Ionic Compounds
Jean Brainard, Ph.D.
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Printed: February 16, 2015
AUTHOR
Jean Brainard, Ph.D.
www.ck12.org
C HAPTER
Chapter 1. Ionic Compounds
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Ionic Compounds
• Describe ionic compounds and how they form.
• Explain how ionic compounds are named.
• Identify properties and uses of ionic compounds.
The truck pictured above is spreading crystals of the salt calcium chloride on a snowy road to prevent ice from
forming. The crystals lower the freezing point of water so when the snow melts, it won’t turn to ice. Do you know
why calcium chloride lowers the freezing point of water? The answer has to do with the type of compound that
calcium chloride is. Calcium chloride is an ionic compound.
What Are Ionic Compounds?
All compounds form when atoms of different elements share or transfer electrons. Compounds in which electrons
are transferred from one atom to another are called ionic compounds. In this type of compound, electrons actually
move between the atoms, rather than being shared between them. When atoms give up or accept electrons in this
way, they become charged particles called ions. The ions are held together by ionic bonds, which form an ionic
compound. Ionic compounds generally form between elements that are metals and elements that are nonmetals. For
example, the metal calcium (Ca) and the nonmetal chlorine (Cl) form the ionic compound calcium chloride (CaCl2 ).
In this compound, there are two negative chloride ions for each positive calcium ion. Because the positive and
negative charges cancel out, an ionic compound is neutral in charge.
Q: Now can you explain why calcium chloride prevents ice from forming on a snowy road?
A: If calcium chloride dissolves in water, it breaks down into its ions (Ca2+ and Cl− ). When water has ions dissolved
in it, it has a lower freezing point. Pure water freezes at 0°C. With calcium and chloride ions dissolved in it, it won’t
freeze unless the temperature reaches -29°C or lower.
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Crystals
Many compounds form molecules, but ionic compounds form crystals instead. A crystal consists of many alternating
positive and negative ions bonded together in a matrix. Look at the crystal of sodium chloride (NaCl) in the Figure
1.1. The sodium and chloride ions are attracted to each other because they are oppositely charged, so they form ionic
bonds.
FIGURE 1.1
Sodium chloride crystals are cubic in
shape. Other ionic compounds may have
crystals with different shapes.
You can watch a short video at the following URL to see how sodium and chlorine combine to form sodium chloride:
http://www.youtube.com/watch?v=c-nMmwfTV-w .
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/79973
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Chapter 1. Ionic Compounds
Naming Ionic Compounds
Ionic compounds are named for their positive and negative ions. The name of the positive ion always comes first,
followed by the name of the negative ion. For example, positive sodium ions and negative chloride ions form the
compound named sodium chloride. Similarly, positive calcium ions and negative chloride ions form the compound
named calcium chloride.
Q: What is the name of the ionic compound that is composed of positive barium ions and negative iodide ions?
A: The compound is named barium iodide.
Properties of Ionic Compounds
The crystal structure of ionic compounds is strong and rigid. It takes a lot of energy to break all those ionic bonds.
As a result, ionic compounds are solids with high melting and boiling points. You can see the melting and boiling
points of several different ionic compounds in the Table 1.1. To appreciate how high they are, consider that the
melting and boiling points of water, which is not an ionic compound, are 0°C and 100°C, respectively.
TABLE 1.1: Melting and Boiling Points of a Few Ionic Compounds
Ionic Compound
Sodium chloride (NaCl)
Calcium chloride (CaCl2 )
Barium oxide (BaO)
Iron bromide (FeBr3 )
Melting Point (°C)
801
772
1923
684
Boiling Point (°C)
1413
1935
2000
934
Solid ionic compounds are poor conductors of electricity. The strong bonds between their oppositely charged ions
lock them into place in the crystal. Therefore, the charged particles cannot move freely and carry electric current,
which is a flow of charge. But all that changes when ionic compounds dissolve in water. When they dissolve, they
separate into individual ions. The ions can move freely, so they can carry current. Dissolved ionic compounds are
called electrolytes.
The rigid crystals of ionic compounds are brittle. They are more likely to break than bend when struck. As a result,
ionic crystals tend to shatter easily. Try striking salt crystals with a hammer and you’ll find that they readily break
into smaller pieces. You can learn more about the properties of ionic compounds by watching the video at this URL:
http://www.youtube.com/watch?v=buWrSgs_ZHk
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/5081
Uses of Ionic Compounds
Ionic compounds have many uses. Some are shown in the Figure 1.2. Many ionic compounds are used in industry.
The human body needs several ions for good health. Having low levels of the ions can endanger important functions
such as heartbeat. Solutions of ionic compounds can be used to restore the ions.
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FIGURE 1.2
Summary
• Ionic compounds consist of oppositely charged ions that are held together by ionic bonds. The opposite
charges cancel out so ionic compounds have a net neutral charge. Ionic compounds form when metals transfer
valence electrons to nonmetals.
• Ionic compounds exist as crystals rather than molecules. A crystal consists of many alternating positive and
negative ions bonded together in a matrix.
• Ionic compounds are named for their positive metal ion first, followed by their negative nonmetal ion.
• Ionic compounds are solids with high melting and boiling points. They are good conductors of electricity but
only when dissolved in water. Their crystals are rigid and brittle.
• Ionic compounds have many uses in industry. They are also needed by humans and other living things.
Vocabulary
• crystal: Rigid, lattice-like framework of many ions bonded together that is formed by some compounds such
as table salt (NaCl).
• ionic compound: Compound that forms when oppositely charged metal and nonmetal ions are held together
in a crystal by ionic bonds.
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Chapter 1. Ionic Compounds
Practice
Watch the tutorial on ionic compounds at the following URL, and then answer the questions below.
http://www.youtube.com/watch?v=yyWuGmX-C1k
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/82373
1. How can you predict the ions and their charges that different elements will form?
2. What type of formula is used to represent an ionic compound? How is it written? Give an example.
3. What ions form when calcium reacts with bromine? What is the name of the ionic compound that results?
What is the formula for this compound?
Review
1.
2.
3.
4.
5.
6.
What is an ionic compound?
Which classes of elements make up ionic compounds?
What are crystals?
How are ionic compounds named? Give an example.
Describe properties of ionic compounds.
The ionic compound sodium fluoride is added to toothpaste to prevent tooth decay. Describe the composition
and structure of this compound.
References
1. Salt: Tony L Wong; Illustration: Christopher Auyeung (CK-12 Foundation). Sodium chloride crystals are
cubic in shape. CC BY 2.0
2. Fireworks: Epic Fireworks; Battery: Lukas A, CZE; Others: Christopher Auyeung (CK-12 Foundation). Uses
of calcium, barium, and cobalt chloride. Fireworks: CC BY 2.0; Battery: Public Domain; Others: CC BY-NC
3.0
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