220A Chapter 8

Covalent BondingPlanning GuideCovalent BPlanning G8

Most elements exist in chemical compounds, where elements are held together by chemical bonds.

Lessons and Objectives Print Resources

For the Student For the Teacher

B-2 8.1 Molecular Compounds p 222–2258.1.1 Identify the information a molecular

formula provides. 8.1.2 Describe the representative units that

define molecular compounds and ionic compounds.

Reading and Study Workbook Lesson 8.1

Lesson Assessment 8.1 p 225

Teaching Resources, Lesson 8.1 Review

Teacher Demo, p 224: Molecular Structures and Formulas

A-1, A-2, B-2, G-2

8.2 The Nature of Covalent Bonding p 226–238

8.2.1 Explain the result of electron sharing in covalent bonds.

8.2.2 Describe how coordinate covalent bonds are different from other covalent bonds.

8.2.3 Identify some exceptions to the octet rule.8.2.4 Explain how the strength of a covalent

bond is related to its bond dissociation energy.

8.2.5 Describe how resonance structures are used.

Reading and Study Workbook Lesson 8.2

Lesson Assessment 8.2 p 238Quick Lab: Strengths of

Covalent Bonds, p 238

Teaching Resources, Lesson 8.2 Review

Class Activity, p 227: Representing Molecules

Class Activity, p 229 Bonding for Second Row Elements

Teacher Demo, p 234: A Resonance Hybrid

Teacher Demo, p 235: Bond Energies

B-2, E-2 8.3 Bonding Theories p 240–2468.3.1 Describe the relationship between atomic

and molecular orbitals.8.3.2 Describe how VSEPR theory helps predict

the shapes of molecules.8.3.3 Identify the ways in which orbital

hybridization is useful in describing molecules.

Reading and Study Workbook Lesson 8.3

Lesson Assessment 8.3 p 246

Teaching Resources, Lesson 8.3 Review

Class Activity, p 242: Making Molecular Models

A-1, B-2, E-2 8.4 Polar Bonds and Molecules p 247–2538.4.1 Describe how electronegativity values

determine the charge distribution in a polar molecule.

8.4.2 Evaluate the strengths of intermolecular attractions compared with the strengths of ionic and covalent bonds.

8.4.3 Explain why the properties of covalent compounds are so diverse.

Reading and StudyWorkbook Lesson 8.4Lesson Assessment 8.4p 253

Small-Scale Lab: Paper Chromatography of Food Dyes p 254

Class Activity, p 249: A Magnetic Analogy

Teacher Demo, p 250: Observing Evidence of Polarity

Teacher Demo, p 252: Evidence of Hydrogen Bonding

Essential Questions1. How is the bonding in molecular compounds

different from the bonding in ionic compounds?2. How do electrons affect the shape of a

molecule?3. What factors affect molecular properties?

Study Guide p 255STP p 261Reading and Study

Workbook Self-Check and Vocabulary Review Chapter 8

Most elements exist in chemical compounds, where elements are held together by chemical bondsM t l t i t i h i l

Introducing the BIGIDEA: BONDING AND INTERACTIONS

Essential Questions1 How is the bonding in molecular compounds

Study Guide p 255STP p 261

Assessing the BIGIDEA: BONDING AND INTERACTIONS

NSES

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For the StudentQuick Lab, p 238

170-g (6 oz) can of food• 2 454-g (16 oz) cans of • food3 No. 25 rubber bands• metric ruler• coat hanger• plastic grocery bag• paper clip• graph paper• motion detector (optional)•

Small-Scale Lab, p 254pencil• ruler• scissors• toothpicks• 4 different colors of food • coloringplastic cup• 0.1% NaCl solution• chromatography paper• various colored candies• markers, and powdered • drinks70% 2-propanol• 0.8M CH• 3COOH

For the TeacherTeacher Demo, p 224

molecular model kit or • foam balls and toothpicks

Teacher Demo, p 23510- to 15-cm piece of • clean magnesium ribbona small piece of charcoal• tongs• Bunsen burner• filters for viewing •

Class Activity, p 249magnets• iron filings• cardboard•

Teacher Demo, p 252ring stand• 2 burets• 2 beakers• buret clamps• turpentine• water• rubber rod• glass rod• fur cloth•

Teacher Demo, p 234small piece of copper metal• concentrated nitric acid• evaporating dish• fume hood •

Class Activity, p 240ball-and-stick molecular • model kits or foam balls and toothpicks

Teacher Demo, p 252four 300-mL stoppered • flasks each containing 100 mL of one of these liquids: glycerol, ethylene glycol, water, ethanol

Digital Resources

Editable Worksheets PearsonChem.com

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Lab 11: Molecular ModelsSmall-Scale Lab Manual Lab 11:

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Formula of a Hydrated Compound

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(includes editable worksheets) • Lesson Reviews • Practice Problems • Interpret Graphs • Vocabulary Review • Chapter Quizzes and Tests • Lab Record Sheets

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A-1, A-2, B-1, B-2, E-2, G-2

National Science Education Standards 220

8Covalent Bonding

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Water droplets result from attractions between water molecules.

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1 CONTENT AND LANGUAGE Write the chapter title on the board. Below the title, write co-valent bonding. Ask students to predict the meaning of the title based on what they have learned previously about valence electrons and ionic bonding. Explain that the prefix co- means “with” or “together” and the verb valere means “to be strong.” Explain that this chapter is about the strength of the bond between two electrons that hold together two atoms.

BEGINNING

LOW Create a picture or symbol to represent the phrase. If literate, create a native language definition.

HIGH Write the phrase and introduce it to the class while saying it out loud.

INTERMEDIATE: LOW/HIGH Write simplified English definitions of the phrase and present them to the class.

ADVANCED: LOW/HIGH Write the definition of the phrase and present it to the class.

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virtual lab tour in which polar and non-polar molecules are studied in a simulated laboratory environment.

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BONDING AND INTERACTIONS

Essential Questions:1. How is the bonding in molecular

compounds different from the bonding in ionic compounds?

2. How do electrons affect the shape of a molecule?

3. What factors affect molecular properties?

BIGIDEA

What’s That Alarm?A family woke up in the middle of the night to the sound of a piercing alarm. The family thought it must be the fire alarm. They quickly evacuated the house and called 9-1-1. While they waited for the fire department to arrive, they didn’t see smoke or any other signs of fire coming from the house.

The fire department inspected the home and told the family that a compound con-taining carbon and oxygen atoms caused the alarm. Carbon dioxide (CO2) is made of carbon and oxygen, but the fire department confirmed that it wasn’t carbon dioxide. Are other molecules besides carbon dioxide made of carbon and oxygen? What was this mystery substance, and why would it set off an alarm?

Connect to the BIGIDEA As you read about covalent bonding, think about how there could be different molecules made of carbon and oxygen atoms.

CHEMYSTERY

NATIONAL SCIENCE EDUCATION STANDARDS

B-2, B-4

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Understanding by DesignStudents are building toward an understanding of covalent bonding by studying the interaction of particles in compounds.

PERFORMANCE GOALS At the end of Chapter 8, students will be able to answer the essential questions by applying their knowledge of covalent bonding. Students will also understand bond polarity and be able to identify bond type using electronegativities.

ESSENTIAL QUESTIONS Read the essential questions aloud. Ask What is a way, other than gaining or losing electrons, that atoms might attain a noble gas configuration? (sharing electrons) Ask Do you think electrostatic forces will affect the shape of a group of atoms that share electrons? Why? (Sample answer: Yes. If the pull of an atom is greater, the electron would spend more time near it, affecting the shape of the group.) Ask Can you predict how this might affect the properties of the group of atoms? (Engage students in a discussion of reasonable answers.)

BIGIDEA Use the photo of the water drops on a flower to help students connect to

the concepts they will learn in this chapter. Activate prior knowledge by reviewing how electrostatic forces among particles cause particles to either attract or repel each other. Ask What shape are the water drops? (round) Ask What forces might hold them in this shape? (the electrostatic forces among particles) Ask How could the particles of water be attracted to each other if all the particles are alike? (Different parts of the particles have different charges. This causes one water particle to be attracted to another.)

CHEMYSTERY Have students read over the CHEMystery. Connect the

CHEMystery to the Big Idea of Bonding, Interactions, and Naming Compounds by explaining that bonding can occur among different types of atoms in different ways. For example, a particle of water, H2O, contains two hydrogen atoms and one oxygen atom. The ratio of hydrogen atoms to oxygen atoms in water is always 2:1. However, hydrogen and oxygen can also combine in other ratios. Have students consider other ratios of carbon and oxygen that might solve the CHEMystery.

Introduce the ChapterCONNECTIONS TO IONIC BONDING Students have learned about ionic bonds, in which an electrostatic force between oppositely charged atoms holds particles together. Use this activity to introduce the concept of joining atoms together by sharing electrons.

Activity You will need 2 egg cartons and 14 table tennis balls or balls of modeling clay. Cut the top off the egg cartons. Then cut away four cups from each egg carton so that each has only 8 cups (a model octet). Give two students each an egg carton. One should have 7 balls with 1 cup empty. The other should have 1 ball with 7 cups empty. Ask How can you model the ionic bonding of sodium and chlorine with the balls? (A chlorine atom has 7 valence electrons, and a sodium atom has 1. The person with 7 balls can take 1 from the other person so that each atom is stable.) Next, give two other students the egg cartons. Both should have 7 balls and 1 cup empty to model chlorine atoms. Ask How can you model bonding with two chlorine atoms? (Some students may correctly answer that they can each share one of their electrons.) Guide students to share an “electron.” Explain that students have modeled the difference between ionic and covalent bonding.

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Oxygen(O2)

Nitrous oxide(N2O)

Helium(He)

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Molecular Compounds8.1

Figure 8.1 Comparing Gas Particles

Key Questions What information does a

molecular formula provide?

What representative units define molecular compounds and ionic compounds?

Vocabulary

Q: How are atoms joined together to make compounds with different struc-tures? This toy model is made from cubes joined together in units by sticks. Although the types of pieces are limited, you can make many different toy models depending on how many pieces you use and how they are arranged. In this lesson, you will learn how atoms are joined together to form units called molecules.

Molecules and Molecular Compounds What information does a molecular formula provide?

In nature, only the noble gas elements, such as helium and neon, exist as uncombined atoms. They are monatomic; that is they consist of single atoms, as shown in Figure 8.1. But not all elements are monatomic. For example, a key component of the air you breathe is oxygen gas, O2. As you might guess from the chemical formula, O2 represents two oxygen atoms that are bonded together.

In Chapter 7, you learned about ionic compounds, which are generally crystalline solids with high melting points. Other compounds, however, have very different properties. For example, water (H2O) is a liquid at room tem-perature. Carbon dioxide (CO2) and nitrous oxide (N2O) are both gases at room temperature. The attractions that hold together the atoms in O2, H2O, CO2, and N2O cannot be explained by ionic bonding. These bonds do not involve the transfer of electrons.

Helium, which is less dense than air, is often used to inflate balloons.

Scuba divers breathe compressed air, a mixture that contains oxygen gas.

Nitrous oxide (also known as laughing gas) is sometimes used as a mild anesthetic in dental procedures.

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1 CONTENT AND LANGUAGE As a class, brainstorm the various words that mean the same as bond. (Sample answers: link, union, tie, attachment, connection) Then have students identify where in this lesson synonyms are used. Then have students identify words that mean the opposite of bond. Repeat this exercise with the word noble.

2 FRONTLOAD THE LESSON Have students draw a table with column heads labeled Atoms, Molecules, Diatomic Molecules, and Molecular Compounds. Tell students they will use their tables to organize the names, symbols, and structural drawings of each example illustrated in the lesson.

3 COMPREHENSIBLE INPUT As a class, summarize the paragraph titled Representing Molecules using a two-column table. Under the two column heads labeled Always True and Never True, construct a list of key points.

Key Objectives8.1.1 IDENTIFY the information a molecular

formula provides.8.1.2 DESCRIBE the representative units that

define molecular compounds and ionic compounds.

Additional Resources• Reading and study Workbook, Lesson 8.1• Core Teaching Resources, Lesson 8.1 Review

EngageCHEMISTRY YOU YOYY U&& Discuss how students

used to create objects when playing with logs, blocks, craft sticks, etc. Ask If everyone in the room had the same set of toy logs and connector pieces, will everyone in the room create the same model? (no) How might the models differ from one another? (The new models could be different shapes and sizes.) Ask Do you think two atoms can be joined together in more than one way? (Guide students to predict that it depends on the type of atoms that are joined together.)

Activate Prior KnowledgeHave students create a three column table. In the first column, instruct students to write a bulleted list of what they learned in Chapter 7 about ionic bonding. In the second column, students should create a bulleted list of what they learned about metallic bonding. As a class, preview this lesson and begin a bulleted list in the third column about covalent bonding.

Additional Resources• Reading and study Workbook, Lesson 8.1• Core Teaching Resources, Lesson 8.1 Review

Structural formula

Space-fillingmolecular model

Ball-and-stickmolecular model

Molecular formula Perspectivedrawing

NH H

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Ammonia Molecule

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Figure 8.2 Butane Butane (C4H10) is commonly used in lighters and household torches. The butane torch shown here is being used to caramelize sugar on a dessert.

Sharing Electrons Recall that ionic bonds form when the combining atoms give up or accept electrons. Another way that atoms can combine is by sharing electrons. Atoms that are held together by sharing elec-trons are joined by a covalent bond. In a covalent bond, a “tug of war” for electrons takes place between the atoms, bonding the atoms together. In Lesson 8.2, you will learn about the different types of covalent bonds.

In Figure 8.1, the representative units shown for oxygen and dinitrogen monoxide are called molecules. A molecule is a neutral group of atoms joined together by covalent bonds. Oxygen gas consists of oxygen mol-ecules; each oxygen molecule consists of two covalently bonded oxygen atoms. An oxygen molecule is an exam-ple of a diatomic molecule —a molecule that contains two atoms. Other elements found in nature in the form of diatomic molecules include hydrogen, nitrogen, and the halogens. Molecules can also be made of atoms of different elements. A compound composed of molecules is called a molecular compound. Water is an example of a molecular com-pound. The molecules in water are all the same; each water molecule is a tightly bound unit of two hydrogen atoms and one oxygen atom.

Representing Molecules A molecular formula is the chemical formula of a molecular compound. A molecular formula shows how many atoms of each element a substance contains. The molecular formula of water is H2O. Notice that a subscript written after an element’s symbol indicates the number of atoms of each element in the molecule. If there is only one atom, the subscript 1 is omitted. The molecular formula of car-bon dioxide is CO2. This formula represents a molecule containing one carbon atom and two oxygen atoms.

Butane, shown in Figure 8.2, is also a molecular compound. The molecular formula for butane is C4H10. According to this formula, one molecule of butane contains four carbon atoms and ten hydrogen atoms. A molecular formula reflects the actual number of atoms in each mol-ecule. The subscripts are not necessarily lowest whole-number ratios. Note that molecular formulas also describe molecules consisting of atoms of one element. For example, an oxygen molecule consists of two oxygen atoms bonded together; its molecular formula is O2.

A molecular formula does not tell you about a molecule’s struc-ture. In other words, it does not show either the arrangement of the vari-ous atoms in space or which atoms are covalently bonded to one another. A variety of diagrams and molecu-lar models, some of them illustrated in Figure 8.3, can be used to show the arrangement of atoms in a molecule. Diagrams and models like these will be used throughout this textbook.

Figure 8.3 Representations of an Ammonia Molecule The formula NH3 tells you the composition of an ammonia molecule, but it does not reveal the arrangement of the atoms. Molecular models and structural formulas specify the bonds between atoms and the arrangement of those atoms.

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Foundations for ReadingBUILD VOCABULARY Explain that the prefix di- is a Greek word that means “two.” Have students predict the meaning of diatomic molecule. (a molecule containing two atoms) Have students preview the lesson to verify their prediction. Students should take note of any other words with a common prefix.

READING STRATEGY Encourage students to examine the illustrations carefully as they read. Have students compare the models shown in this lesson with representations of ionic bonding illustrated in Chapter 7. Have students summarize any similarities or differences they notice.

Explain

Molecules and Molecular CompoundsSTART A CONVERSATION As a class, brainstorm ways in which people join objects together, for instance, attaching papers with a paper clip, attaching a leg of a table to a tabletop with screws, setting a diamond in a ring with prongs, using glue to fix a broken vase, etc. Ask What typically guides a person to choose one method for attaching objects over another? (Sample answers: whether the attachment is permanent or temporary, expensive or inexpensive, etc.) Guide the discussion toward the concept of stability. Use an analogy involving tape and superglue to help students understand the strength of an ionic or covalent bond. Develop a sense of awareness that the bonding that occurs between two elements varies in strength. Some of the resulting molecules are stronger, meaning more stable, than others. Tell students to keep this in mind as they progress through the chapter.

USE CONTEXT CLUES Have students read the first paragraph. Ask What is the meaning of monatomic? (elements that consist of a single atom) Ask Use what you learned about the prefix di- from your vocabulary exercise to predict the meaning of the prefix mono-. (one) Guide students to rely on their language development skills to help them understand difficult or unfamiliar passages.

USING VISUALS Direct students to Figure 8.1. Point out that some other elements are most stable when two atoms are joined together. Ask Which of the three examples are monatomic molecules? (helium) Ask Is H2 a monatomic or diatomic molecule? (diatomic molecule) Ask Do you think H2 could be an ionic compound? (No, ionic bonds do not form between two atoms of the same element.) Tell students that H2 is a molecule held together not by the transfer of an electron but by the sharing of a pair of electrons.

Differentiated Instruction L1 STRUGGLING STUDENTS Prior to introducing molecular compounds, have students practice identifying elements. To help students interpret a molecular compound, have students assign a colored pencil to each element and rewrite the molecular formula using the colored pencils. Have students write the subscript “1” for single atoms in a compound until they comprehend that it is not necessary. ELL ENGLISH LANGUAGE LEARNERS Prepare three-dimensional models of ammonia similar to the two models in Figure 8.3. Write the molecular and structural formulas for ammonia on large cards. Hold the models and cards up one by one. Slowly and clearly point out the information that each model or formula provides. Have students make their own drawings indicating the key points.L3 ADVANCED STUDENTS Have students create a poster or models of the various ways that the bonding in a hydrogen molecule can be represented: molecular formula, structural formula, electron-dot structure, and orbital diagram showing the overlap of the 1s orbitals. Then have students research the octet rule and repeat the activity for a chlorine molecule.

Carbon dioxide (CO2)1 molecule of CO2

contains 2 oxygen atoms 1 carbon atom

Ethanol (C2H6O)1 molecule of C2H6O

contains 6 hydrogen atoms

2 carbon atoms 1 oxygen atom

Water (H2O)1 molecule of H2O

contains 2 hydrogen atoms

1 oxygen atom

Hydrogen atom (H) Oxygen atom (O) Carbon atom (C)

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Figure 8.4 shows the chemical formulas and structures of some other molecular compounds. The arrangement of the atoms within a molecule is called its molecular structure. Carbon dioxide, for exam-ple, is a gas produced by the complete burning of carbon. It is found in Earth’s atmosphere and is dissolved in seawater. The molecular structure of carbon dioxide shows how the three atoms are arranged in a row. It also shows how the carbon atom in each molecule is in the middle between the two oxygen atoms. The molecular structure of water shows how the oxygen atom is in the middle between the hydrogen atoms. However, the atoms in water are not arranged in a row. Instead, the hydrogen atoms are mainly on one side of the water molecule. The molecular structure of ethanol (C2H6O) is more com-plicated. As you can see in the model, each carbon is bonded to four atoms, each hydrogen is bonded to one atom, and the one oxygen is bonded to two atoms.

Comparing Molecular and Ionic Compounds

What representative units define molecular compounds and ionic compounds?You have now seen how formulas can be used to describe molecular compounds and ionic compounds. Each type of compound contains atoms of different elements that are combined chemically. However, the formulas describe different representative units. The repre-sentative unit of a molecular compound is a molecule. For an ionic compound, the representative unit is a formula unit. Recall that a formula unit is the lowest whole-number ratio of ions in an ionic com-pound. It is important not to confuse formula units with molecules. A molecule is made up of two or more atoms that act as a unit. No such discrete units exist in an ionic compound, which consists of a contin-uous array of ions. So there is no such thing as a molecule of sodium chloride or a molecule of magnesium chloride. Instead, these com-pounds exist as collections of positively and negatively charged ions arranged in repeating three-dimensional patterns.

Figure 8.4 Molecular Formulas and Structures The formula of a molecular compound indicates the numbers and kinds of atoms in each molecule of the compound. Use Models Which of these molecules has the greatest number of oxygen atoms?

Q: Similar to how you can make different types of toy models, there are thou-sands of different types of molecular structures. How are atoms joined together to make compounds with different structures?

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ExplainUSE AN ANALOGY Point out that the recipe for a cake provides information about what type and how much of each ingredient is needed. Varying the types and amounts of ingredients leads to cakes that have different tastes and textures. Ask How does a molecular formula provide similar information about a compound? (The subscripts in a molecular formula tell the number of atoms of each element in a compound.)

Misconception AlertStudents often confuse empirical (simplest) formulas and molecular formulas. Explain that the subscripts in a molecular formula are not necessarily written in the smallest whole-number ratio of the atoms. For example, ethane is correctly represented by the formula C2H6

because the molecule actually contains two carbon atoms and six hydrogen atoms. It would be incorrect to represent ethane by its empirical formula, which is CH3.

CHEMISTRY YOU YOYY U&& Atoms join together by forming covalent bonds. Changing the number, type, and/or order of the atoms changes the compound. Atoms can also be joined by ionic bonds.

Explore

Teacher DemoPURPOSE Students see the structure associated with molecular formulas.

MATERIALS molecular model kit or foam balls and toothpicks

PROCEDURE Construct models of these pairs of molecules: O2 and O3; CO and CO2; H2O and H2O2. Show students each pair and ask them how many of each type of atom is in each molecule. Write the corresponding chemical formulas on the chalkboard.

EXPECTED OUTCOMES Students will see that the addition of one more oxygen atom to O2, CO, and H2O results in new molecules that have very different structures.

ExplainComparing Molecular and Ionic CompoundsUSE VISUALS Direct students to examine Figure 8.5. Ask How do the submicroscopic illustrations of NaCl and H2O differ? (The ions of sodium and chlorine are separate and arrayed in a regular pattern; two hydrogen atoms and one oxygen atom are bonded together in one molecule.) Compare the formulas for NaCl and H2O. (They look similar, but NaCl represents just one unit in the array of sodium and chloride ions, whereas H2O represents one separate molecule of water.)

Check for Understanding

BIGIDEA BONDING AND INTERACTIONS Give each student an index card. Assess students’ knowledge of how bonding in molecular compounds differs from bonding in ionic compounds. Ask students to state the type of compound associated with electron sharing, and the type of compound associated with electron transfer. (molecular compound; ionic compound)

ADJUST INSTRUCTION If students are having difficulty answering the question, have them re-read the previous page and then repeat the exercise.

Array ofsodium ions andchloride ions

Formula unit ofsodium chloride:

Chemical formula: NaCl

Na Cl

Collection ofwater molecules

Molecule of water:

Chemical formula: H2O

HHO

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Lesso8.11. Identify What information does a molecular

formula provide?

2. Compare How is the representative unit of a molecular compound different from the representa-tive unit of an ionic compound?

Identify 3. What are the only elements that exist in nature as uncombined atoms? What term is used to describe such elements?

Compare and Contrast 4. Describe how the mol-ecule whose formula is NO is different from the molecule whose formula is N2O.

Apply Concepts 5. Give an example of a diatomic molecule found in Earth’s atmosphere.

Identify 6. What information does a molecular structure give?

Figure 8.5 Molecular and Ionic Compounds Water, which is a molecular compound, and sodium chloride, which is an ionic compound, are compared here.Interpret Diagrams How do molecular compounds differ from ionic compounds?

Molecular compounds tend to have relatively lower melting and boiling points than ionic compounds. Many molecular compounds are gases or liquids at room temperature. In contrast to ionic com-pounds, which are formed from a metal combined with a nonmetal, most molecular compounds are composed of atoms of two or more nonmetals. For example, one atom of carbon can combine with one atom of oxygen to produce one molecule of a compound known as carbon monoxide. Carbon monoxide is a poisonous gas produced by burning gasoline in internal combustion engines or in household gas appliances and furnaces. Figure 8.5 illustrates some differences between molecular and ionic compounds, using water and sodium chloride as examples.

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CRITICAL THINKING Have students explain why two hydrogen atoms bond by sharing rather than by electron transfer to form a hydrogen molecule. (Hydrogen atoms cannot transfer electrons, so there is no way to determine which hydrogen atom would donate and which would receive an electron. However, if the hydrogen atoms share their electrons, they can each achieve the stable arrangement of a helium atom.)

Evaluate

Informal AssessmentHave students number a sheet of paper from 1 to 6. Write the following statements on the board. Have students indicate on their papers if the following statements are true or false:1. All molecular compounds are composed of

atoms of two or more different elements. (true)2. All compounds contain molecules. (false)3. No elements exist as molecules. (false)4. Most molecular compounds are composed of

two or more nonmetallic elements. (true)5. Atoms in molecular compounds share electrons.

(true)6. Ionic compounds tend to have lower melting and

boiling points than molecular compounds. (false)

Then have students complete the 8.1 Lesson Check.

ReteachUse the statements in the Informal Assessment above as a basis for review. Have students locate where in the lesson the concept for each statement is explained. Use the given material to prove each statement as true or provide an example to show that the statement is false.

AnswersFIGURE 8.4 carbon dioxide FIGURE 8.5 Molecular compounds are made up of

molecules and usually have lower melting and boiling points than ionic compounds, which are made up of ions.

Lesson Check Answers1. Molecular formulas show how

many atoms of each element one molecule of a compound contains.

2. The representative unit of a molecular compound is a molecule. The representative unit of an ionic compound is a formula unit.

3. noble gases; monatomic

4. NO has 1 N atom and 1 O atom; N2O has 2 N atoms and 1 O atom.

5. nitrogen (N2) or oxygen (O2) 6. Molecular structures show the

arrangement of atoms in a molecule.