8.2 the Nature of Covalent Bonding 8.2

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8.2 The Nature of Covalent Bonding 8.2

1 FOCUS Connecting to Your World You know that without oxygen Guide for Reading to breathe, you could not live. But did you know that oxygen plays Objectives another important role in your life? High in the atmosphere, a different Key Concepts • What is the result of electron 8.2.1 Describe how electrons are form of oxygen, called ozone, forms a layer that filters sharing in covalent bonds? out harmful radiation from the sun. The colors in • How do electron dot structures shared to form covalent bonds this map indicate the concentrations of ozone in represent shared electrons? and identify exceptions to the • various parts of Earth’s atmosphere. In this How do atoms form double or triple covalent bonds? octet rule. section, you will learn how oxygen atoms can • How are coordinate covalent 8.2.2 Demonstrate how electron dot join in pairs to form the oxygen you breathe and bonds different from other structures represent shared can also join in groups of three oxygen atoms to covalent bonds? • How is the strength of a electrons. form ozone. covalent bond related to its bond dissociation energy? 8.2.3 Describe how atoms form dou- • How are oxygen atoms bonded in ozone? ble or triple covalent bonds. • What are some exceptions to 8.2.4 Distinguish between a cova- The Octet Rule in Covalent Bonding the octet rule? Recall that when ionic compounds form, electrons tend to be transferred lent bond and a coordinate Vocabulary so that each ion acquires a noble gas configuration. A similar rule applies single covalent bond covalent bond and describe for covalent bonds. In covalent bonds, electron sharing usually occurs structural formula how the strength of a covalent so that atoms attain the electron configurations of noble gases. For example, unshared pair each hydrogen atom has one electron. But a pair of hydrogen atoms share bond is related to its bond dis- double covalent bond these two electrons when they form a covalent bond in a hydrogen triple covalent bond sociation energy. molecule. Each hydrogen atom thus attains the electron configuration coordinate covalent bond 8.2.5 Describe how oxygen atoms of helium, a noble gas with two electrons. Combinations of atoms of the polyatomic ion are bonded in ozone. nonmetals and metalloids in Groups 4A, 5A, 6A, and 7A of the periodic bond dissociation energy table are likely to form covalent bonds. In this case the atoms usually resonance structure acquire a total of eight electrons, or an octet, by sharing electrons, so that Reading Strategy Guide for Reading the octet rule applies. Identifying Main Idea/Details List the main idea in the para- Build Vocabulary L2 graph following the heading Single Covalent Bonds The Octet Rule in Covalent Bonding. Word Parts The word structure comes The hydrogen atoms in a hydrogen molecule are held together mainly by As you read, list examples of from the Latin verb struere, which how this rule is followed when a the attraction of the shared electrons to the positive nuclei. Two atoms held single covalent bond, a double means “to build.” A structural formula together by sharing a pair of electrons are joined by a single covalent bond. covalent bond, and a triple cova- is one that shows how a molecule is Hydrogen gas consists of diatomic molecules whose atoms share only one lent bond form. pair of electrons, forming a single covalent bond. built, that is, how the atoms are joined together by chemical bonds. Ask students to define structural steel. (Steel is H ϩ H ⎯→ HH shared pair of electrons used to form the skeleton of buildings.) Hydrogen Hydrogen Hydrogen atom atom molecule Reading Strategy L2 1s H Directed Reading/Thinking Activity

H Have students read the red headings 1s Hydrogen molecule throughout the section ﬁrst and ask themselves questions about what they Section 8.2 The Nature of Covalent Bonding 217 might learn when they read the text.

2 INSTRUCT Section Resources Print Technology •Guided Reading and Study Workbook, •Interactive Textbook with ChemASAP, Explain that scientists attribute the Section 8.2 Problem-Solving 8.8, 8.10; Simulation 6, thinning of the ozone layer, in part, to •Core Teaching Resources, Assessment 8.2 the action of compounds called chlo- Section 8.2 Review •Go Online, Section 8.2 roﬂuorcarbons (CFCs), which have •Probeware Laboratory Manual, been released into the atmosphere. Quick Lab, p. 226 Ask, Why is ozone important in the •Transparencies, T87–T89 atmosphere? (It ﬁlters out radiation that could harm living beings on Earth.)

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Section 8.2 (continued) An electron dot structure such as H:H represents the shared pair of electrons of the covalent bond by two dots. The pair of shared electrons forming the covalent bond is also often represented as a dash, as in H⎯H The Octet Rule in for hydrogen. A structural formula represents the covalent bonds by dashes For: Links on Valence Covalent Bonding Electrons and shows the arrangement of covalently bonded atoms. In contrast, the Visit: www.SciLinks.org molecular formula of hydrogen, H , indicates only the number of hydrogen Discuss L2 2 Web Code: cdn-1082 atoms in each molecule. Write electron configurations for car- The halogens also form single covalent bonds in their diatomic mole- bon, nitrogen, oxygen, fluorine, and cules. Fluorine is one example. Because a fluorine atom has seven valence neon on the chalkboard. Ask, How electrons, it needs one more to attain the electron configuration of a noble many electrons would carbon, nitro- gas. By sharing electrons and forming a single covalent bond, two fluorine atoms each achieve the electron configuration of neon. gen, oxygen, and fluorine need to ...... ...... share in order to achieve the same F.. F.. .F...F... or .F.. F... electron configuration as neon? (4, 3, Fluorine Fluorine Fluorine atom atom 2, and 1 respectively) molecule 1s 2s 2p F CLASS Activity F Representing Molecules L2 1s 2s 2p Purpose Students practice different ways to represent molecules. Materials paper and pencil Fluorine molecule Procedure Divide students into In the F molecule, each fluorine atom contributes one electron to com- groups of three or four. Have them 2 plete the octet. Notice that the two fluorine atoms share only one pair of practice drawing molecular diagrams, valence electrons. A pair of valence electrons that is not shared between structural formulas, electron-dot struc- atoms is called an unshared pair, also known as a lone pair or a nonbond- tures, and orbital diagrams for mole- ing pair. You can draw electron dot structures for molecules of compounds in cules such as OF2, SCl2, N2H4, CCl4, much the same way that you draw them for molecules of diatomic ele- CHCl3, and C2H6. ments. Water (H2O) is a molecule containing three atoms with two single covalent bonds. Two hydrogen atoms share electrons with one oxygen Single Covalent Bonds atom. The hydrogen and oxygen atoms attain noble-gas configurations by sharing electrons. As you can see in the electron dot structures below, the oxygen atom in water has two unshared pairs of valence electrons...... . . . . . 2H .O. .O.. .H or .O H H H Hydrogen Oxygen atoms atom Water molecule

Download a worksheet on Valence 1s 2s 2p Electrons for students to complete, O and ﬁnd additional teacher support from NSTA SciLinks. 1s 1s HH Water molecule

Checkpoint What does a structural formula represent?

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Differentiated Instruction Special NeedsL1 Pair each student with a study partner. Have any atom, 1A to 7A, indicates the number of them use the periodic table and quiz each valence electrons that atom has, and that it is other on writing electron dot structures for valence electrons that appear in the electron single atoms and bonded atoms. Make sure dot structures. they understand that the Group number for

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You can draw the electron dot structure for ammonia (NH3), a suffocat- Discuss L2 ing gas, in a similar way. The ammonia molecule has one unshared pair of First review the molecular and struc- electrons. tural formulas, electron dot structures, H . H.. and orbital diagrams for fluorine, . ϩ . . . . . 3H .N. .N.. .H or .N H water, and ammonia molecules. If pos- H H Hydrogen Nitrogen sible, display physical models. Call atoms atom Ammonia molecule attention to the fact that fluorine has one half-filled orbital and forms one s 2s 2p 1 bond, oxygen has two half-filled orbit- N als and forms two bonds, and nitrogen has three and forms three bonds. Tell 1s 1s 1s H HH Ammonia molecule students carbon has two. Ask, How many covalent bonds do you think

Methane (CH4) contains four single covalent bonds. The carbon atom carbon forms?(Students may logically has four valence electrons and needs four more valence electrons to attain say two.) Tell students that CH2 does a noble-gas configuration. Each of the four hydrogen atoms contributes not represent a stable molecule, but one electron to share with the carbon atom, forming four identical carbon- hydrogen bonds. As you can see in the electron dot structure below, meth- CH4 (methane) is a stable molecule. ane has no unshared pairs of electrons. Explain the concept of electron promotion, which allows carbon to form four H . H.. single covalent bonds. Point out that . ϩ . . . . 4H C. H.C.. .H or H C H elements in groups 3A and 4A pro- H H Hydrogen Carbon mote s electrons to p orbitals, increas- atoms atom Methane molecule ing their bonding capacity. For 2s and 2p 1s example, boron’s electron configura- 2 2 1 C tion is 1s 2s 2p . Based on this configuration, students might infer that 1s 1s 1s 1s boron can form only one covalent H H H H bond. However, the chloride of boron is BCl3 rather than BCl. The promotion of one 2s electron to the 2p orbital allows for the formation of three bonds. Boron does not achieve a noble-gas Methane molecule configuration, but it does achieve When carbon forms bonds with other atoms, it usually forms four added stability by forming three bonds bonds. You would not predict this based on carbon’s electron configura- rather than one. tion, shown below.

1s 2 2s 2 2p 2

Checkpoint What is the electron dot structure of a methane molecule?

Section 8.2 The Nature of Covalent Bonding 219

Facts and Figures Expanding the Octet Nonmetals in the third row and beyond such phosphorus to form ﬁve bonds. The amount Answers to... as phosphorus, sulfur, and iodine can form of energy needed to promote an electron is Checkpoint more than four bonds because they have less than the energy released with the forma- It shows the empty d orbitals. Phosphorus, for example, tion of an extra bond. Sulfur can promote covalent bonds as dashes and can unpair a 3s electron and promote it to an one 3s electron and one 3p electron and shows the arrangement of empty 3d orbital. The promotion allows form two extra bonds. covalently bonded atoms. H Checkpoint H C H H

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Section 8.2 (continued) If you tried to form covalent C ⎯ H bonds for methane by combining the two 2p electrons of the carbon with two 1s electrons of hydrogen atoms,

you would incorrectly predict a molecule with the formula CH2 (instead of CH ). The formation of four bonds by carbon can be simply explained. One CLASS Activity 4 of carbon’s 2s electrons is promoted to the vacant 2p orbital to form the Bonding for Second Row following electron configuration. Elements L2 2 Purpose Students gain understand- 1s 2s and 2p ing of covalent bonding and distin- This electron promotion requires only a small amount of energy. The pro- guish between covalent and ionic motion provides four electrons of carbon that are capable of forming cova- bonding. lent bonds with four hydrogen atoms. Methane, the carbon compound formed by electron sharing of carbon with four hydrogen atoms, is much Procedure Have students draw elec- more stable than CH2. The stability of the resulting methane more than tron dot structures for each element in compensates for the small energy cost of the electron promotion. There- the second row of the periodic table: fore, formation of methane (CH4) is more energetically favored than the Li, Be, B, C, N, O, and F. Then have them formation of CH2. answer the following: •Predict how many bonds each atom must form to attain a noble-gas CONCEPTUAL PROBLEM 8.1 configuration. (1, 2, 3, 4, 3, 2, 1) •Can lithium form a covalent bond Drawing an Electron Dot Structure and reach stability? (no) Hydrochloric acid (HCl (aq)) is prepared by dissolving gaseous •Which elements can reach stability by hydrogen chloride (HCl (g)) in water. Hydrogen chloride is a diatomic molecule with a single covalent bond. Draw the electron forming covalent bonds? (C, N, O, F) dot structure for HCl. •Can fluorine form an ionic bond? (yes) •Are the bonds in nitrogen mole- 1 Analyze Identify the relevant concepts. 2 Solve Apply concepts to the situation. .. .. cules (N ) ionic or covalent? . . . . . 2 In a single covalent bond, a hydrogen and a H Cl.. . H.Cl.. . (covalent) chlorine atom must share a pair of electrons. Hydrogen Chlorine Hydrogen Each must contribute one electron to the atom atom chloride molecule CONCEPTUAL PROBLEM 8.1 bond. First, draw the electron dot structures In the electron dot structures, the hydrogen for the two atoms. Then show the electron atom and the chlorine atom are each correctly Answers sharing in the compound they produce. shown to have an unpaired electron. Through electron sharing, the hydrogen and chlorine 7. a. Cl Cl b. Br Br c.I I atoms are shown to attain the electron config- 8. a. HHO O b. Cl P Cl urations of the noble gases helium and argon, Cl respectively. Practice Problems Plus L2 Practice Problems The following covalent molecules have 7. Draw electron dot structures for each molecule. only single covalent bonds. Draw an a. chlorine b. bromine c. iodine electron dot structure for each. 8. The following molecules have single covalent Problem Solving 8.8 Solve F N F bonds. Draw an electron dot structure for each. Problem 8 with the help of an a. NF interactive guided tutorial. 3 F a. H2O2 b. PCl3 withChemASAP b. SBr2 Br S Br 220 Chapter 8

Facts and Figures Inventing Electron Dot Structures Gilbert Newton Lewis (1875–1946) was an the theory of acids and bases by deﬁning an American chemist who invented electron acid as an electron pair acceptor and a base dot structures, which are often called Lewis as an electron pair donor. The deﬁnitions structures or diagrams in his honor. These encompass all Brønsted-Lowry acid-base structures supported Lewis’s theory of the reactions and include many others not previ- electron pair in chemical bonding. As a pro- ously categorized as acid-base reactions. fessor of physical chemistry, he expanded

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Double and Triple Covalent Bonds Double and Triple Sometimes8.1 atomsMolecular bond by sharing more Compounds than one pair of electrons. Atoms Covalent Bonds form double or triple covalent bonds if they can attain a noble gas structure Simulation 6 Simulate the Discuss L2 by sharing two pairs or three pairs of electrons. A bond that involves two covalent bonding between shared pairs of electrons is a double covalent bond. A bond formed by sharing molecules. To introduce the discussion of multiple three pairs of electrons is a triple covalent bond. covalent bonds, use the electron dot Connecting to Your World withChemASAP You might think that an oxygen atom, withThese six toy valence models electrons, are made would of Guide for Reading structure for the nitrogen molecule. formthe same a double basic bondpieces, by joined sharing together two of by its sticks electrons in units with in anotherdifferent oxygen ways. Key Concepts Ask students to show how the struc- atom.Even when the toy models are made of the same kind of pieces, the units • How are the melting points and ture of diatomic nitrogen can satisfy . they form can be different because of the differ- ...... boiling points of molecular .O. .O. .OO.. . (The nitrogen atoms can . . ent numbers. and. or arrangements..O O.. of the compounds different from the octet rule. Oxygen Oxygen wooden barsOxygen joining them. Atoms can ionic compounds? share six electrons.) Have them com- atom atom molecule join in ways that are like these models, • What information does a pare the bonding in ammonia with the molecular formula provide? s p with different possible ways of join- bonding in nitrogen gas. Then intro- 1s 2 2 Vocabulary ing the atoms. In this section, you O covalent bond duce the oxygen molecule. Ask stu- will learn how atoms can share elec- molecule dents to draw a structure that obeys O trons to form a bond, called a cova- diatomic molecule the octet rule. Ask, Why doesn’t oxy- 1s s p 2 2 lent bond,Oxygen and howmolecule the atoms join to molecular compound gen form a triple bond? (Each oxygen form units called molecules. molecular formula In such an arrangement, all the electrons within the molecule would be needs to share only two electrons to Reading Strategy paired. Experimental evidence, however, indicates that two of the electrons achieve a stable electron configuration.) Building Vocabulary As you in O2 are still unpaired. Thus, the bonding in the oxygen molecule (O2) does read the section, write a definition Explain that although a double bond in not obey the octet rule. You cannot draw an electron dot structure that ade- of each key term in your own the oxygen molecule fulfills the octet quately describes the bonding in the oxygen molecule. words. An element whose molecules contain triple bonds is nitrogen (N ), a rule, it does not fit with experimental Molecules and Molecular Compounds 2 major component of Earth’s atmosphere illustrated in Figure 8.7. In the evidence that shows that the oxygen In nature, matter takes many forms. The noble gas elements, such as nitrogen molecule, each nitrogen atom has one unshared pair of electrons. molecule contains two unpaired elec- helium and neon shown in Figure 8.1, exist as atoms. They are monatomic; A single nitrogen atom has five valence electrons. Each nitrogen atom in that is, they consist of single atoms. You learned in the previous chapter trons. Thus the structure of O2 is an the nitrogen molecule must gain three electrons to have the electron con- that atoms of some elements combine to form salts. Salts, such as sodium exception to the octet rule. Help stu- figuration of neon. chloride (NaCl), are crystalline solids with high melting points. dents draw the electron dot structure . . . .. Other compounds,.N. however,.N. can have. very.. . different. properties.. Hydro- . . N..N or N N Figure 8.7 Oxygen and nitrogen and orbital diagram for carbon dioxide. gen chloride (HCl), for example, is a gas at room temperature. Water (H O) Nitrogen Nitrogen Nitrogen 2 are the main components of Ask, What type of bonds does carbon is a liquid at roomatom temperature.atom These two compounds are so different molecule Earth’s atmosphere. The oxygen form with the two oxygen atoms in from salts that you might correctly suspect that electrostatic attraction molecule is an exception to the s 2s 2p between ions fails1 to explain their bonding. Such compounds are not ionic. octet rule. It has two unpaired CO2? (double covalent bonds) Note that Their combiningN atoms do not give up electrons or accept electrons as electrons. Three pairs of electrons carbon can form single, double, and tri- are shared in a nitrogen readily as sodiumN does in combining with chlorine. ple bonds, but a quadruple bond is molecule. Instead, a “tugs of war” for the electrons takes place between the atoms, 1 2s 2p Nitrogen molecule impossible because of geometric bonding the atoms together. The atoms held together by sharing electrons restrictions. Have students draw dia- are joined by a covalent bond. grams for hydrogen cyanide (HCN) and formaldehyde (H2CO). Ask, What kind of bonds does carbon form in each Figure 8.1 The noble gases, including helium and neon, are monatomic. That of these molecules? (HCN: one single means they exist as single atoms. Helium, carbon-to-hydrogen bond and one tri- being less dense than air, is often used to ple carbon-to-nitrogen bond; H CO: two inflate balloons. The colors produced in 2 what we commonly call neon lights are a single carbon-to-hydrogen bonds and result of passing an electric current one double carbon-to-oxygen bond) As through one or more noble gases. the discussion proceeds, provide models for the molecules discussed. Section 8.2 The Nature of Covalent Bonding 221

Differentiated Instruction Less Proﬁcient ReaderL1 Divide students into groups of three or four. OF2. Have them do the same for SCl2 and Have them draw electron dot formulas for CCl4. When students have mastered these, oxygen and ﬂuorine, then combine the continue the exercise with CHCl3, N2H4, and drawings into the electron dot formula for C2H6.

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Section 8.2 (continued) Table 8.1 The Diatomic Elements Relate L1 Chemical Electron dot Earth’s atmosphere is approximately 80 Name formula structure Properties and uses percent nitrogen gas, but surprisingly Greenish-yellow reactive toxic gas. Compounds of fluorine, .. .. Fluorine F . . a halogen, are added to drinking water and toothpaste to few nitrogen compounds exist com- 2 ..F. .F.. pared with the numerous compounds promote healthy teeth. .. .. Greenish-yellow reactive toxic gas. Chlorine is a halogen Chlorine Cl . . of oxygen, which constitutes only 20 2 .Cl.. Cl.. . used in household bleaching agents. percent of the atmosphere. Students Dense red-brown liquid with pungent odor. Compounds ...... may correctly surmise that the triple Bromine Br2 .Br Br. of bromine, a halogen, are used in the preparation of .. .. photographic emulsions. bond in N2 is harder to break than a Dense gray-black solid that produces purple vapors; a .. .. double bond and considerably harder Iodine I . . halogen. A solution of iodine in alcohol (tincture of iodine) 2 ..I...I. to break than a single bond. Thus, N2 is is used as an antiseptic. a stable molecule. Plant and animal life ⎯ Colorless, odorless, tasteless gas. Hydrogen is the Hydrogen H2 H H depends on nitrogen, but in order to lightest known element. Colorless, odorless, tasteless gas. Air is almost 80% Nitrogen N . . be usable to living systems, the ele- 2 NN nitrogen by volume. ment must be converted to a com- Colorless, odorless, tasteless gas that is vital for life. Air is Oxygen O Inadequate pound, a process called nitrogen fixing. 2 about 20% oxygen by volume. Nitrogen fixing occurs naturally when lightning provides the energy for Up to this point in your textbook, the examples of single and triple atmospheric nitrogen to react with covalent bonds have involved diatomic molecules. Table 8.1 lists the prop- oxygen to form nitrogen oxides. The erties and uses of the elements that exist as diatomic molecules. Single, oxides dissolve in rain and fall to the double, and triple covalent bonds can also exist between unlike atoms. For example, consider carbon dioxide (CO ), which is present in the atmo- ground where they can be utilized 2 by plants. Nitrogen fixing bacteria in sphere and is used to carbonate many soft drinks as shown in Figure 8.8. The carbon dioxide molecule contains two oxygens, each of which shares two the soil are also able to convert electrons with carbon to form a total of two carbon–oxygen double bonds. atmospheric nitrogen to usable . . . . . . . ...... compounds. ..O. .C. .O.. ..OO..C.. .. or ..OOC .. Oxygen Carbon Oxygen Carbon dioxide atom atom atom molecule

s p 1s 2 and 2 C

OO 1s s p p s 1s Figure 8.8 Carbon dioxide gas is 2 2 2 2 Carbon dioxide molecule soluble in water and is used to carbonate many beverages. A The two double bonds in the carbon dioxide molecule are identical to carbon dioxide molecule has two each other. Carbon dioxide is an example of a triatomic molecule, which is carbon–oxygen double bonds. a molecule consisting of three atoms.

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Coordinate Covalent Bonds Coordinate Covalent Carbon monoxide (CO) is an example of a type of covalent bonding differ- Bonds ent from that seen in water, ammonia, methane, and carbon dioxide. A car- Discuss L2 bon atom needs to gain four electrons to attain the electron configuration of neon. An oxygen atom needs two electrons. Yet it is possible for both A coordinate covalent bond is an atoms to achieve noble-gas electron configurations by a type of bonding exception to the rule that covalent called coordinate covalent bonding. To see how, begin by looking at the bonding occurs between two half- double covalent bond between carbon and oxygen. empty orbitals of two atoms. In a coor- . . . ...... C. .O.. .C..O.. dinate covalent bond, one atom has an Carbon Oxygen Carbon empty orbital and the other an orbital atom atom monoxide filled with an electron pair that is not 1s 2s 2p yet involved in a chemical bond. Note C that the bonding still involves only one O pair of electrons and one pair of orbit- 1s 2s 2p als but one atom provides both of the Carbon monoxide molecule shared electrons. With the double bond in place, the oxygen has a stable configuration but the carbon does not. As shown below, the dilemma is solved if the oxygen Discuss L2 also donates one of its unshared pairs of electrons for bonding. Ask students to compare the electron ...... C..O.. .C..O. dot structure for ammonia (NH3) and + Carbon monoxide for a hydrogen ion (H ). After forming molecule the three nitrogen-to-hydrogen bonds, A coordinate covalent bond is a covalent bond in which one atom contrib- nitrogen has an unshared pair of elec- utes both bonding electrons. In a structural formula, you can show coordinate covalent bonds as arrows that point from the atom donating the pair trons. The hydrogen ion has no elec- of electrons to the atom receiving them. The structural formula of carbon trons available for bonding. Ask, How monoxide, with two covalent bonds and one coordinate covalent bond, is Figure 8.9 The polyatomic do you explain the existence of the

" ammonium ion (NH ), present C O. In a coordinate covalent bond, the shared electron pair comes 4 + (The empty in ammonium sulfate, is an ammonium ion, NH4 ? from one of the bonding atoms. Once formed, a coordinate covalent bond is + important component of orbital of the H must overlap the ﬁlled like any other covalent bond. fertilizer for field crops, home orbital in ammonia. The electrons from The ammonium ion (NH4 ) consists of atoms joined by covalent gardens, and potted plants. bonds, including a coordinate covalent bond. A polyatomic ion, such as nitrogen are attracted to both the nitro- NH4 , is a tightly bound group of atoms that has a positive or negative gen nucleus and the hydrogen nucleus. A charge and behaves as a unit. The ammonium ion forms when a pos- bond forms when electrons are simulta- itively charged hydrogen ion (H ) attaches to the unshared neously attracted to two nuclei. This type electron pair of an ammonia molecule (NH ). Most plants 3 of bond is called a coordinate covalent need nitrogen that is already combined in a compound rather bond.) Ask students to explain the than molecular nitrogen (N2) to grow. The fertilizer shown in Figure 8.9 contains the nitrogen compound ammonium sulfate. bonding in CO2 and then explain the bonding in CO using coordinate cova- unshared electron pair H lent bonding. H.. H.. . . . . H N.. H H.N.. .H or H N H H H H Hydrogen Ammonia Ammonium ion molecule ion (proton) (NH ) 3 (NH 4 )

Checkpoint What is a polyatomic ion?

223

Answers to...

Checkpoint a group of atoms that has a positive or negative charge and behaves as a unit

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Section 8.2 (continued) Most polyatomic cations and anions contain both covalent and coordinate covalent bonds. Therefore, compounds containing polyatomic ions Discuss L2 include both ionic and covalent bonding. As another example, draw the electron dot structure of the polyatomic Have students write the electron dot 2 ion SO3 . Start by drawing the electron dot structures for the oxygen and structure for SO2. Emphasize that the sulfur atoms, and the two extra electrons indicated by the charge. Then join structure should satisfy the bonding two of the oxygens to sulfur by single covalent bonds. requirements of all three atoms. Stu- O S O O S O dents should ﬁnd that, to satisfy the O O octet rule for all the atoms, they must Then join the remaining oxygen by a coordinate covalent bond, with write a structure in which one oxygen sulfur donating one of its unshared pairs to oxygen, and add the two extra atom is double bonded to sulfur. The electrons. Put brackets about the structure and indicate the 2 charge, other oxygen is single bonded by a giving the result shown...... ...... 2 coordinate covalent bond in which the .O. .S...O... .. c O.. S.. O.. d . . . . electrons are donated by sulfur. Point O... .O.. out that experimental evidence indi- Each of the atoms of the completed structure has eight valence electrons, cates that both sulfur-oxygen bonds satisfying the octet rule. Without the two extra electrons, two of the oxygens are identical. Explain that this evidence would be electron-deﬁcient. Table 8.2 lists electron dot structures of some common compounds indicates that the bonding in SO2 must with covalent bonds. be some intermediate between a single and double bond. Ask, How does Table 8.2 the formation of a coordinate cova- Some Common Molecular Compounds lent bond differ from that of a covalent bond? (In a covalent bond, each Chemical Name formula Structure Properties and uses atom provides one electron, in a coordi- Hydrogen H

nate covalent bond, both electrons are peroxide . ¬ ... Colorless, unstable liquid when pure. It is used as rocket

H2O2 .O¬O. provided by the same atom.) .. ¬ fuel. A 3% solution is used as a bleach and antiseptic. H . . L2 Sulfur . ¬ . Relate ..O Su dioxide SO2 .. O. Have students think of everyday exam- .. Oxides of sulfur are produced in combustion of petroleum Sulfur .. ples in which the shape of an object is O. products and coal. They are major air pollutants in industrial trioxide .. .. u areas. Oxides of sulfur can lead to respiratory problems. as important as its composition. For SO3 .O¬ S . u .. example, several keys might be made . O... of the same metal, but only one will ﬁt .. .. Nitric oxide NO .O¬N. into a particular lock. . Oxides of nitrogen are major air pollutants produced by the .. . Nitrogen . ¬ combustion of fossil fuels in automobile engines. They irri- .O Nu dioxide NO2 .. tate the eyes, throat, and lungs. ..O..

Nitrous .. Colorless, sweet-smelling gas. It is used as an anesthetic N O . ¬u ¬ . oxide 2 .O.. N¬ N. commonly called laughing gas. Hydrogen HCN¬ ¬ . Colorless, toxic gas with the smell of almonds. cyanide H C ¬ N Hydrogen .. HF ¬ . ﬂuoride H .F. Two hydrogen halides, all extremely soluble in water. Hydro- gen chloride, a colorless gas with pungent odor, readily dis- Hydrogen .. HCI H¬ Cl. solves in water to give a solution called hydrochloric acid. chloride ..

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Remember, the electron dot structure for a neutral molecule contains CONCEPTUAL PROBLEM 8.2 the same number of electrons as the total number of valence electrons in the combining atoms. The negative charge of a polyatomic ion shows the Answers _ number of electrons in addition to the valence electrons of the atoms 9. HO present. Because a negatively charged polyatomic ion is part of an ionic _ compound, the positive charge of the cation of the compound balances F these additional electrons. 10. F B F F 2_ 2_ CONCEPTUAL PROBLEM 8.2 O O 11. a. O S O b. O C O

Drawing the Electron Dot Structure of a Polyatomic Ion O O _ The polyatomic hydronium ion (H3O ), which is found in acidic mixtures such as lemon juice, contains a coordinate covalent bond. 12. H O C The H3O ion forms when a hydrogen ion is attracted to an unshared O electron pair in a water molecule. Draw the electron dot structure of the hydronium ion. Practice Problems Plus L2 Draw the electron dot structure of 12Analyze Identify the relevant concepts. Solve Apply the concepts to this situation. the polyatomic chlorate anion, ClO3 . H O forms by the addition of a hydrogen ion H _ 3 H.. H.. to a water molecule. Draw the electron dot H . . . . O Cl O O.. H HHO.. or H O.. H structure of the water molecule. Then, add the O Hydrogen Water Hydronium hydrogen ion. Oxygen must share a pair of ion molecule ion (proton) (H O) electrons with the added hydrogen ion to form 2 (H 3 O ) a coordinate covalent bond. The oxygen in the hydronium ion has eight TEACHER Demo valence electrons, and each hydrogen shares two valence electrons. This satisﬁes the needs L2 of both hydrogen and oxygen for valence elec- Bond Energies trons. The water molecule is electrically neu- Purpose Students observe two reac- tral, and the hydrogen ion has a positive tions in which bonds are formed and charge. The combination of these two species compare ionic and covalent bond must have a charge of 1 , as is found in the energies. hydronium ion. Materials 10- to 15- cm piece of clean Practice Problems magnesium ribbon, a small piece of

9. Draw the electron dot structure of the hydroxide 12. Draw the electron dot structure for the hydro- charcoal, tongs, Bunsen burner, filters ion (OH ). gen carbonate ion (HCO3 ). Carbon is the cen- for viewing 10. Draw the electron dot structure of the poly- tral atom, and hydrogen is attached to oxygen in Safety Wear safety goggles and lab this polyatomic anion. atomic boron tetrafluoride anion (BF4 ). apron. 11. Draw the electron dot structures for sulfate Procedure Using tongs, hold the (SO 2) and carbonate (CO 2). Sulfur and 4 3 piece of magnesium in a Bunsen carbon are the central atoms, respectively. Problem-Solving 8.10 Solve burner. CAUTION! Tie back long hair Problem 10 with the help of an and loose clothing and wear safety gog- interactive guided tutorial. gles. Do not look directly at the flame. withChemASAP Observe through filters. Discuss with students the large amount of heat and light given off in the formation of MgO. Write the balanced equation on the Section 8.2 The Nature of Covalent Bonding 225 board. → 2Mg + O2 2MgO + energy Using tongs, place a small piece of charcoal in the Bunsen burner flame and try to ignite it. Write the balanced chemical equation. → C + O2 CO2 + energy Expected Outcomes Students note that much less energy is given off in forming CO2 than in forming MgO. Ask, What kind of bonds are in MgO? (ionic) What kind of bonds are in CO2? (covalent) Ionic bond energies are, in general, greater than covalent bond energies.

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Quick LAB Section 8.2 (continued) Bond Dissociation Strengths of Covalent Bonds Energies Purpose Procedure To compare and contrast Sensor version available in the Probeware the stretching of rubber Lab Manual. Quick LAB bands and the dissociation 1. Bend the coat hanger to fit over the top energy of covalent bonds. of a door. The hook should hang down on one side of the door. Measure the Materials L2 length of the rubber bands (in cm). Strengths of Covalent Bonds • 1 170-g (6-oz) can of Hang a rubber band on the hook Objective After completing this activ- food created by the coat hanger. ity, students will understand that the • 2 454-g (16-oz) cans of 2. Place the 170-g can in the plastic bag. food dissociation energy of a covalent bond Use the paper clip to fasten the bag to • 3 No. 25 rubber bands the end of the rubber band. Lower the increases in order from single bond to you estimate best fits the points for metric ruler bag gently until it is suspended from • each set of data. (Your graph should double bond to triple bond. the end of the rubber band. Measure • coat hanger have three separate lines.) The x-axis and record the length of the stretched Skills Focus Solving, interpreting • plastic grocery bag and y-axis intercepts of the lines should rubber band. Using different combina- paper clip pass through zero, and the lines should • tions of food cans, repeat this process Prep Time 10 minutes graph paper extend past 1 kg on the x-axis. Deter- • three times with the following masses: Advance Prep Obtain the necessary mine the slope of each line in cm/kg. • pencil 454 g, 624 g, and 908 g. materials in advance. You may wish to • motion detector (optional) 3. Repeat Step 2, first using two rubber Analyze and Conclude ask students to help with this task by bands to connect the hanger and the 1. Assuming the rubber bands are mod- bringing cans of food, coat hangers, paper clip, and then using three. els for covalent bonds, what can you and plastic grocery bags to class. 4. Graph the length difference: (stretched conclude about the relative strengths of single, double, and triple bonds? Class Time 30 minutes rubber band) (unstretched rubber band) on the y-axis versus mass (kg) on 2. How does the behavior of the rubber Teaching Tips the x-axis for one, two, and three rub- bands differ from that of covalent ber bands. Draw the straight line that bonds? •Use tape to secure the hanger in place if necessary. •Have groups of students record the Bond Dissociation Energies data and then create a graph as a A large quantity of heat is released when hydrogen atoms combine to form class using an overhead projector. hydrogen molecules. This suggests that the product is more stable than the

Expected Outcome As the mass of reactants. The covalent bond in the hydrogen molecule (H2) is so strong the load increases, the stretch of the that it would take 435 kJ of energy to break apart all of the bonds in 1 mole (about 2 grams) of H . (You will study the mole, abbreviated mol, in Chap- rubber band or bands increases. 2 ter 12.) The energy required to break the bond between two covalently For a given mass, a single rubber band bonded atoms is known as the bond dissociation energy. This is usually stretches farther than a double and a expressed as the energy needed to break one mole of bonds, or 6.02 1023 double rubber band stretches farther bonds. The bond dissociation energy for the H2 molecule is 435 kJ/mol. than a triple. A large bond dissociation energy corresponds to a strong covalent Analyze and Conclude bond. A typical carbon–carbon single bond has a bond dissociation energy of 347 kJ/mol. Typical carbon–carbon double and triple bonds have bond 1. Triple covalent bonds are stronger dissociation energies of 657 kJ/mol and 908 kJ/mol, respectively. Strong than double covalent bonds, which carbon–carbon bonds help explain the stability of carbon compounds. are stronger than single covalent Compounds with only C ⎯ C and C ⎯ H single covalent bonds, such as bonds. methane, tend to be quite unreactive. They are unreactive partly because the dissociation energy for each of these bonds is high. 2. The change in bond dissociation energies in going from a carbon-car- 226 Chapter 8 bon single bond to a carbon-carbon double bond to a carbon-carbon triple bond is nearly constant. The change in length of one, two, and For Enrichment L3 three rubber bands, as given by the Have students draw a graph of actual bond slopes of the lines, is not constant. It dissociation energies versus number of is large going from one to two rub- bonds for carbon-carbon single, double, and ber bands and small going from two triple bonds. Have students write a state- to three rubber bands. ment that relates this graph to their conclu- sions in the activity.

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Resonance Resonance Ozone in the upper atmosphere blocks harmful ultraviolet radiation from Discuss L2 the sun. At the lower elevations shown in Figure 8.10, it contributes to smog. Although the electron dot structures of The ozone molecule has two possible electron dot structures. O2 on page 221 show that all electrons ...... d ...... are paired, experimental evidence .O...O..O.. ..O..O.O. .. reveals that two electrons are Notice that the structure on the left can be converted to the one on the unpaired. Ask, Would you consider right by shifting electron pairs without changing the positions of the oxy- oxygen a reactive gas? (yes) What gen atoms. could account for oxygen’s reactiv- As drawn, these electron dot structures suggest that the bonding in ity? (Unpaired electrons are reactive.) ozone consists of one single coordinate covalent bond and one double covalent bond. Because earlier chemists imagined that the electron pairs After a brief discussion of the other rapidly flip back and forth, or resonate, between the different electron dot exceptions to the octet rule that are structures, they used double-headed arrows to indicate that two or more presented in this section, remind stu- structures are in resonance. dents that many bonds do follow the Double covalent bonds are usually shorter than single bonds, so it was octet rule. believed that the bond lengths in ozone were unequal. Experimental mea- surements show, however, that this is not the case. The two bonds in ozone Relate L2 are the same length. This result can be explained if you assume that the actual bonding in the ozone molecule is the average of the two electron dot Students may be interested in knowing structures. The electron pairs do not actually resonate back and forth. that substances containing unpaired The actual bonding of oxygen atoms in ozone is a hybrid, or mixture, of electrons can be identified through a the extremes represented by the resonance forms. phenomenon called paramagnetism. The two electron dot structures for ozone are examples of what are still referred to as resonance structures. A resonance structure is a structure that When molecules with unpaired elec- occurs when it is possible to draw two or more valid electron dot structures trons are placed in a magnetic field, that have the same number of electron pairs for a molecule or ion. Reso- they tend to be drawn into the field. nance structures are simply a way to envision the bonding in certain mol- These substances are paramagnetic. In ecules. Although no back-and-forth changes occur, double-headed arrows contrast, molecules in which all elec- are used to connect resonance structures. trons are paired tend to be pushed Checkpoint What notation is used to show that the two covalent bonds in from a magnetic field. These sub- O3 are the same? stances are diamagnetic. Paramagnetism should not be con- fused with ferromagnetism, which is the familiar attraction of metals such as iron, cobalt and nickel to a magnetic field. The property of paramagnetism is the evidence that shows that the oxygen molecule has unpaired and thus cannot be described exactly by appli- cation of the octet rule. However, the oxygen bond does have double bond Figure 8.10 Although ozone high above the character; its bond length and bond ground forms a protective layer that absorbs energy are similiar to those of double ultraviolet radiation from the sun, at lower elevations ozone is a pollutant that contributes bonds in other molecules that conform to smog. to the octet rule.

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Answers to...

Checkpoint double-headed arrows

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Section 8.2 (continued) Exceptions to the Octet Rule The octet rule provides guidance for drawing electron dot structures. For Exceptions to the Octet some molecules or ions, however, it is impossible to draw structures that satisfy the octet rule. The octet rule cannot be satisﬁed in molecules Rule whose total number of valence electrons is an odd number. There are also molecules in which an atom has fewer, or more, than a complete octet of

valence electrons. The nitrogen dioxide (NO2) molecule, for example, con- TEACHER Demo tains a total of seventeen, an odd number, of valence electrons. Each oxygen contributes six electrons and the nitrogen contributes five. Two

A Resonance Hybrid L2 plausible electron dot structures can be drawn for the NO2 molecule. Purpose Students observe the forma- ...... “ ¬ . .O N O... tion of nitrogen dioxide and write its ...... ¬ “ . resonance structures. .O.. N O. Materials small piece of copper 1s 2s 2p metal, concentrated nitric acid, evapo- O rating dish, fume hood N Safety Wear safety goggles, gloves, and lab apron. Perform the experiment O under an efﬁcient hood. 1s 2s 2p Nitrogen dioxide molecule Procedure Place a small piece of cop- An unpaired electron is present in each of these structures, both of which per in an evaporating dish and cover it fail to follow the octet rule. It is impossible to draw an electron dot structure for NO that satisfies the octet rule for all atoms. Yet, NO does exist as with concentrated HNO3. While stu- 2 2 dents observe the reaction, write the a stable molecule. In fact, it is produced naturally by lightning strikes of the sort shown in Figure 8.11. balanced equation for the reaction on A number of other molecules also have an odd number of electrons. In the board. these molecules, as in NO2, complete pairing of electrons is not possible. It Figure 8.11 Lightning is one is not possible to draw an electron dot structure that satisfies the octet rule. Cu + 4HNO3 → means by which nitrogen and Examples of such molecules include chlorine dioxide (ClO ) and nitric Cu(NO ) + 2H O + 2NO + energy oxygen in the atmosphere 2 3 2 2 2 produce nitrogen dioxide. oxide (NO). Several molecules with an even number of valence electrons, such as Tell students that NO2 is one of the pollutants in automobile exhaust. It gives some compounds of boron, also fail to follow the octet rule. This may hap- smog its reddish-brown color and is pen because an atom acquires less than an octet of eight electrons. The boron atom in boron trifluoride (BF3), for example, is deficient by two elec- very reactive and poisonous. Have stu- trons, and therefore is an exception to the octet rule. Boron trifluoride dents try to write the electron dot readily reacts with ammonia to make the compound BF3·NH3. In doing so, structure for NO2. They will be unable the boron atom accepts the unshared electron pair from ammonia and to ﬁnd a way to arrange the 17 elec- completes the octet. trons around the central nitrogen and ...... F. H .F. H the two oxygen atoms so that the octet ...... F ¬ B ϩ .N ¬ H . ¬ d ¬ rule is obeyed. Refer students to the .. F.. B N H . . . . two resonance structures on this page, F.. H .F... H each with an unpaired electron on the nitrogen atom. Ask them to draw Checkpoint Give two examples of exceptions to the octet rule. another plausible resonance structure. (They could draw a structure with the unpaired electron on an oxygen atom.) Expected Outcome A reddish-brown 228 Chapter 8 gas is produced.

Facts and Figures Free Radicals Nitrogen dioxide has an odd number of electron on oxygen atoms. When free radi- valence electrons (17), so one electron must cals interact, they share their unpaired elec- be unpaired. Molecules with unpaired electrons and create a dimer. Thus, dinitrogen trons are called free radicals and tend to be tetroxide, N2O4 consists of two nitrogen reactive. Two resonance structures for NO2 dioxide molecules joined by a N-N covalent appear on this page. These have the bond. The bond is temperature dependent; unpaired electron on the nitrogen atom. Two at higher temperatures the bond breaks as other possible structures place the unpaired shown in an equilibrium diagram on p. 557.

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Figure 8.12 Phosphorus pentachloride, used as a 3 ASSESS chlorinating and dehydrating agent, and sulfur hexafluoride, used as an insulator for electrical equipment, are exceptions to the octet rule. Interpreting Diagrams Evaluate Understanding L2 How many valence electrons does the sulfur in sulfur Ask student to write electron dot struc- hexafluoride (SF6 )have for the structure shown in the figure? tures for SBr2 and C3H8. Have pairs of students decide whether their structures are correct and work together to Cl F ƒ Cl ƒ √ F ≈ √ F correct any errors. Have students Cl P √ S ƒ ≈ F ƒ ≈ F define a coordinate covalent bond in Cl Cl F their own words. (A coordinate covalent bond is formed when one atom contrib- Phosphorus pentachloride Sulfur hexafluoride utes both bonding electrons.) Ask students to explain resonance structures. A few atoms, especially phosphorus and sulfur, sometimes expand the (Resonance structures occur when two or octet to include ten or twelve electrons. Consider phosphorus trichloride more valid electron dot structures can be (PCl3) and phosphorus pentachloride (PCl5). Both are stable compounds written for a molecule.) in which all of the chlorines are bonded to a single phosphorus atom.

Covalent bonding in PCl3 follows the octet rule because all the atoms have eight valence electrons. However, as shown in Figure 8.12, the electron Elements Handbook

dot structure for PCl5 can be written so that phosphorus has ten valence electrons. CFCs react with ozone, thinning the ozone layer. Banning the use of CFCs should help decrease the amount of 8.2 Section Assessment CFCs reaching Earth’s ozone layer.

13. Key Concept What electron conﬁgurations 22. Use the bond dissociation energies of H2 do atoms usually achieve by sharing electrons to (435 kJ/mol) and of a typical carbon–carbon form covalent bonds? bond (347 kJ/mol) to decide which bond is 14. Key Concept How is an electron dot structure stronger. Explain your reasoning. used to represent a covalent bond? 15. Key Concept When are two atoms likely to If your class subscribes to the form a double bond between them? A triple bond? Interactive Textbook, use it to Handbook 16. Key Concept How is a coordinate covalent review key concepts in Section 8.2. bond different from other covalent bonds? Ozone Read the feature on ozone on page R31 of 17. Key Concept How is the strength of a covalent the Elements Handbook. Describe the effect of CFCs with ChemASAP bond related to its bond dissociation energy? on the ozone layer. Explain why the United States has banned the use of CFCs in aerosols. 18. Key Concept Draw the electron dot resonance structures for ozone and explain how they describe its bonding. Answers to... 19. Key Concept List three ways in which the octet rule can sometimes fail to be obeyed. Checkpoint NO2, BF3 20. What kinds of information does a structural for- Figure 8.12 In sulfur hexaﬂuoride, mula reveal about the compound it represents? Assessment 8.2 Test yourself the sulfur atom must have twelve 21. Draw electron dot structures for the following on the important concepts valence electrons. molecules, which have only single covalent bonds. of Section 8.2.

a. H2S b. PH3 c. ClF withChemASAP

Section 8.2 The Nature of Covalent Bonding 229

Section 8.2 Assessment 13. the conﬁgurations of noble gases sponds to a strong covalent bond. There are also molecules in which an 14. Two dots represent each covalent bond. 18. O O O ←→ O O O atom has fewer, or more, than a com- 15. when they can attain a noble-gas struc- The actual bonding of oxygen atoms in plete octet of valence electrons. ture by sharing two pairs or three pairs ozone is a hybrid, or mixture, of the ex- 20. the arrangement of atoms in a molecule of electrons tremes represented by the resonance 21. a. S H b. HPH c. Cl F 16. The shared electron pair comes from one of forms. H H the bonding atoms. In other covalent bonds 19. The octet rule cannot be satisﬁed in 22. The H-H bond is stronger because it has a each bonding atom provides an electron. molecules whose total number of va- greater dissociation energy. 17. A large bond dissociation energy corre- lence electrons is an odd number.

Covalent Bonding 229