gives water different properties than the oxygen or hydrogen from which it formed ( Figure 9.1).
Understanding how electron structures of atoms merge to form new patterns is understanding the changes that matter
itself undergoes, the topic of this chapter.

9.1 COMPOUNDS AND Compounds with one type of chemical bond, as you will see,
have molecules that are electrically neutral groups of atoms held
CHEMICAL CHANGE
together strongly enough to be considered independent units.
There are more than one hundred elements listed in the peri- For example, water is a compound. The smallest unit of water
odic table, and all matter on Earth is made of these elements. that can exist alone is an electrically neutral unit made up of two
However, very few pure elements are found in your surround- hydrogen atoms and one oxygen atom held together by chemical
ings. The air you breathe, the liquids you drink, and all the bonds. The concept of a molecule will be expanded as chemical
other things around you are mostly compounds, substances bonds are discussed.
made up of combinations of elements. Water, sugar, gasoline, Compounds occur naturally as gases, liquids, and sol-
and chalk are examples of compounds, and each can be bro- ids. Many common gases occur naturally as molecules made
ken down into the elements that make it up. Examples of ele- up of two or more atoms. For example, at ordinary tempera-
ments are hydrogen, carbon, and calcium. Why and how these tures, hydrogen gas occurs as molecules of two hydrogen atoms
elements join together in different ways to form the different bound together. Oxygen gas also usually occurs as molecules of
compounds that make up your surroundings is the subject of two oxygen atoms bound together. Both hydrogen and oxygen
this chapter. occur naturally as diatomic molecules (di- means “two”). Oxygen
You have already learned that elements are made up of sometimes occurs as molecules of three oxygen atoms bound
atoms that can be described by the modern atomic theory. You together. These triatomic oxygen molecules (tri- means “three”)
can also consider an atom to be the smallest unit of an element are called ozone. The noble gases are unique, occurring as single
that can exist alone or in combination with other elements. Com- atoms called monatomic (mon- or mono- means “one”) ( Figure 9.2).
pounds are formed when atoms are held together by an attractive These monatomic particles are sometimes called monatomic
force called a chemical bond. The chemical bond binds indi- molecules since they are the smallest units of the noble gases that
vidual atoms together in a compound. A molecule is generally can exist alone. Helium and neon are examples of the mona-
thought of as a tightly bound group of atoms that maintains its tomic noble gases.
identity. More specifically, a molecule is defined as the smallest When molecules of any size are formed or broken down
particle of a compound, or a gaseous element, that can exist and into simpler substances, new materials with new properties are
still retain the characteristic chemical properties of a substance. produced. This kind of a change in matter is called a chemical

230 CHAPTER 9 Chemical Bonds 9-2