Download Understanding Covalent Bonding: Result, Octet Rule, and Multiple Bonds and more Study notes Chemistry in PDF only on Docsity!
A-1, A-2, B-2, G-
National Science Education Standards
CHEMISTRY & YOUYOYY U
à (^) of electronsshared pair
H H Hydrogen molecule
1 s
Hydrogen atom
Hydrogen atom
Hydrogen molecule
1 s
H (^) H H H
shared pair of electrons ydrogen molec
C
226 $IBQUFSt-FTTPO
Key Questions What is the result of elec- tron sharing in covalent bonds? How are coordinate covalent bonds different from other covalent bonds? What are some exceptions to the octet rule? How is the strength of a covalent bond related to its bond dissociation energy? How are resonance struc- tures used? Vocabulary t TJOHMFDPWBMFOUCPOE tTUSVDUVSBMGPSNVMB tVOTIBSFEQBJS tEPVCMFDPWBMFOUCPOE tUSJQMFDPWBMFOUCPOE tDPPSEJOBUFDPWBMFOUCPOE tQPMZBUPNJDJPO tCPOEEJTTPDJBUJPOFOFSHZ tSFTPOBODFTUSVDUVSF
Q: What is the difference between the oxygen you breathe and the oxygen in ozone in the atmosphere? Our atmosphere contains two different mol- ecules that are both made of oxygen atoms. One is the oxygen that our cells need to survive. The other molecule containing only oxygen atoms is the ozone that protects us from the sun but also contributes to smog. The colors in this map indicate the concentrations of ozone in various parts of Earth’s atmosphere. In this lesson, you will learn how oxygen atoms can join to form the oxygen you breathe and can also join to form ozone.
8.2 The Nature of Covalent BondingThe N
The Octet Rule in Covalent Bonding What is the result of electron sharing in covalent bonds? Recall that when ionic compounds form, electrons tend to be transferred so that each ion acquires a noble gas configuration. A similar rule applies for covalent bonds. In covalent bonds, electron sharing usually occurs so that atoms attain the electron configurations of noble gases. For example, a single hydrogen atom has one electron. But a pair of hydrogen atoms shares electrons to form a covalent bond in a diatomic hydrogen molecule. Each hydrogen atom, thus, attains the electron configuration of helium, a noble gas with two electrons. Combinations of atoms of the nonmetals and metalloids in Groups 4A, 5A, 6A, and 7A of the periodic table are likely to form covalent bonds. The combined atoms usually acquire a total of eight electrons, or an octet, by sharing electrons, so that the octet rule applies. Single Covalent Bonds The hydrogen atoms in a hydrogen molecule are held together mainly by the attraction of the shared electrons to the positive nuclei. Two atoms held together by sharing one pair of electrons are joined by a single covalent bond. Hydrogen gas consists of diatomic molecules whose atoms share only one pair of electrons, forming a single covalent bond.
226 Chapter 8 • Lesson 2
LESSON 8.
Focus on ELL
1 CONTENT AND LANGUAGE Have each student prepare a set of flash cards for the vocabulary words in this lesson. Each card should list the vocabulary word, its definition, and the page number where it can be found. Cycle through the index cards a few times, reading the words aloud as a class. Point out familiar prefixes, poly- and dis- and co- , and remind students of their meanings. (many; not; with or together) 2 FRONTLOAD THE LESSON Divide the class into three groups. Have one group preview all the models in the lesson; another group, the tables, charts, and photos; and the third, the sample problems. Ask each group to answer a series of questions, such as What is this lesson about? Is there anything I have already learned that will help me learn this material? Have each group present their answers to the class, as well as any symbols or vocabulary that is unfamiliar. 3 COMPREHENSIBLE INPUT Re-draw the electron dot figures used throughout lesson 8.2, using a separate color for each different element’s electrons to help demonstrate the concepts associated with each figure. Have students view the Kinetic Art animation of covalent bonding.
Key Objectives 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 the how resonance structures are used.
Additional Resources
- Reading and Study Workbook, Lesson 8.
- Core Teaching Resources, Lesson 8.2 Review
- Probeware Laboratory Manual, Quick Lab, p. 238
Engage
CHEMISTRY && YOUYOYY U Explain that scientists
attribute the thinning of the ozone layer, in part, to the action of compounds called chlorofluorocarbons (CFCs), which have been released into the atmosphere. Ask Why is ozone important in the atmosphere? (It filters out radiation that could harm living things on Earth.)
Activate Prior Knowledge Prior to beginning this lesson, ask students to recall what they know about electron configuration and the rules that govern it. Have student volunteers demonstrate the proper use of the aufbau principle, the Pauli exclusion principle, and Hund’s rule.
Additional Resources
- Reading and Study Workbook, Lesson 8.
- Core Teaching Resources, Lesson 8.2 Review
- Probeware Laboratory Manual, Quick Lab, p. 238
à Hydrogen atoms
Oxygen atom Water molecule
Water molecule
2H O O H H
O H H
or
H H
1 s 1 s
1 s^2 s^^2 p O
shared pairs of electrons
1 s^2 s^^2 p F F 1 s (^) 2 s 2 p
Fluorine molecule
Fluorine atom
Fluorine atom
F à F Fluorine molecule
F F or F F
shared pair of electrons
shared pair of electrons
Covalent Bonding 227
See covalent bonding animated online. ART
KINETIC
A T
K
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 Ŀ H for hydrogen. A structural formula represents the covalent bonds as dashes and shows the arrangement of covalently bonded atoms. In contrast, the molecular formula of hydrogen, H 2 , indicates only the number of hydrogen atoms in each molecule. The halogens also form single covalent bonds in their diatomic mol- ecules. Fluorine is one example. Because a fluorine atom has seven valence electrons, it needs one more to attain the electron configuration of a noble gas. By sharing electrons and forming a single covalent bond, two fluorine atoms each achieve the electron configuration of neon.
In the F 2 molecule, each fluorine atom contributes one electron to com- plete the octet. Notice that the two fluorine atoms share only one pair of valence electrons. A pair of valence electrons that is not shared between atoms is called an unshared pair, also known as a lone pair or a nonbonding pair. In F 2 , each fluorine atom has three unshared pairs of electrons. You can draw electron dot structures for molecules of compounds in much the same way that you draw them for molecules of diatomic elements. Water (H 2 O) is a molecule containing three atoms with two single cova- lent bonds. Two hydrogen atoms share electrons with one oxygen atom. The hydrogen and oxygen atoms attain noble-gas configurations by sharing elec- trons. As you can see in the electron dot structures below, the oxygen atom in water has two unshared pairs of valence electrons.
LESSON 8.
Covalent Bonding 227
Foundations for Reading
BUILD VOCABULARY Explain that the word structure comes from the Latin verb struere , which means “to build.” Convey that a structural formula is one that shows how the atoms of a molecule are joined together by chemical bonds. READING STRATEGY Identify the main idea in the paragraph titled The Octet Rule in Covalent Bonding. Have students create and complete a KWL chart, with corresponding examples, to support how this rule is applied to a single covalent bond, a double covalent bond, and a triple covalent bond.
Explain
The Octet Rule in Covalent Bonding APPLY CONCEPTS Write electron configurations for carbon, nitrogen, oxygen, fluorine, and neon on the chalkboard. Ask How many electrons would carbon, nitrogen, oxygen, and fluorine need to share in order to achieve the same electron configuration as neon? (4, 3, 2, and 1, respectively)
Explore
Class Activity
PURPOSE Students practice different ways to represent molecules. MATERIALS paper and pencil PROCEDURE Divide students into groups of three or four. Have them practice drawing molecular diagrams, structural formulas, electron-dot structures, and orbital diagrams for molecules such as OF 2 , SCl 2 , N 2 H 4 , CCl 4 , CHCl 3 , and C 2 H 6.
Differentiated Instruction
L1 (^) STRUGGLING STUDENTS Pair each student with a study partner. Have them use the periodic table and quiz each other on writing electron dot structures for single atoms and bonded atoms. Make sure they understand that for groups 1, 2, and 13–18, the number in the ones place indicates the number of valence electrons that atom has, and that it is valence electrons that appear in the electron dot structures. L1 (^) SPECIAL NEEDS Recreate the electron dot structures on large posters or overheads. Show students with a colored marker how to draw the dots. Point out where on the periodic table the element is located; circle the element and circle the group number. Then explain the reasoning for the placement and the number of dots. Make sure the noble gas group on their periodic table is highlighted.
H á Hydrogen atom
Cl Chlorine atom
H Cl Hydrogen chloridemolecule
Through electron sharing, the hydrogen and chlorine atoms attain the electron configurations of the noble gases helium and argon, respectively.
Covalent Bonding 229
If you tried to form covalent C ĿH 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 CH 2 (instead of CH 4 ). The formation of four bonds by carbon can be explained by the fact that one of carbon’s 2s electrons is promoted to the vacant 2p orbital to form the fol- lowing electron configuration:
This electron promotion requires only a small amount of energy. The promo- tion provides four electrons of carbon that are capable of forming covalent bonds with four hydrogen atoms. Methane, the carbon compound formed by electron sharing of carbon with four hydrogen atoms, is much more sta- ble than CH 2. The stability of the resulting methane more than compensates for the small energy cost of the electron promotion. Therefore, formation of methane (CH 4 ) is more energetically favored than the formation of CH 2.
1 s^2 2 s and 2 p
T^ Sample^ Problem^ 8. UTOR
CHEM
T
C^ M
Drawing an Electron Dot Structure Hydrochloric acid (HCl (aq)) is prepared by dissolving gaseous hydrogen chloride (HCl (g)) in water. Hydrogen chloride is a diatomic molecule with a single covalent bond. Draw the electron dot structure for HCl.
Analyze^ Identify the relevant concepts.^ In a single covalent bond, a hydrogen and a chlorine atom must share a pair of electrons. Each must contribute one electron to the bond. Then show the electron sharing in the compound they produce.
Solve Apply concepts to the problem.
7. Draw electron dot structures for each molecule. a.chlorine b. bromine c. iodine 8. The following molecules have single covalent bonds. Draw an electron dot structure for each. a. H 2 O 2 b. PCl 3
Draw the electron dot structure for the hydrogen chloride molecule.
Draw the electron dot structures for the hydrogen and chlorine atoms.
F
F
NF
Br SBr
Cl Cl
Br Br
I I H O OH Cl Cl
PCl
LESSON 8.
Covalent Bonding 229
Explore
Class Activity
PURPOSE Students gain understanding of covalent bonding and distinguish between covalent and ionic bonding. PROCEDURE Have students draw electron dot structures for each element in the second row of the periodic table: Li, Be, B, C, N, O, and F. Then have them answer the following:
- Predict how many bonds each atom must form to attain a noble gas configuration. (1, 2, 3, 4, 3, 2, 1)
- Can lithium form a covalent bond and reach stability? (no)
- Which elements can reach stability by forming covalent bonds? (C, N, O, F)
- Can fluorine form an ionic bond? (yes)
- Are the bonds in nitrogen molecules, N 2 , ionic or covalent? (covalent)
Sample Practice Problem The following covalent molecules have only single covalent bonds. Draw an electron dot structure for each. a. NF 3
b. SBr 2
Answers
- a.
b. c.
- a. b.
Check for Understanding
What is the result of electron sharing in covalent bonds? Assess students’ understanding of electron sharing in covalent bonds by asking them to answer the key question in two sentences or less, using their own words. (Sample answer: Atoms share electrons in order to form an octet. The result is a stable molecule.) ADJUST INSTRUCTION If students are having difficulty writing their summaries, review the differences between ionic and covalent compounds. Point out that ions gain or lose electrons to achieve an ionic compound with a noble gas configuration, whereas atoms share electrons to achieve a molecular compound with a noble gas configuration.
à
1 s^2 s^^2 p N N 1 s (^) 2 s 2 p
Nitrogen atom
Nitrogen atom
Nitrogen molecule
or
Nitrogen molecule
N N N N N N
2 s and 2 p
O O
C
1 s (^) 2 s 2 p
1 s
2 p 2 s^1 s Carbon dioxide molecule
O à O Oxygen atom
or Carbon atom
Oxygen atom
Carbon dioxide molecule
C à O CO O C O
230 $IBQUFSt-FTTPO
Figure 8.7 Carbon Dioxide Carbon dioxide gas is soluble in water and is used to carbonate many beverages. A carbon dioxide molecule has two carbon–oxygen double bonds.
Figure 8. Oxygen and Nitrogen Oxygen and nitrogen are the main components of Earth’s atmosphere. The oxygen molecule is an exception to the octet rule. It has two unpaired electrons. Three pairs of electrons are shared in a nitrogen molecule.
Double and Triple Covalent Bonds Sometimes atoms bond by sharing more than one pair of electrons. Atoms form double or triple cova- lent bonds if they can attain a noble gas structure by sharing two pairs or three pairs of electrons. A double covalent bond is a bond that involves two shared pairs of electrons. Similarly, a bond formed by sharing three pairs of electrons is a triple covalent bond. Carbon dioxide (CO 2 ) is used to carbonate many soft drinks like the one shown in Figure 8.7. The carbon dioxide molecule contains two oxygens, each of which shares two electrons with carbon to form a total of two carbon– oxygen double bonds.
The two double bonds in the carbon dioxide molecule are identical to each other. Carbon dioxide is an example of a triatomic molecule, which is a molecule consisting of three atoms. An example of an element whose molecules contain triple bonds is nitro- gen (N 2 ), a major component of Earth’s atmosphere, illustrated in Figure 8.8. A single nitrogen atom has five valence electrons. Each nitrogen atom in the nitrogen molecule must share three electrons to have the electron configura- tion of neon. In the nitrogen molecule, each nitrogen atom has one unshared pair of electrons.
LESSON 8.
230 Chapter 8 • Lesson 2
Explain
START A CONVERSATION To introduce the discussion of multiple covalent bonds, use the electron dot structure for the nitrogen molecule. Ask How does the structure of diatomic nitrogen satisfy the octet rule? (The nitrogen atoms can share six electrons.) Have students compare the bonding in ammonia with the bonding in nitrogen gas. Then introduce the oxygen molecule. Instruct students to draw a structure that obeys the octet rule. Ask Why doesn’t oxygen form a triple bond? (Each oxygen atom needs to share only two electrons to achieve a stable electron configuration.) Explain that although a double bond in the oxygen molecule fulfills the octet rule, it does not fit with experimental evidence that shows that the oxygen molecule contains two unpaired electrons. Thus the structure of O (^2) is an exception to the octet rule. USE MODELS Help students draw the electron dot structure and orbital diagram for carbon dioxide. Ask What type of bonds does carbon form with the two oxygen atoms in CO 2? (double covalent bonds) Note that carbon can form single, double, and triple bonds, but a quadruple bond is impossible because of geometric restrictions. Have students draw diagrams for hydrogen cyanide (HCN) and formaldehyde (H (^2) CO). Ask What kind of bonds does carbon form in each of these molecules? (HCN: one single carbon-to-hydrogen bond and one triple carbon-to-nitrogen bond; H 2 CO: two single carbon-to-hydrogen bonds and one double carbon-to-oxygen bond) As the discussion proceeds, provide models for the molecules discussed, or allow students time to come up with a way to model the examples with materials in the classroom or home.
Inventing Electron Dot Structures
Gilbert Newton Lewis (1875–1946) was an American chemist who invented electron dot structures, which are often called Lewis structures or diagrams in his honor. These structures supported Lewis’s theory of the electron pair in chemical bonding. As a professor of physical chemistry, he expanded the theory of acids and bases by defining an acid as an electron pair acceptor and a base as an electron pair donor. The definitions encompass all Brønsted-Lowry acid-base reactions and include many others not previously categorized as acid-base reactions.
à
Hydrogen ion (proton)
Ammonia molecule (NH 3 )
Ammonium ion (NH 4 à
Hà
H H N H H
H H N+^ H H
or
H N H H æè
à
unshared electron pair coordinate covalent bond
232 $IBQUFSt-FTTPO
Figure 8.9 Ammonia Fertilizers Most plants need nitrogen that is already combined in a compound rather than molecular nitrogen (N 2 ) to grow. The polyatomic ammonium ion (NH 4 à), present in ammonium hydroxide, also called aqua ammonia, is an important component of fertilizer for field crops, home gardens, and potted plants.
Coordinate Covalent Bonds How are coordinate covalent bonds different from other covalent bonds? Carbon monoxide (CO) is an example of a type of covalent bonding dif- ferent from that seen in water, ammonia, methane, and carbon dioxide. A carbon atom needs to gain four electrons to attain the electron con- figuration of neon. An oxygen atom needs two electrons. Yet it is pos- sible for both atoms to achieve noble-gas electron configurations by a type of bonding called coordinate covalent bonding. To see how, begin by looking at the double covalent bond between carbon and oxygen.
1 s^2 s^^2 p C O 1 s (^) 2 s 2 p (^) Carbon monoxide molecule
O Carbon atom
Oxygen atom
Carbon monoxide molecule
C à C O
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 also donates one of its unshared pairs of electrons for bonding.
Carbon monoxide molecule
C O CO
A covalent bond in which one atom contributes both bonding electrons is a coordinate covalent bond. In a structural formula, you can show coordinate covalent bonds as arrows that point from the atom donating the pair of electrons to the atom receiving them. The structural formula of carbon monoxide, with two covalent bonds and one coordinate cova- lent bond, is. In a coordinate covalent bond, the shared electron pair comes from one of the bonding atoms. Once formed, a coordinate covalent bond is like any other covalent bond. The ammonium ion (NH 4 à), which is often found in fertilizers like the one in Figure 8.9, consists of atoms joined by covalent bonds, including a coordinate covalent bond. A polyatomic ion, such as NH 4 à, is a tightly bound group of atoms that has a positive or negative charge and behaves as a unit. The ammonium ion forms when a positively charged hydrogen ion (Hà) attaches to the unshared electron pair of an ammonia molecule (NH 3 ).
C O
LESSON 8.
232 Chapter 8 • Lesson 2
Explain
Coordinate Covalent Bonds APPLY CONCEPTS Explain that a coordinate covalent bond is an exception to the rule that covalent bonding occurs between two half-empty orbitals of two atoms. Show students that in a coordinate covalent bond, one atom has an empty orbital and the other, an orbital filled with an electron pair that is not yet involved in a chemical bond. Note that the bonding still involves only one pair of electrons and one pair of orbitals, but one atom provides both of the shared electrons. CRITICAL THINKING Ask students to compare the electron dot structure for ammonia, NH 3 , and for a hydrogen ion, H +^. After forming the three nitrogen- to-hydrogen bonds, nitrogen has an unshared pair of electrons. The hydrogen ion has no electrons available for bonding. Ask How do you explain the existence of the ammonium ion, NH 4 +^? (The empty orbital of the H +^ must overlap the filled orbital in ammonia. The electrons from nitrogen are attracted to both the nitrogen nucleus and the hydrogen nucleus. A bond forms when electrons are simultaneously attracted to two nuclei. This type of bond is called a coordinate covalent bond.)
Check for Understanding
How are coordinate covalent bonds different from other covalent bonds? Assess students’ understanding of coordinate covalent bonds by having them write a one-minute response that explains the bonding in CO 2 and then explains the bonding in CO using coordinate covalent bonding. ADJUST INSTRUCTION If students are having difficulty understanding the difference between normal covalent bonding and coordinate covalent bonding, construct a model of each element. Then construct a model of each compound to give students a visual image of the difference in how the electrons are shared.
T Sample^ Problem^ 8. UTOR
CHEM
T
C^ M
Remember to always include the charge when drawing electron dot structures of polyatomic ions.
á Hydrogenion (proton)
moleculeWater (H 2 O)
H á
H O H Hydroniumion (H 3 O à )
or
H æè H O H
á H æè H O H
á
Remember, the charge of a negative polyatomic ion is equal to the number of electrons that are in addition to the valence electrons of the atoms present. Since a negatively charged polyatomic ion is part of an ionic compound, the positive charge of the cation of the compound balances the additional electrons.
Covalent Bonding 233
Most polyatomic cations and anions contain covalent and coordinate covalent bonds. Therefore, compounds containing polyatomic ions include both ionic and covalent bonding. As another example, draw the electron dot structure of the polyatomic ion SO 32 Ź. First, draw the electron dot structures for the oxygen and sulfur atoms, and the two extra electrons indicated by the charge. Then, join two of the oxygens to sulfur by single covalent bonds.
Next, join the remaining oxygen by a coordinate covalent bond, with sul- fur donating one of its unshared pairs to oxygen, and add the two extra elec- trons. Put brackets around the structure and indicate the 2Ź charge.
Each of the atoms now has eight valence electrons, satisfying the octet rule. Without the extra electrons, two of the oxygens would be electron-deficient. Table 8.2 lists electron dot structures of some common compounds with covalent bonds.
O S O^ à^ S O
O O
O
à (^) æè
2 Ź O S O
O OS O
O
10. Draw the electron dot structures for sulfate (SO 42 Ź) and carbonate (CO 32 Ź). Sulfur and carbon are the central atoms, respectively.
Drawing the Electron Dot Structure of a Polyatomic Ion The H 3 Oà^ ion forms when a hydrogen ion is attracted to an unshared electron pair in a water molecule. Draw the electron dot structure of the hydronium ion.
Analyze^ Identify the relevant concepts.^ Each atom must share electrons to satisfy the octet rule.
Solve^ Apply the concepts to the problem.
Draw the electron dot structure of the water molecule and the hydrogen ion. Then, draw the electron dot structure of the hydronium ion. The oxygen must share a pair of electrons with the added hydrogen ion to form a coordinate covalent bond.
Check that all the atoms have the electrons they need and that the charge is correct.
9. Draw the electron dot structure of the hydroxide ion (OHŹ).
The oxygen in the hydronium ion has eight valence electrons, and each hydrogen shares two valence electrons, satisfying the octet rule. The water molecule is neutral, and the hydrogen ion has a positive charge, giving the hydronium ion a charge of 1à.
_
O O
O
Cl
H O
_
O
O
O SO
2 _
O
O
O C
2 _
LESSON 8.
Covalent Bonding 233
Answers
a.
b.
APPLY CONCEPTS Have students write the electron dot structure for SO 2. Emphasize that the structure should satisfy the bonding requirements of all three atoms. Students should find that, to satisfy the octet rule for all the atoms, they must write a structure in which one oxygen atom is double bonded to sulfur. The other oxygen is single bonded by a coordinate covalent bond in which the electrons are donated by sulfur. Point out that experimental evidence indicates that both sulfur-oxygen bonds are identical. Explain that this evidence indicates that the bonding in SO 2 must be some intermediate between a single and double bond. Ask How does the formation of a coordinate covalent bond differ from that of a covalent bond? (In a covalent bond, each atom provides one electon. In a coordinate covalent bond, both electrons are provided by the same atom.)
Sample Practice Problem Draw the electron dot structure of the polyatomic chlorate anion, ClO 3 −^.
Foundations For Math
PATTERNS Students may start to realize that an element’s electron configuration and its location in the periodic table has a useful relationship. Guide students to realize that the number of bonds typically formed by atoms in a covalent compound is equal to the atom’s number of valence shell electrons that the atom must gain or lose in order to achieve an octet. In Sample Problem 8.2, as well as in other examples in this lesson, the number of valence electrons equals the number in the ones column of the Group number. Have students note this pattern on their periodic table and encourage students to continue to look for these types of patterns as they work through this chapter.
ONO
ONO
B
H à H H
B
H H H
F F F
N
F F F
N
Nitrogen dioxide molecule
Covalent Bonding 235
Figure 8. Nitrogen Dioxide Lightning is one means by which nitrogen and oxygen in the atmosphere produce nitrogen dioxide.
Exceptions to the Octet Rule
What are some exceptions to the octet rule? The octet rule provides guidance for drawing electron dot structures. For some molecules or ions, however, it is impossible to draw structures that sat- isfy the octet rule. The octet rule cannot be satisfied in molecules whose total number of valence electrons is an odd number. There are also molecules in which an atom has less, or more, than a complete octet of valence electrons. The nitrogen dioxide (NO 2 ) molecule, for example, con- tains a total of seventeen, an odd number, of valence electrons. Each oxygen contributes six electrons and the nitrogen contributes five. Two plausible elec- tron dot structures can be drawn for the NO 2 molecule.
An unpaired electron is present in each of these structures, both of which fail to follow the octet rule. It is impossible to draw an electron dot structure for NO 2 that satisfies the octet rule for all atoms. Yet, NO 2 does exist as a sta- ble molecule. In fact, it is produced naturally by lightning strikes of the sort shown in Figure 8.10. A number of other molecules also have an odd number of electrons. In these molecules, as in NO 2 , complete pairing of electrons is not possible. It is not possible to draw an electron dot structure that satisfies the octet rule. Examples of such molecules include chlorine dioxide (ClO 2 ) and nitric oxide (NO). Several molecules with an even number of valence electrons, such as some compounds of boron, also fail to follow the octet rule. This outcome may occur because an atom acquires less than an octet of eight electrons. The boron atom in boron trifluoride (BF 3 ), for example, is deficient by two elec- trons and, therefore, is an exception to the octet rule. Boron trifluoride read- ily reacts with ammonia to make the compound BF 3 ·NH 3. In doing so, the boron atom accepts the unshared electron pair from ammonia and completes the octet.
LESSON 8.
Covalent Bonding 235
Teacher Demo
PURPOSE Students observe two reactions in which bonds are formed and compare ionic and covalent bond energies. MATERIALS 10- to 15-cm piece of clean magnesium ribbon, a small piece of charcoal, tongs, Bunsen burner, filters for viewing SAFETY Wear safety goggles and lab apron. Tie back long hair and loose clothing. PROCEDURE Using tongs, hold the piece of magnesium in a Bunsen burner flame. CAUTION! Tie back long hair and loose clothing and wear safety goggles. Do not look directly at the flame. 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 board. 2Mg + O 2 → 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 + O 2 → CO 2 + energy EXPECTED OUTCOMES Students note that much less energy is given off in forming CO 2 than in forming MgO. Ask What kind of bonds are in MgO? (ionic) What kind of bonds are in CO 2? (covalent) Ionic bond energies are, in general, greater than covalent bond energies.
Paramagnetism
Substances containing unpaired electrons can be identified through a phenomenon called paramagnetism. When molecules with unpaired electrons are placed in a magnetic field, they tend to be drawn into the field. These substances are paramagnetic. In contrast, molecules in which all electrons are paired tend to be pushed from a magnetic field. These substances are diamagnetic. Paramagnetism differs from 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 electrons and thus cannot be described exactly by application of the octet rule. However, the oxygen bond does have a double bond character; its bond length and bond energy are similar to those of double bonds in other molecules that conform to the octet rule.
S
F F
F
F
F F
Sulfur hexafluoride
P
Cl
Cl
Cl
Cl
Cl
Phosphorus pentachloride
236 $IBQUFSt-FTTPO
A few atoms, especially phosphorus and sulfur, sometimes expand the octet to ten or twelve electrons. Consider phosphorus trichloride (PCl 3 ) and phosphorus pentachloride (PCl 5 ). Both are stable compounds in which all of the chlorine atoms are bonded to a single phosphorus atom. Covalent bonding in PCl 3 follows the octet rule because all the atoms have eight valence electrons. However, as shown in Figure 8.11, the electron dot structure for PCl 5 can be written so that phosphorus has ten valence electrons. The octet is also expanded in sulfur hexafluoride (SF 6 ). The elec- tron dot structure for SF 6 can be written so that sulfur has twelve valence electrons.
Bond Dissociation Energies How is the strength of a covalent bond related to its bond dissociation energy? A large quantity of heat is released when hydrogen atoms combine to form hydrogen molecules. This release of heat suggests that the product is more stable than the reactants. The covalent bond in the hydrogen molecule (H 2 ) is so strong that it would take 435 kJ of energy to break apart all of the bonds in 1 mole (6.02ñ 1023 bonds or about 2 grams) of H 2. (You will study the mole, abbreviated mol, in Chapter 12.) The energy required to break the bond between two covalently bonded atoms is the bond dissociation energy. The units for this energy are often given in kJ/mol, which is the energy needed to break one mole of bonds. For example, the bond dissoci- ation energy for the H 2 molecule is 435 kJ/mol.
Table 8. Bond Dissociation Energies and Bond Lengths for Covalent Bonds Bond Bond dissociation energy (kJ/mol) Bond length (pm) HĿH 435 74 CĿH 393 109 CĿO 356 143
CãO 736
CǎO 1074 113
CĿC 347 154
CãC 657 133
CǎC 908
C (^) ĿN 305 147 Cl (^) ĿCl ^199 NĿN 140 O ĿH 464 96 O (^) ĿO
Figure 8. Exceptions to the Octet Rule Phosphorus pentachloride and sulfur hexafluoride, are exceptions to the octet rule. Interpret Diagrams How many valence electrons does the sulfur in sulfur hexafluoride (SF 6 ) have for the structure shown in the figure?
LESSON 8.
236 Chapter 8 • Lesson 2
Explain
Bond Dissociation Energies SUMMARIZE Direct students’ attention to Table 8.3. Explain the units used to measure bond energy and bond length. Have students write a summary statement using any patterns they notice between bonds, bond energy, and bond length.
Extend
Have students draw a graph of actual bond dissociation energies versus number of bonds for carbon-carbon single, double, and triple bonds. Have students write a statement that relates this graph to their conclusions in the lab activity.
Connect to PHYSICS
Have students investigate the relationship between potential energy and bond length. Students should be prepared to state the relationship orally in class as well as demonstrate their findings using two magnets. (Bond length is the measure at which the potential energy of the combined atoms is at its lowest.)
Check for Understanding
How is the strength of a covalent bond related to its bond dissociation energy? Assess students’ understanding of covalent bond strength by providing students with a copy of Table 8.3. Have students write down the three strongest covalent bonds in the table and the three weakest. (Strongest: C triple bond to O; C double bond to C; C double bond to O; Weakest: O single bond to O; N single bond to N; Cl single bond to Cl) ADJUST INSTRUCTION If students are struggling to understand this concept, have students create a new the table sorted in order of greatest to least bond energy. As a class, discuss any noticeable patterns.
Quick Lab
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PROBLEMS LessonCheck
ONLINE R O B L E M M^ S
O^ E 8.2 Lesso
11. Identify What electron configurations do atoms usually achieve by sharing electrons to form covalent bonds? 12. Compare How is a coordinate covalent bond different from other covalent bonds? 13. Explain How is the strength of a covalent bond related to its bond dissociation energy? 14. List List three ways in which the octet rule can sometimes fail to be obeyed. 15. Identify Draw the electron dot resonance structures for ozone and explain how they describe its bonding. 16. ExplainHow is an electron dot structure used to represent a covalent bond? 17. InferWhen are two atoms likely to form a double bond between them? A triple bond? 18. IdentifyWhat kinds of information does a structural formula reveal about the compound it represents? 19. CompareUse the bond dissociation energies of H 2 and of a typical carbon–carbon bond to decide which bond is stronger. Explain your reasoning. 20. Use ModelsDraw electron dot structures for the following molecules, which have only single covalent bonds: a. H 2 S b. PH 3 c. ClF
Analyze and Conclude
1. Analyze Experimental ResultsAssuming the rubber bands are models for covalent bonds, what can you conclude about the relative strengths of single, double, and triple bonds? 2. EvaluateHow does the behavior of the rubber bands differ from that of covalent bonds?
Purpose To compare and Strengths of Covalent Bonds
contrast the stretching of rub- ber bands and the dissociation energy of covalent bonds
Materials r 1 170-g (6-oz) can of food r 2 454-g (16-oz) cans of food r 3 No. 25 rubber bands r metric ruler r coat hanger r plastic grocery bag r paper clip r graph paper r motion detector (optional)
3. Repeat Step 2, first using two rub- ber bands to connect the hanger and the paper clip, and then using three. 4. Graph the length differ- ence: (stretched rubber band) – (unstretched rubber band) on the y-axis versus mass (kg) on the x-axis for one, two, and three rubber bands. Draw the straight line that you esti- mate best fits the points for each set of data. (Your graph should have three separate lines.) The x-axis and y-axis intercepts of the lines should pass through zero, and the lines should extend past 1 kg on the x-axis. Determine the slope of each line in cm/kg.
Procedure
1. Bend the coat hanger to fit over the top of a door. The hook should hang down on one side of the door. Measure the length of the rubber bands (in cm). Hang a rubber band on the hook cre- ated by the coat hanger. 2. Place the 170-g can in the plastic bag. Use the paper clip to fasten the bag to the end of the rubber band. Lower the bag gently until it is suspended from the end of the rubber band. Measure and record the length of the stretched rubber band. Using different combina- tions of food cans, repeat this process three times with the following masses: 454 g, 624 g, and 908 g.
O O O OO O
H S H
H
H PH
Cl F
LESSON 8.
238 Chapter 8 • Lesson 2
Explore
OBJECTIVE After completing this activity, students will understand that the dissociation energy of a covalent bond increases in order from single bond to double bond to triple bond. SKILLS FOCUS Solving, interpreting PREP TIME 10 minutes CLASS TIME 30 minutes MATERIALS Obtain the necessary materials in advance. You may wish to ask students to help with this task by bringing cans of food, coat hangers, and plastic grocery bags to class. TEACHING TIP Use tape to secure the hanger in place if necessary. Have groups of students record the data and then create a graph as a class, using an overhead projector. EXPECTED OUTCOMES As the mass of the load increases, the stretch of the rubber band or bands increases. For a given mass, a single rubber band stretches farther than a double, and a double rubber band stretches farther than a triple. ANALYZE AND CONCLUDE
1. Triple covalent bonds are stronger than double covalent bonds, which are stronger than single covalent bonds. 2. The change in bond dissociation energies in going from a carbon-carbon 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 three rubber bands, as given by the slopes of the lines, is not constant. It is large going from one to two rubber bands and small going from two to three rubber bands.
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Lesson Check Answers
11. the configurations of noble gases 12. The shared electron pair comes from one of the bonding atoms. In other covalent bonds, each bonding atom provides an electron. 13. A large bond dissociation energy corresponds to a strong covalent bond. 14. The octet rule cannot be satisfied in molecules 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. 15. The actual bonding of oxygen atoms in ozone is a hybrid, or mixture, of the extremes represented by the resonance forms. 16. Two dots represent each covalent bond. 17. when they can attain a noble gas structure by sharing two pairs or three pairs of electrons 18. the arrangement of atoms in a molecule 19. The H-H bond is stronger because it has a greater dissociation energy. 20. a.
b.
c.
21st Century Learning (^) To be successful in the 21st century, students need skills and learning experiences that extend beyond subject matter mastery. The following project helps students build the following 21st Century Skills: Financial , Economic , Business , and Entrepreneurial Literacy ; Creativity and Innovation ; Communication and Collaboration ; Information , Media , and Technology Literacy ; Initiative and Self - Direction ; and Productivity and Accountability. CLEANER COATINGS Tell students that due to the social and political viewpoint towards ecological awareness, controlling emissions and waste is an ever increasing concern in all areas of manufacturing. Pose the following challenge to students: As members of a powder coating company’s sales department, it is your job to convince a major plastics manufacturer to replace their liquid coating process with powder coating. Create “marketing teams” of 4–6 people to develop a multimedia marketing campaign designed to convince the plastics manufacturer of the benefits of switching to powder coating. It should include a comparison of current powder coating and liquid coating processes from the standpoint of environmental impact and worker safety. Record the campaign on DVD for submission.
Negatively charged paint adheres to the positively charged metal surface.
Electrostatic spray gun nozzle
Chemistry & You 239
GLOSSY FINISH Powder coating can produce a smooth, glossy paint finish.
Powder Coating
Have you ever admired a new car with its glossy, smooth paint? Car manufacturers use a special process to apply paint to a car. This process is called powder coating or electrostatic spray painting. In powder coating, a custom-designed spray nozzle wired up to an electric power supply imparts a negative charge to the paint droplets as they exit the spray gun. The negatively charged droplets are attracted to the grounded, positively-charged, metal surface. Painting with attractive forces is very efficient, because almost all the paint is applied to the car body and very little is wasted. Powder coating isn’t just for cars. The process has many different applications, including the painting of motorcycles, outdoor furniture, exercise equipment, office furniture, and metal fencing. An eye-catching paint finish isn’t the only benefit of powder coating, however. This process is also environmentally friendly. Since the paint is actually attracted to its intended surface, the amount of wasted paint is much lower compared to traditional spray painting. Also, the amount of toxic volatile organic compounds (VOCs) released is minimal, if there are any at all.
CHEMISTRYY (^) & YOU:YYY TECHNOLOGY
Take It Further
1. Analyze BenefitsPowder coating is being used for more and more applications, partly because of its many benefits. Research other advantages of powder coating that are not mentioned here. 2. Infer Powder coating results in a smooth surface, usually without drips and runs. Given what you have learned about attractive forces, why do you think drips and runs are avoided during powder coating?
APPLYING THE POWDER This worker is using an electrostatic spray gun to apply powder to the metal. Any powder that does not stick to the part can be collected and reused. Once the powder is applied, the part is baked in an oven to cure the paint.
ATTRACTIVE PAINT The paint almost wraps around the metal, sticking to any available charged surface.
Chemistry & You 239
CHEMISTRY & YOU
Answers
- Answers may vary, but possible benefits include the ability to recover unused or over-sprayed powder, and the production of a strong, scratch-resistant paint finish.
- Since the paint is attracted to the metal, excess paint that might normally form drips is instead attracted to metal that is not yet covered in paint.
CHEMISTRY (^) && YOUYOY U The automotive industry is an example of an industry that is strongly swayed by consumer concerns. Pose the following question to students: What are some likely consumer concerns that have had or could potentially have an effect on the types of technological advancements in the automotive industry, and how did the automotive industry respond to those concerns? You may need to guide students in the following ways:
- Safety developments include shoulder restraints, front and side curtain airbags, reinforced frames, anti-lock brakes, enhanced traction systems, in-trunk release latches, xenon headlamps, and LED tail lamps.
- Fuel efficiency developments include computer controlled engines, fuel injection, automated manual and continually variable transmissions, reductions in gross vehicular weight
- Emissions regulation developments include catalytic converters, low carbon fuels (ethanol and CNG), hybrid engines, exhaust gas recirculation, and evaporative emission systems.
Explain
START A CONVERSATION Direct students to read about the powder coating process highlighted in this feature and to examine the photographs. Ask What is the basic underlying chemistry-related principle applied in the powder coating process? (Opposite charges are inherently attracted to each other.) Ask Do you know of other types of coating processes used in other industries? (Sample answers: electroplating, galvanizing, Parkerizing, bluing, painting, etc.) Ask What do you think the benefits are for using powder coating over one of these other processes? (Sample answers : durability of the surface, even coating, longevity, avoiding corrosion issues due to weather, etc.)
