LESSON 7.2

National Science Education Standards

A-1, B-2, B-3

CHEMISTRY YOU YOYY U&

Ionic and Metallic Bonding 201

Key Questions What is the electrical charge

of an ionic compound ?

What are three properties of ionic compounds?

Vocabularyionic compoundionic bondchemical formulaformula unitcoordination number

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.

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 of cations and anions. Although they are composed of ions, ionic com-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 configu-ration of neon.) The chlorine atom has seven valence electrons and can eas-ily 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.

octet

1s22s22p6 3s1 1s22s22p63s23p5 1s22s22p6 1s22s22p63s23p6

1s22s22p6 1s22s22p63s23p6

octet

octet octet

Na

Ne Ar

Na Cl Cl

Ionic Bonds and Ionic CompoundsIonic BIonic C7. 2


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.2Key Objectives

7.2.1 EXPLAIN the electrical charge of an ionic compound.

7.2.2 DESCRIBE three properties of ionic compounds.

Additional ResourcesReading 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

EngageCHEMISTRY YOU YOYY 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 KnowledgeHave 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)

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.

18 n0

18 e

Chloride ion (Cl )

Sodium ion (Na )

Structures of sodiumion and chloride ion

Arrangement of Na ionsand Cl ions in a crystal

of sodium chloride

Crystals of sodium chloride12 n0

10 e

17 p

11 p

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Figure 7.6 Formation of Aluminum BromideAluminum metal and the nonmetal bromine react violently to form the ionic solid aluminum bromide.

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.

Formula Units The ionic compound sodium chloride is com-posed of equal numbers of sodium cations (Na ) and chloride anions (Cl ). 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 phys-ical unit. As shown in Figure 7.7, the ions in solid sodium chlo-ride 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 Na ion to each Cl ion). 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 magne-sium cations (Mg2 ) and chloride anions (Cl ). In magnesium chloride, the ratio of magnesium cations to chloride anions is 1:2 (one Mg2 ion to two Cl ions). Therefore, the formula unit for magnesium chloride is MgCl2. The compound has twice as many chloride anions (each with a 1 charge) as magnesium cations (each with a 2 charge), so it is electrically neutral. In aluminum bromide, the ratio of aluminum cations to bromide anions is 1:3 (one Al3 ion to three Br ions), so the formula unit is AlBr3. Again, the compound is electrically neutral.

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?

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.2 Foundations for ReadingBUILD 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 CompoundsAPPLY 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.)

Check for UnderstandingThe 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.

Sample Problem 7.1T U T O R

CHEM

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The formula of the compound formed is K2O (potassium oxide).

Mg2

Mg2

K and

K

K

K

O2

N 3

Mg2

N 3

N

N

Mg

Mg and

Mg

Mg

N

KO

O

The formula of the compound formed is Mg3N2 (magnesium nitride).

a.

b.


Predicting Formulas of Ionic CompoundsUse electron dot structures to predict the formulas of the ionic compounds formed from the following elements:

potassium and oxygena. magnesium and nitrogenb.

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 Apply the concepts to this problem.

Use electron dot structures to deter-10. mine formulas of the ionic compounds formed when

potassium reacts with iodine.a. aluminum reacts with oxygen.b.

What is the formula of the ionic com-11. pound composed of calcium cations and chloride anions?

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.

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.

What is the formula of the ionic com111

Apply the octet rule to determine

how many electrons each atom

gains or loses.

Start with the atoms.

Start with the atoms.

Express the electron dot structure as a formula.


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.2Sample Practice ProblemUse 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)

Explore

Properties of Ionic Compounds

Class ActivityPURPOSE Students detect the presence of ions in water samples.

MATERIALS tap water samples, 10-mL graduated cylinder, potassium thiocyanate solution, dilute ethanoic acid, sodium oxalate solution, dropper

PROCEDURE Have students bring water samples 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.

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.

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.

AnswersFIGURE 7.7 Each ion is surrounded by ions of

opposite charge.10. a. The formula is KI.

b. The formula is Al2O3.11. CaCl2

Fluorite (CaF2)

Aragonite (CaCO3)

Beryl (BeAl2(SiO3)6)

Barite (BaSO4)

Hematite (Fe2O3)

Wulfenite (PbMoO4)

Cinnabar (HgS)

Grossularite (Ca3Al2(SiO4)3)

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Figure 7.8 Crystalline SolidsThe beauty of crystalline solids, such as these, comes from the orderly arrangement of their component ions.

Properties of Ionic Compounds 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 of about 800°C. Ionic compounds generally have high melting points.

Q: Would you expect to find sodium chloride in underground rock deposits as a solid, liquid, or gas? Explain.

CHEMISTRY YOU YYYYY&

Go online to learn about properties of ionic crystals

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Class ActivityPURPOSE Students observe different types of crystals.

MATERIALS crystals of ionic compounds, watch glasses, magnifying glasses

SAFETY Use only nontoxic crystals. Remind students to not touch the crystals.

PROCEDURE Pass around watch glasses with crystals 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.

EXPECTED OUTCOME Students should observe the geometries of different crystal structures.

CHEMISTRY YOU YOYY U&& Sample answer: I would expect NaCl in underground rock deposits to exist as a crystalline solid because NaCl is an ionic compound.

ExtendUSE 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.)

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 CaCl and TiO2, having students direct you as you number each ion.

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.

Sodium chloride (NaCl)

Cl

NaCl

CsCesium chloride(CsCl)

a b

Ti4O2

T

Rutile (TiO2)


Figure 7.9 Coordination NumbersSodium 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.

The coordination number of an ion is the number of ions of oppo-site charge that surround the ion in a crystal. Figure 7.9a shows the three-dimensional arrangement of ions in NaCl. The coordination number of Nais 6 because each Na ion is surrounded by six Cl ions. The coordination number of Cl is also 6 because each Cl ion is surrounded by six Na 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 Cs ion is surrounded by eight Cl ions, and each Cl ion is surrounded by eight Cs 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 cat-ion (Ti4 ) is 6. Each Ti4 ion is surrounded by six O2 ions. The coordina-tion number of the anion (O2 ) is 3. Each O2 ion is surrounded by three Ti4 ions.

Figure 7.10 RutileTitanium dioxide, or rutile, forms tetragonal crystals. In TiO2, each Ti4 ion has a coordination number of 6, while each O2 ion has a coordination number of 3.

See crystal structures of ionic compounds online.

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.2ExplainCRITICAL 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

Class ActivityPURPOSE Students investigate properties of different classes of ionic compounds.

MATERIALS library or Internet access

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.

Check for UnderstandingBIGIDEA 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.

Flow of electrons

Power sourceCurrent meter

Molten salt(801°C—1412°C)

Inert metalelectrodes

To (+)electrode

To (–)electrode

Flow ofelectrons

Cl

Na

–+

a

b

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Figure 7.11 Molten NaClSodium 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.

Another characteristic property of ionic compounds relates to conduc-tivity. Ionic compounds can conduct an electric current when melted 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 com-pounds also conduct electric current if they are dissolved in water. When dis-solved, the ions are free to move about in the solution.


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OBJECTIVE After completing this activity, students will be able to show that ions in solution conduct an electric current.

Misconception AlertStudents may think that the solutions contain only one ion. Clarify that each solution contains both cations and anions.

SKILLS FOCUS Observing, experimenting, concluding

PREP TIME 20 minutes

CLASS TIME 30 minutes

MATERIALS D-cell batteries, masking tape, 30-cm 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. 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.

2. The solutions in which bubbles were produced also conduct an electric current. These solutions contain ions.

3. 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.

Af

Quick Lab

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 students write a summary of what happens.

Quick Lab


12. Describe How can you describe the electri-cal charge of an ionic compound?

13. Identify What properties characterize ionic compounds?

Apply Concepts 14. Write the correct chemical formula for the compounds formed from each pair of elements.

potassium and sulfura. calcium and oxygenb. sodium and oxygenc. aluminum and nitrogend.

Describe 15. Write formulas for each compound.barium chloridea. magnesium oxideb. lithium oxidec. calcium fluorided.

Describe 16. How can you describe the arrange-ment of sodium ions and chloride ions in a crystal of sodium chloride?

Relate Cause and Effect 17. Why do ionic com-pounds conduct electric current when they are melted or dissolved in water?

Apply Concepts 18. Read about restoring electro-lytes on page R4 of the Elements Handbook. Write electron configurations for the two princi-pal ions found in body fluids.

BIGIDEA BONDING AND INTERACTIONS

Which pairs of elements are likely to form ionic 19. compounds? Explain your choices and write the formulas for the compounds that will form.

Cl, Br a. c. Li, ClK, He b. d. I, Na

LessonCheckPR O B L E M

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Analyze and ConcludeObserve 1. Which samples produced bubbles of gas? Which samples did not

produce bubbles of gas?Draw Conclusions 2. Which samples conducted an electric current? What do

these samples have in common?Predict 3. Would you expect the same results if you used only one battery? If

you used six batteries? Explain your answers.

Solutions Containing Ions

Look for the production of bub-3. bles. They are a sign that the solution conducts electric current.

Repeat Steps 2 and 3 with tap 4. water, vinegar, and concentrated solu-tions of table sugar, table salt, and baking soda.

Procedure Tape the batteries together so the 1.

positive end of one touches the nega-tive end of another. Tape the bare end of one wire to the positive terminal of the battery assembly and the bare end of the other wire to the negative terminal. CAUTION Bare wire ends can be sharp and scratch skin. Handle with care.

Half fill the cup with distilled water. 2. Hold the bare ends of the wires close together in the water.

Purpose To show that ions in solution conduct an electric current

Materials3 D-cell batteriesmasking tape2 30-cm lengths of bell wire with ends scraped bareclear plastic cupdistilled watertap watervinegar (acetic acid, C2H4O2)table sugar (sucrose, C12H22O11)table salt (sodium chloride, NaCl)baking soda (sodium hydro-gen carbonate, NaHCO3)


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Connect to 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.

Evaluate

Informal AssessmentCreate 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.

Lesson Check Answers12. electrically neutral13. 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. AlN15. a. BaCl2 b. MgO c. Li2O d. CaF2

16. Acceptable answers should describe a solid containing positive sodium ions and negative chloride ions

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

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On Your Own

Ionic CrystalsWhat 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 (Na ) and chloride anions (Cl ).

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.

Think About ItCompare and Contrast 1. Describe

the shapes of the crystals of sea salt and Epsom salts. How are they the same? How are they different?

Identify 2. Epsom salts are magnesium sulfate (MgSO4) crystals. A magnesium sulfate formula unit consists of a magnesium cation and a sulfate (SO4

2 ) anion. What is the charge on the magnesium cation?

Control Variables 3. What factors do you think affect crystal growth? Identify two possible factors, then repeat the activity to test your hypotheses.

For this activity you will need 1. 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.

Mix together 1/4 cup of the sea salt and 1/4 cup of hot water 2. 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.

Place both pie plates in the refrigerator for three hours. Once 3. the crystals have formed, compare the shapes of the crystals made from the two substances. Record your observations.

CHEMISTRY YOU: EVERYDAY MATTERY YYY&

208 Chapter 7 • Chemistry & You

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YO

UCHEMISTRY YOU YOYY 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.

ExplainAPPLY 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.)

Extend

Connect to 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.

AnswersTHINK ABOUT IT

1. Both crystals have an orderly shape. The sea salt crystals look like little cubes. The Epson salt crystals are shaped like needles.

2. 2+3. Possible factors are rate of cooling and purity of

the solution.

LESSON 7.2

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