Key Objectives
7.2.1 EXPLAIN the electrical charge of an ionic
compound.
7.2.2 DESCRIBE three properties of ionic
compounds.
CHEMISTRY
Y U
YO
&YOU
Additional Resources
Q: Where does table salt come from? Sodium chloride, or table salt, has been
used by people for centuries to add flavor to food and for preserving food.
In some countries, salt is obtained by the evaporation of seawater. In other
countries, salt is mined from rock deposits deep underground. In this lesson,
you will learn how cations and anions combine to form stable compounds
such as sodium chloride.
Key Questions
What is the electrical charge
of an ionic compound ?
What are three properties of
ionic compounds?
Vocabulary
tionic compound
tionic bond
tchemical formula
tformula unit
tcoordination number
Reading and Study Workbook, Lesson 7.2
Available Online or on Digital Media:
• Teaching Resources, Lesson 7.2 Review
• Probeware Laboratory Manual, Section 7.2
• Virtual Chemistry Labs, Lab 2
Formation of Ionic Compounds
What is the electrical charge of an ionic compound ?
Sodium chloride, or table salt, is an ionic compound consisting of sodium
cations and chloride anions. An ionic compound is a compound composed
Although they are composed of ions, ionic comof cations and anions.
pounds are electrically neutral. The total positive charge of the cations
equals the total negative charge of the anions.
Ionic Bonds Anions and cations have opposite charges and attract one
another by means of electrostatic forces. The electrostatic forces that hold ions
together in ionic compounds are called ionic bonds.
Sodium chloride provides a simple example of how ionic bonds are
formed. Consider the reaction between a sodium atom and a chlorine atom.
The sodium atom has a single valence electron that it can easily lose. (If the
sodium atom loses its valence electron, it achieves the stable electron configuration of neon.) The chlorine atom has seven valence electrons and can easily gain one electron. (If the chlorine atom gains a valence electron, it achieves
the stable electron configuration of argon.) When sodium and chlorine react
to form a compound, the sodium atom transfers its one valence electron to
the chlorine atom. Thus, sodium and chlorine atoms combine in a one-to-one
ratio, and both ions have stable octets.
Na
2
2
Naà
Cl
6
1
1s 2s 2p 3s
2
2
6
2
5
1s 2s 2p 3s 3p
2
2
6
1s 2s 2p
2
Cl
Ź
2
6
1s 2s 2p 3s23p6
ì
ì
octet
octet
1s22s22p6
1s22s22p63s23p6
octet
ì
Ar
ì
Ne
octet
Ionic and Metallic Bonding 201
Engage
&
CHEMISTRY
Y
YO
YOU
U Have students examine
the opening photograph along with Figure 7.7.
Discuss how the poisonous elements sodium and
chlorine can react to form harmless table salt. Ask
What characteristics of sodium and chlorine atoms
allow them to form a stable compound? (Sodium
atoms can lose an electron easily, and chlorine
atoms can accept an electron easily. The resulting
ions are attracted to the oppositely charged ions.)
Remind students that NaCl is an example of an ionic
compound.
Access Prior Knowledge
Have students review the patterns in Tables 6.1 and
6.2. Ask What term is used to describe the ability
of an atom of an element to attract electrons?
(electronegativity)
National Science Education Standards
A-1, B-2, B-3
Focus on ELL
1 CONTENT AND LANGUAGE Write the vocabulary terms on the board along with
their phonetic spellings and pronounce them slowly. Discuss the meaning of each
word in each term, and then explain the meaning of the combined term. Have
students produce a half-page illustration or example for each vocabulary term.
2 FRONTLOAD THE LESSON Have small groups of students read aloud and discuss
the lesson headings and key questions. Then have them write at least one sentence
for each page summarizing what they will study on that page.
3 COMPREHENSIBLE INPUT Explain to students that they will study the interaction
of ions in this lesson. As a visual analogy, show students two magnets with opposite
poles attracting each other. Explain that this attraction is similar to the attraction
between oppositely charged ions. Be sure to stress that the attraction between the
ions is due to electrical charge, not magnetic force.
Ionic and Metallic Bonding
201
LESSON 7.2
7.2
Ionic B
Bonds and
Compounds
Ionic C
LESSON 7.2
Figure 7.6 shows the reaction of aluminum (Al) and bromine
(Br2) to form the ionic compound aluminum bromide (AlBr3).
Each aluminum atom has three valence electrons to lose. Each
bromine atom has seven valence electrons and readily gains one
additional electron. Therefore, when aluminum and bromine
react, three bromine atoms combine with each aluminum atom.
Foundations for Reading
BUILD VOCABULARY Have students skim the lesson
to locate the definition of each vocabulary term.
Then have them define each term in their own words.
READING STRATEGY Have students rewrite the
headings as how, why, or what questions about
ionic compounds. As they read, have them write
answers to their questions.
Explain
Formation of Ionic Compounds
APPLY CONCEPTS Explain that the formation of
positive ions and negative ions are simultaneous
and interdependent processes. Explain that an ionic
compound is the result of the transfer of electrons
from one set of atoms to another set of atoms.
Emphasize that an ionic compound consists entirely
of ions. Have students name some substances
that they think are ionic compounds. As a class,
determine which substances are in fact ionic
compounds, and why.
CRITICAL THINKING Direct students’ attention
to Figure 7.7. Ask Why are crystalline ionic
compounds generally so rigid and brittle? (The
crystal is rigid because it is held together by a
specific three-dimensional array of relatively strong
attractive forces between anions and cations,
which is accompanied by minmal repulsion of like
ions. The crystal is brittle because the attractive
interactions are specifically arranged within the
crystal structure. If this arrangement is disturbed, as
it would be if the crystal were hit with a hammer,
repulsion between ions of the same charge can
force the crystal to fragment.)
Figure 7.6 Formation of Aluminum Bromide
Aluminum metal and the nonmetal bromine
react violently to form the ionic solid aluminum
bromide.
Figure 7.7 Formation of Sodium Chloride
Sodium cations and chloride anions form a
repeating three-dimensional array in sodium
chloride (NaCl).
Infer How does the arrangement of ions in a
sodium chloride crystal help explain why the
compound is so stable?
Formula Units The ionic compound sodium chloride is composed of equal numbers of sodium cations (Naà) and chloride
anions (ClŹ). Chemists represent the composition of substances
by writing chemical formulas. A chemical formula shows the
numbers of atoms of each element in the smallest representative
unit of a substance. For example, NaCl is the chemical formula
for sodium chloride.
Note that the formula NaCl does not represent a single physical unit. As shown in Figure 7.7, the ions in solid sodium chloride are arranged in an orderly pattern. Ionic compounds do
not exist as single discrete units, but as collections of positively
and negatively charged ions arranged in repeating patterns.
Therefore, the chemical formula of an ionic compound refers
to a ratio known as a formula unit. A formula unit is the lowest
whole-number ratio of ions in an ionic compound. For sodium
chloride, the lowest whole-number ratio of the ions is 1:1 (one
Naà ion to each ClŹion). Thus, the formula unit for sodium
chloride is NaCl. Although ionic charges are used to derive the
correct formula, they are not shown when you write the formula
unit of the compound.
The ionic compound magnesium chloride contains magnesium cations (Mg2à) and chloride anions (ClŹ). In magnesium
chloride, the ratio of magnesium cations to chloride anions is
1:2 (one Mg2à ion to two ClŹions). Therefore, the formula unit
for magnesium chloride is MgCl2. The compound has twice as
many chloride anions (each with a 1Ź charge) as magnesium
cations (each with a 2à charge), so it is electrically neutral. In
aluminum bromide, the ratio of aluminum cations to bromide
anions is 1:3 (one Al3à ion to three BrŹ ions), so the formula unit
is AlBr3. Again, the compound is electrically neutral.
Chloride ion (Clź)
18 eź
17 pá
18 n0
Sodium ion (Naá)
10 eź
11 pá
12 n0
Structures of sodium
ion and chloride ion
Arrangement of Naá ions
and Clź ions in a crystal
of sodium chloride
Crystals of sodium chloride
202 $IBQUFSt-FTTPO
Check for Understanding
The Essential Question How do ionic compounds form?
Assess students’ understanding of the process of ionic bond formation by asking
them to complete the following cloze prompt:
Ionic bonds form when one or more _______ is transferred from a ________ to a(n)
_________ so that both ions form a __________. (electrons, cation, anion, stable
octet)
ADJUST INSTRUCTION If students are having difficulty answering this question, have
them reread the last paragraph on page 201 and examine the diagram below it. Then
provide students another opportunity to complete the prompt.
202
Chapter 7 • Lesson 2
TU
TOR
Sample Problem 7.1
Sample Practice Problem
Predicting Formulas of Ionic Compounds
Use electron dot structures to predict the formulas of the ionic compounds formed from
the following elements:
a. potassium and oxygen
b. magnesium and nitrogen
— Analyze Identify the relevant concepts. Atoms of metals lose valence electrons
when forming an ionic compound. Atoms of nonmetals gain electrons. Enough atoms of
each element must be used in the formula so that electrons lost equals electrons gained.
˜ Solve
Explore
Apply the concepts to this problem.
Start with the atoms.
Properties of Ionic Compounds
a. K and O
In order to have a completely filled
valence shell, the oxygen atom must
gain two electrons. These electrons
come from two potassium atoms,
each of which loses one electron.
Express the electron dot structure as
a formula.
Class Activity
Ká
K
á
O
Ká
K
O
2Ľ
PURPOSE Students detect the presence of ions in
water samples.
MATERIALS tap water samples, 10-mL graduated
The formula of the compound formed is K2O (potassium oxide).
Each nitrogen atom needs three
electrons to have an octet, but each
magnesium atom can lose only two
electrons. Three magnesium atoms are
needed for every two nitrogen atoms.
Express the electron dot structure as
a formula.
from home to test for ions associated with water
hardness. Test 2-mL samples as follows. Add three
drops of potassium thiocyanate (KSCN) to the first
sample. Add three drops of dilute ethanoic acid,
CH3COOH, and three drops of sodium oxalate,
Na2C2O4, to the second sample. Mix well.
Mg2á
Mg
N
N
3Ľ
N
3Ľ
Mg2á
Mg á
N
Mg
Mg2á
The formula of the compound formed is Mg3N2 (magnesium nitride).
Apply the octet rule to determine
how many electrons each atom
gains or loses.
10. Use electron dot structures to determine formulas of the ionic compounds
formed when
a. potassium reacts with iodine.
b. aluminum reacts with oxygen.
cylinder, potassium thiocyanate solution, dilute
ethanoic acid, sodium oxalate solution, dropper
PROCEDURE Have students bring water samples
b. Mg and N
Start with the atoms.
Use electron dot structures to determine chemical
formulas for the ionic compounds formed when the
following elements combine:
a. magnesium and chlorine (MgCl2)
b. aluminum and sulfur (Al2S3)
c. calcium and iodine (CaI2)
EXPECTED OUTCOME A red color from the iron(III)
thiocyanate ion, Fe(SCN)2+, indicates the presence
of Fe3+ ions. A white precipitate of calcium oxalate,
CaC2O4, indicates the presence of Ca2+ ions.
11.
1 What is the formula of the ionic com11
com
pound composed of calcium cations
and chloride anions?
Ionic and Metallic Bonding 203
Water Hardness
Water hardness varies with location and source, and is primarily a result of a high
calcium and magnesium ion content, although other metal ions contribute to the
problem. The United States Geological Survey (USGS) considers water to be hard if
it contains greater than 60 mg/L of calcium carbonate. People who live in areas with
hard water often see this problem manifested as a build-up of limescale on tubs,
sinks, faucets, and other surfaces frequently exposed to water.
Generally, water from groundwater sources is harder than water from surface
sources. In the United States, most northeastern, southern, and northwestern states
have predominantly soft water. Generally, hard water of varying degrees is found in
the southwestern and midwestern states.
Answers
FIGURE 7.7 Each ion is surrounded by ions of
opposite charge.
10. a. The formula is KI.
b. The formula is Al2O3.
11. CaCl2
Ionic and Metallic Bonding
203
LESSON 7.2
CHEM
Explore
CHEMISTRY
Class Activity
Y
&YOU
Q: Would you
ou expect to ffin
find
sodium chloride in underground
undergro
rock deposits as a solid, liquid, or
gas? Explain.
PURPOSE Students observe different types of
crystals.
MATERIALS crystals of ionic compounds, watch
glasses, magnifying glasses
NCEP
TS
EXPECTED OUTCOME Students should observe the
IN
of ionic compounds. Have the students examine the
crystals with magnifying glasses and write down
their observations. Make a list of these observations
and then discuss them in terms of the underlying
ionic lattice structures.
N
PROCEDURE Pass around watch glasses with crystals
What are three properties of ionic compounds?
Figure 7.8 shows the striking beauty of the crystals of some ionic compounds.
Most ionic compounds are crystalline solids at room temperature. The
component ions in such crystals are arranged in repeating three-dimensional
patterns. In solid sodium chloride, each sodium ion is surrounded by six
chloride ions, and each chloride ion is surrounded by six sodium ions. In
this arrangement, each ion is attracted strongly to each of its neighbors, and
repulsions are minimized. The large attractive forces result in a very stable
structure. This stability is reflected in the fact that NaCl has a melting point
Ionic compounds generally have high melting points.
of about 800°C.
Figure 7.8 Crystalline Solids
The beauty of crystalline solids, such
as these, comes from the orderly
arrangement of their component ions.
ACTI
O
SAFETY Use only nontoxic crystals. Remind students
to not touch the crystals.
CO
LESSON 7.2
Properties of Ionic Compounds
Go online to learn
about properties of
a
iionic compounds.
Fluorite (CaF2)
Grossularite (Ca3Al2(SiO4)3)
Aragonite (CaCO3)
Barite (BaSO4)
Wulfenite (PbMoO4)
Beryl (Be3Al2(SiO3)6)
Hematite (Fe2O3)
Cinnabar (HgS)
geometries of different crystal structures.
&
CHEMISTRY
Y
YOU Sample answer: I would
expect NaCl in underground rock deposits to exist as
a crystalline solid because NaCl is an ionic
compound.
Extend
USE VISUALS Point out the formulas for barite and
aragonite in Figure 7.8. Note that those compounds
contain anions that are polyatomic ions, which
means that they contain more than one type of
atom. Tell students that they will learn more about
polyatomic ions in Chapter 8. (If students ask about
the formulas with multiple cations, explain that
those formulas represent complex minerals.)
204 $IBQUFSt-FTTPO
Differentiated Instruction
ELL ENGLISH LANGUAGE LEARNERS Encourage students to look up and define
terms used to describe ionic compounds. Students should define terms such as crystal
and formula unit in English and their native language.
L1 STRUGGLING STUDENTS Students may have difficulty visualizing the ions that
represent the coordination number for the example crystals in Figures 7.9 and 7.10.
Provide students with copies of both figures, and project the figures on the board.
Select a Na+ ion in NaCl and number the Cl− ions around it. Then, select a Cl− ion and
number the Na+ ions around it. Repeat with CsCl and TiO2, having students direct
you as you number each ion.
204
Chapter 7 • Lesson 2
L3 ADVANCED STUDENTS Have students research how minerals are classified.
Suggest that their written reports include information about the physical properties
of minerals and how these properties are used in mineral identification. Encourage
students to include drawings, photos, or samples of minerals.
Explain
See crystal structures of
ionic compounds online.
ET
KIN IC
ART
CRITICAL THINKING Point out that the rusting
of iron is the production of iron oxide from iron
metal and oxygen gas. Point out that Fe3+ is a
stable cation of Fe. Ask What is the stable anion of
oxygen? (O2−) Ask What compound is formed from
these two ions? (The compound formed from these
ions is iron(III) oxide, or Fe2O3.)
Explore
Figure 7.9
Coordination Numbers
Sodium chloride and cesium
chloride form cubic crystals.
a. In NaCl, each ion has a
coordination number of 6.
b. In CsCl, each ion has a
coordination number of 8.
Class Activity
PURPOSE Students investigate properties of
different classes of ionic compounds.
MATERIALS library or Internet access
a
Sodium chloride
(NaCl)
ź
Cl
Naá
b
Cl
Cesium chloride
(CsCl)
ź
Csá
Figure 7.10 Rutile
Titanium dioxide, or rutile, forms
tetragonal crystals. In TiO2,
each Ti4à ion has a coordination
number of 6, while each O2Ź ion
has a coordination number of 3.
PROCEDURE Divide the class into groups. Have each
group choose a different class of ionic compounds
to research and write about. For example, one
group could work with oxides while another group
works with sulfides. Initially, each student should
work alone to discover information such as where
the compounds occur in nature, how they are
produced, their physical and chemical properties,
and any important uses. Finally, students in each
group can pool their information to prepare a class
display or report.
EXPECTED OUTCOMES Students will discover that
different classes of ionic compounds share some
properties with other ionic compounds and have
some unique properties.
Rutile (TiO2)
O2ź
T 4á
Ti
Ionic and Metallic Bonding 205
Check for Understanding
BIGIDEA Bonding and Interactions To assess students’ knowledge about
ionic bonds and ionic compounds, have students write one-minute responses to the
following questions:
• Given what you have learned so far, write or illustrate your own definition of
an ionic bond. (an electical attraction between ions of opposite charge)
• Explain why crystals of different ionic compounds have different shapes. (The
shapes reflect different geometric arrangements of anions and cations.)
ADJUST INSTRUCTION If students demonstrate difficulty answering the second
question, review Figure 7.10 and discuss how each crystal’s shape is determined by
the coordination number of an ion. Then, encourage them to go online and watch
the Concepts in Action feature about properties of ionic crystals.
Ionic and Metallic Bonding
205
LESSON 7.2
The coordination number of an ion is the number of ions of opposite charge that surround the ion in a crystal. Figure 7.9a shows the threedimensional arrangement of ions in NaCl. The coordination number of Naà
is 6 because each Naà ion is surrounded by six ClŹ ions. The coordination
number of ClŹ is also 6 because each ClŹ ion is surrounded by six Naà ions.
Cesium chloride (CsCl) has a formula unit that is similar to that of NaCl. As
Figure 7.9b illustrates, both compounds have cubic crystals, but their internal
crystal structures are different. Each Csà ion is surrounded by eight ClŹ ions,
and each ClŹ ion is surrounded by eight Csà ions. Therefore, each anion and
each cation in cesium chloride has a coordination number of 8.
Figure 7.10 shows the crystalline form of titanium dioxide (TiO2), also
known as rutile. In this compound, the coordination number for the cation (Ti4à) is 6. Each Ti4à ion is surrounded by six O2Ź ions. The coordination number of the anion (O2Ź) is 3. Each O2Ź ion is surrounded by three
Ti4à ions.
LESSON 7.2
Another characteristic property of ionic compounds relates to conducIonic compounds can conduct an electric current when melted
tivity.
or dissolved in water. When sodium chloride is melted, the orderly crystal
structure breaks down. As Figure 7.11a shows, if a voltage is applied across
this molten mass, cations migrate freely to one electrode and anions migrate
to the other. This movement of ions allows electric current to flow between
the electrodes through an external wire. For a similar reason, ionic compounds also conduct electric current if they are dissolved in water. When dissolved, the ions are free to move about in the solution.
Explore
Quick Lab
OBJECTIVE Af
After completing this activity, students
will be able to show that ions in solution conduct
an electric current.
Misconception Alert
Power source
Flow of
electrons
Students may think that the solutions contain only
one ion. Clarify that each solution contains both
cations and anions.
Current meter
Flow of
electrons
Inert metal
electrodes
+
–
SKILLS FOCUS Observing, experimenting,
Naà
concluding
PREP TIME 20 minutes
Molten salt
(801°C—1412°C)
CLASS TIME 30 minutes
MATERIALS D-cell batteries, masking tape, 30-cm
a
lengths of bell wire with ends scraped bare, clear
plastic cups, distilled water, tap water, vinegar,
sucrose, sodium chloride, baking soda, conductivity
probe (optional)
SAFETY Students should handle wires with caution.
The wires may become hot during the activity.
EXPECTED OUTCOME When ions are present in
solution, the solution conducts an electric current.
ANALYZE AND CONCLUDE
1.
2.
3.
ClĽ
To (+)
electrode
To (–)
electrode
Figure 7.11 Molten NaCl
Sodium chloride melts about about 800°C. a. If a voltage
is applied to molten NaCl, positive sodium ions move to
the negative electrode, and negative chloride ions move
to the positive electrode. b. This solar facility uses molten
NaCl for its ability to absorb and hold a large quantity of
heat, which is used to generate electricity.
b
Solutions of vinegar, sodium chloride, and
baking soda (and maybe tap water) produced
bubbles. Distilled water and sugar solution (and
maybe tap water) do not produce bubbles.
The solutions in which bubbles were produced
also conduct an electric current. These solutions
contain ions.
Sample answer: A larger number of batteries
will increase the current, which will, in turn,
cause the rate at which the bubbles appear to
increase.
FOR ENRICHMENT Ask What gases form the
bubbles you observe? (hydrogen and oxygen) Ask
What is the source of these gases? (water) Have
students collect the gases produced and check
them for the presence of hydrogen and oxygen.
Hydrogen will burn when a lit splint is placed in it,
and a glowing split will begin to flame when placed
in oxygen.
206 $IBQUFSt-FTTPO
Focus on ELL
4 LANGUAGE PRODUCTION Have students work in groups of four to complete the
Quick Lab. Make sure each group has ELLs of varied language proficiencies, so that
more proficient students can help less proficient ones. Have students work according
to their proficiency level.
BEGINNING:
LOW Make sure students know what the terminal of a battery is. Discuss each step
before groups perform it, using the photo to increase their understanding.
HIGH Recreate the procedure as a numbered list of steps to follow.
INTERMEDIATE: LOW/HIGH Provide students with an organized and labeled chart to
complete during the Analyze and Conclude section.
ADVANCED: LOW/HIGH Perform the enrichment activity as a demo and have
206
Chapter 7 • Lesson 2
students write a summary of what happens.
Purpose To show that ions
in solution conduct an electric
current
Connect to
Procedure
1. Tape the batteries together so the
Materials
positive end of one touches the negar3 D-cell batteries
tive end of another. Tape the bare end
rmasking tape
of one wire to the positive terminal of
r2 30-cm lengths of bell wire the battery assembly and the bare end of
with ends scraped bare
the other wire to the negative terminal.
rclear plastic cup
CAUTION Bare wire ends can be sharp
and scratch skin. Handle with care.
rdistilled water
2. Half fill the cup with distilled water.
rtap water
rvinegar (acetic acid, C2H4O2) Hold the bare ends of the wires close
together in the water.
rtable sugar (sucrose,
NLIN
PR
S
E
O
C12H22O11)
rtable salt (sodium chloride,
NaCl)
rbaking soda (sodium hydrogen carbonate, NaHCO3)
M
OBLE
Extend
Solutions Containing Ions
3. Look for the production of bubbles. They are a sign that the solution
conducts electric current.
4. Repeat Steps 2 and 3 with tap
water, vinegar, and concentrated solutions of table sugar, table salt, and
baking soda.
TECHNOLOGY
Ask for a show of hands of students who are
familiar with the term liquid crystal display (LCD).
Identify any computer monitors or other items in
the classroom that have an LCD screen. Explain
that LCDs use a type of matter called liquid crystals
to display the information or images they see on
the LCD screens. Have students use the internet to
discover the properties of liquid crystals and why
they have such widespread use. Have students report
their findings to the class.
Analyze and Conclude
1. Observe Which samples produced bubbles of gas? Which samples did not
produce bubbles of gas?
2. Draw Conclusions Which samples conducted an electric current? What do
these samples have in common?
3. Predict Would you expect the same results if you used only one battery? If
you used six batteries? Explain your answers.
7.2 LessonCheck
12.
Describe How can you describe the electrical charge of an ionic compound?
13.
Identify What properties characterize ionic
compounds?
14. Apply Concepts Write the correct chemical
formula for the compounds formed from each
pair of elements.
a. potassium and sulfur
b. calcium and oxygen
c. sodium and oxygen
d. aluminum and nitrogen
15. Describe Write formulas for each compound.
a. barium chloride
b. magnesium oxide
c. lithium oxide
d. calcium fluoride
16. Describe How can you describe the arrangement of sodium ions and chloride ions in a
crystal of sodium chloride?
17. Relate Cause and Effect Why do ionic compounds conduct electric current when they are
melted or dissolved in water?
18. Apply Concepts Read about restoring electrolytes on page R4 of the Elements Handbook.
Write electron configurations for the two principal ions found in body fluids.
BIGIDEA BONDING AND INTERACTIONS
19. Which pairs of elements are likely to form ionic
compounds? Explain your choices and write the
formulas for the compounds that will form.
a. Cl, Br
c. Li, Cl
b. K, He
d. I, Na
Ionic and Metallic Bonding 207
Lesson Check Answers
12. electrically neutral
13. usually solids at room temperature;
have high melting points; conduct
an electric current when melted or
dissolved in water
14. a. K2S b. CaO c. Na2O d. AlN
15. a. BaCl2 b. MgO c. Li2O d. CaF2
16. Acceptable answers should describe
a solid containing positive sodium
ions and negative chloride ions
Evaluate
Informal Assessment
Create a game where groups of three students
compete to win the Ionic Crystal award. Give each
group the name of an ionic compound; have one
group member identify the cation, have one group
member identify the anion, and have one group
member determine the ratio of cations to anions in
the given compound. If groups are successful then
have them switch positions in their group and give
them the name of another compound. Repeat again
until each group member has taken turns identifying
a cation, an anion, and the ratio. If the group
successfully completes three rounds of the game,
award them the “Ionic Crystal.” Then have students
complete the 7.2 Lesson Check.
Reteach
Emphasize that an ionic solid is a collection of
independent ions. Explain that there is no joining
of individual particles to form molecules. State that
each ion “belongs” as much to one of its nearest
neighbors as it belongs to any other. Point out that
the arrangement in an ionic crystal is such that each
ion is surrounded by ions of opposite charge, which
produces a strong bonding force. Work through
several examples to be sure students understand this
important point.
in an alternating, regular, and
repeating three-dimensional pattern.
17. The ions are free to move.
18. Na+:1s22s22p6 K+: 1s22s22p63s23p6
19. a. not likely, both form cations
b. not likely, helium is a noble gas
c. LiCl
d. NaI
Ionic and Metallic Bonding
207
LESSON 7.2
Quick Lab
CHEMISTRY & YOU
CHEMISTRY
Y
&
CHEMISTRY
Y
YO
YOU
U Discuss the process of
nucleation with students. Then, model nucleation
and crystal growth using square blocks or metal
cubes. Let one block represent the nucleation
crystal. Stack additional blocks on and around the
nucleation block to form a crystalline shape. Pose
the following question to students: What
interactions do you think occur between the
individual crystals to create the larger crystal? You
may need to guide students in the following ways:
• Have students examine the arrangement of
sodium and chloride ions in Figure 7.9.
• Ions along the edge of one crystal can interact
with ions along the edge of another crystal.
• Have students consider how crystals might share
ions along their edges and faces.
Explain
APPLY CONCEPTS After students read the feature,
start a discussion of properties of ionic crystals. Ask
What keeps the ions in their orderly arrangements?
(The electrostatic forces among them.) Ask Why
don’t ionic crystals conduct electricity? (They do
not have free electrons or ions.) Ask What does
“growing a crystal” mean? (The crystal starts from
a small particle and becomes larger as more and
more ions attach to the crystal.)
GEOLOGY
In nature, large crystals with clearly defined faces
can grow only if there is plenty of free space and the
minerals are available from the environment. Molten
rock can form crystals if it cools slowly enough that
the ions can settle into an orderly arrangement. If the
rock cools quickly, such as during a volcanic eruption,
the rock is glassy instead of crystalline. Have students
look back at the giant crystals on the first page of
this chapter. Explain that these crystals in Mexico’s
Cave of Crystals grew because they were surrounded
by very warm water rich in minerals. When the water
was pumped out of the cave and the crystals were
discovered, the crystals stopped growing.
Answers
THINK ABOUT IT
1.
2.
3.
Both crystals have an orderly shape. The sea
salt crystals look like little cubes. The Epson salt
crystals are shaped like needles.
2+
Possible factors are rate of cooling and purity of
the solution.
208
Ionic Crystals
What ionic crystal is essential to human life, was found
among the funeral offerings of ancient Egyptians, created
and destroyed empires, and is now commonly used to
season foods? If you said table salt, you’d be right! Table
salt, or sodium chloride (NaCl) is an ionic compound
composed of sodium cations (Naà) and chloride
anions (ClŹ).
Crystals of ionic compounds, such as sodium
chloride, can be grown by a process called nucleation.
During nucleation, the ionic compound that is to be
crystallized is dissolved in a solvent, such as water. In the
dissolution process, the positive and negative ions break
away from each other. As the solvent is removed, the ions
join together again to form a repeating three-dimensional
pattern. Sodium chloride has a cubic crystal structure,
but different ionic compounds form crystals with
different shapes. Try the On Your Own activity at home
and compare the shapes of two different ionic crystals.
On Your Own
1. For this activity you will need sea salt, Epsom salts, hot
tap water, 2 pie plates, a measuring cup, and a stirring rod or
spoon. Epsom salts can be found in the medicine department of
most grocery stores.
2. Mix together 1/4 cup of the sea salt and 1/4 cup of hot water
in one of the pie plates. Stir to dissolve most of the salt. In the
other pie plate, mix together 1/4 cup of the Epsom salts and 1/4
of hot water. Again, stir until most of the salt has dissolved.
Extend
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Chapter 7 • Chemistry & You
3. Place both pie plates in the refrigerator for three hours. Once
the crystals have formed, compare the shapes of the crystals
made from the two substances. Record your observations.
208 $IBQUFSt$IFNJTUSZ:PV
Think About It
1. Compare and Contrast Describe
the shapes of the crystals of sea salt and
Epsom salts. How are they the same?
How are they different?
2. Identify Epsom salts are
magnesium sulfate (MgSO4) crystals.
A magnesium sulfate formula unit
consists of a magnesium cation and
a sulfate (SO42Ź) anion. What is the
charge on the magnesium cation?
3. Control Variables What factors do
you think affect crystal growth? Identify
two possible factors, then repeat the
activity to test your hypotheses.