Chapter Nine Chemical Bonds: A Preview Chemical Bonds
1Chapter Nine2Chemical Bonds: A PreviewChemical Bonds Forces called chemical bonds hold atoms togetherin molecules and keep ions in place in solid ioniccompounds. Chemical bonds are electrostatic forces; theyreflect a balance in the forces of attraction andrepulsion between electrically charged particles.Prentice Hall 2005General Chemistry 4th edition, Hill, Petrucci, McCreary, PerryChapter NinePrentice Hall 2005Chapter NineGeneral Chemistry 4th edition, Hill, Petrucci, McCreary, Perry3Electrostatic Attractions andRepulsions4Energy of InteractionNuclei attractelectron(s)Nuclei repelother nucleiPrentice Hall 2005At 74 pm, attractiveforces are at amaximum, energy isat a minimum.Electronsrepel otherelectronsBoth attractions andrepulsions occur;what’s the neteffect? Answer: itdepends on thedistance betweennuclei General Chemistry 4th edition, Hill, Petrucci, McCreary, PerryCloser together;attraction increases.Closer than 74pm, repulsionincreases.Chapter NinePrentice Hall 2005Hydrogen nuclei farapart; just a littleattraction.General Chemistry 4th edition, Hill, Petrucci, McCreary, PerryChapter Nine56The Lewis Theory ofChemical Bonding: An Overview Valence electrons play a fundamental role in chemicalbonding. When metals and nonmetals combine, valence electronsusually are transferred from the metal to the nonmetalatoms, giving rise to ionic bonds. In combinations involving only nonmetals, one or morepairs of valence electrons are shared between the bondedatoms, producing covalent bonds. In losing, gaining, or sharing electrons to form chemicalbonds, atoms tend to acquire the electron configurations ofnoble gases.Prentice Hall 2005General Chemistry 4th edition, Hill, Petrucci, McCreary, PerryChapter NineLewis Symbols In a Lewis symbol, the chemical symbol for the elementrepresents the nucleus and core electrons of the atom. Dots around the symbol represent the valence electrons. In writing Lewis symbols, the first four dots are placedsingly on each of the four sides of the chemical symbol. Dots are paired as the next four are added. Lewis symbols are used primarily for those elements thatacquire noble-gas configurations when they form bonds.Prentice Hall 2005General Chemistry 4th edition, Hill, Petrucci, McCreary, PerryChapter Nine1
7Ionic Bonds and Ionic CrystalsExample 9.1Give Lewis symbols for magnesium, silicon, andphosphorus.Prentice Hall 20058General Chemistry 4th edition, Hill, Petrucci, McCreary, Perry When atoms lose or gain electrons, they mayacquire a noble gas configuration, but do notbecome noble gases. Because the two ions formed in a reaction betweena metal and a nonmetal have opposite charges,they are strongly attracted to one another and forman ion pair. The net attractive electrostatic forces that hold thecations and anions together are ionic bonds. The highly ordered solid collection of ions iscalled an ionic crystal.Chapter NinePrentice Hall 2005General Chemistry 4th edition, Hill, Petrucci, McCreary, PerryChapter Nine910Using Lewis Symbolsto Represent Ionic BondingFormation of a Crystal of Sodium ChlorideNa donates anelectron to Cl Lewis symbols can be used to represent ionicbonding between nonmetals and: the s-blockmetals, some p-block metals, and a few d-blockmetals. Instead of using complete electron configurationsto represent the loss and gain of electrons, Lewissymbols can be used. andoppositesattract.Sodiumreactsviolently inchlorine gas.Prentice Hall 2005General Chemistry 4th edition, Hill, Petrucci, McCreary, PerryChapter NinePrentice Hall 2005General Chemistry 4th edition, Hill, Petrucci, McCreary, PerryChapter Nine1112Energy Changes in IonicCompound FormationExample 9.2Use Lewis symbols to show the formation of ionicbonds between magnesium and nitrogen. What are thename and formula of the compound that results?Na(g) Æ Na (g) e–Cl(g) e–ÆCl–(g)I1 496 kJ/molEA1 –349 kJ/mol From the data above, it doesn’t appear that the formation ofNaCl from its elements is energetically favored. However the enthalpy of formation of the ionic compound is moreimportant than either the first ionization energy or electronaffinity. The overall enthalpy change can be calculated using a stepwise procedure called the Born–Haber cycle.Prentice Hall 2005General Chemistry 4th edition, Hill, Petrucci, McCreary, PerryChapter NinePrentice Hall 2005General Chemistry 4th edition, Hill, Petrucci, McCreary, PerryChapter Nine2
13Energy Changes in Ionic Compound Formation (cont’d)14Example 9.3Use the following data to determine the lattice energy ofMgF2(s): enthalpy of sublimation of magnesium, 146kJ/mol; I1 for Mg, 738 kJ/mol; I2 for Mg, 1451 kJ/mol;bond-dissociation energy of F2(g), 159 kJ/mol F2;electron affinity of F, –328 kJ/mol F; enthalpy offormation of MgF2(s), –1124 kJ/mol. The Born–Haber cycleis a hypotheticalprocess, in which ΔHfis represented byseveral steps. What law can be usedto find an enthalpychange that occurs insteps?ΔHf for NaCl is verynegative because ΔH5—the latticeenergy—is verynegative.Prentice Hall 2005General Chemistry 4th edition, Hill, Petrucci, McCreary, PerryChapter NinePrentice Hall 2005Chapter NineGeneral Chemistry 4th edition, Hill, Petrucci, McCreary, Perry1516Lewis Structures ofSimple MoleculesLewis Structures (cont’d) The shared pairs of electrons in a molecule are calledbonding pairs. In common practice, the bonding pair is represented by adash (—). The other electron pairs, which are not shared, are callednonbonding pairs, or lone pairs. A Lewis structure is a combination of Lewissymbols that represents the formation of covalentbonds between atoms. In most cases, a Lewis structure shows the bondedatoms with the electron configuration of a noblegas; that is, the atoms obey the octet rule. (Hobeys the duet rule.) The shared electrons can be counted for each atomthat shares them, so each atom may have a noblegas configuration.Prentice Hall 2005General Chemistry 4th edition, Hill, Petrucci, McCreary, PerryEach chlorine atom seesan octet of electrons.Chapter NinePrentice Hall 2005Chapter NineGeneral Chemistry 4th edition, Hill, Petrucci, McCreary, Perry1718Some Illustrative CompoundsMultiple Covalent Bonds The covalent bond in which one pair of electrons is sharedis called a single bond.In a double bond two pairs Multiple bonds can also form:of electrons are shared. Note that the two-dimensional Lewis structures do not necessarily show the correctshapes of the three-dimensional molecules. Nor are they intended to do so.In a triple bond three pairsof electrons are shared.Note that each atom obeys the octetrule, even with multiple bonds. The Lewis structure for water may be drawn with all three atoms in a line: H–O–H. We will learn how to predict shapes of molecules in Chapter 10.Prentice Hall 2005General Chemistry 4th edition, Hill, Petrucci, McCreary, PerryChapter NinePrentice Hall 2005General Chemistry 4th edition, Hill, Petrucci, McCreary, PerryChapter Nine3
1920Polar Covalent Bonds andElectronegativityThe Importance of Experimental Evidence The Lewis structurecommonly drawn for oxygenis Electronegativity (EN) is a measure of the ability of anatom to attract its bonding electrons to itself. EN is related to ionization energy and electron affinity. The greater the EN of an atom in a molecule, the morestrongly the atom attracts the electrons in a covalent bond. But oxygen is paramagnetic,and therefore must haveunpaired electrons.Electronegativity generallyincreases from left to rightwithin a period, and it generallyincreases from the bottom to thetop within a group. Lewis structures are a usefultool, but they do not alwaysrepresent molecules correctly,even when the Lewisstructure is plausible.Prentice Hall 2005General Chemistry 4th edition, Hill, Petrucci, McCreary, PerryChapter NinePrentice Hall 2005Chapter NineGeneral Chemistry 4th edition, Hill, Petrucci, McCreary, Perry21Pauling’s ElectronegativitiesExample 9.4Referring only to the periodic table inside the frontcover, arrange the following sets of atoms in theexpected order of increasing electronegativity.(a) Cl, Mg, Si(b) As, N, Sb(c) As, Se, SbIt would be a good idea to rememberthe four elements of highestelectronegativity: N, O, F, Cl.Prentice Hall 2005General Chemistry 4th edition, Hill, Petrucci, McCreary, Perry22Chapter NinePrentice Hall 2005Chapter NineGeneral Chemistry 4th edition, Hill, Petrucci, McCreary, Perry2324Electronegativity Difference andBond TypeElectronegativity Differenceand Bond TypeNo sharp cutoff betweenionic and covalent bonds. Identical atoms have the same electronegativity and share abonding electron pair equally. The bond is a nonpolarcovalent bond. When electronegativities differ significantly, electron pairsare shared unequally. The electrons are drawn closer to the atom of higherelectronegativity; the bond is a polar covalent bond. With still larger differences in electronegativity, electronsmay be completely transferred from metal to nonmetalatoms to form ionic bonds.Prentice Hall 2005General Chemistry 4th edition, Hill, Petrucci, McCreary, PerryChapter NineCs—F bonds are “so polar”that we call the bonds .C—H bonds arevirtually nonpolar.Prentice Hall 2005General Chemistry 4th edition, Hill, Petrucci, McCreary, PerryChapter Nine4
25Depicting Polar Covalent BondsIn nonpolar bonds,electrons are sharedequally.26Example 9.5Use electronegativity values to arrange the followingbonds in order of increasing polarity:Br—Cl, Cl—Cl, Cl—F, H—Cl, I—ClPolar bonds are also depictedby partial positive and partialnegative symbols Unequal sharingin polar covalentbonds.Polar bonds are often depictedusing colors to showelectrostatic potential (blue positive, red negative).Prentice Hall 2005 or with a cross-basedarrow pointing to the moreelectronegative element.General Chemistry 4th edition, Hill, Petrucci, McCreary, PerryChapter NinePrentice Hall 2005General Chemistry 4th edition, Hill, Petrucci, McCreary, PerryChapter Nine2728Writing Lewis Structures:Skeletal StructuresWriting Lewis Structures: A Method1.2. The skeletal structure shows the arrangement ofatoms. Hydrogen atoms are terminal atoms (bonded toonly one other atom). The central atom of a structure usually has thelowest electronegativity. In oxoacids (HClO4, HNO3, etc.) hydrogen atomsare usually bonded to oxygen atoms. Molecules and polyatomic ions usually havecompact, symmetrical structures.Prentice Hall 2005General Chemistry 4th edition, Hill, Petrucci, McCreary, Perry3.4.5.Chapter NineDetermine the total number of valence electrons.Write a plausible skeletal structure and connect the atomsby single dashes (covalent bonds).Place pairs of electrons as lone pairs around the terminalatoms to give each terminal atom (except H) an octet.Assign any remaining electrons as lone pairs around thecentral atom.If necessary (if there are not enough electrons), move oneor more lone pairs of electrons from a terminal atom toform a multiple bond to the central atom.Prentice Hall 2005General Chemistry 4th edition, Hill, Petrucci, McCreary, PerryChapter Nine2930Formal ChargeExample 9.6Write the Lewis structure of nitrogen trifluoride, NF3.Example 9.7Write a plausible Lewis structure for phosgene, COCl2.Example 9.8Write a plausible Lewis structure for the chlorate ion,ClO3–.Prentice Hall 2005General Chemistry 4th edition, Hill, Petrucci, McCreary, PerryChapter Nine Formal charge is the difference between the number ofvalence electrons in a free (uncombined) atom and thenumber of electrons assigned to that atom when bonded toother atoms in a Lewis structure. Formal charge is a hypothetical quantity; a useful tool. Usually, the most plausible Lewis structure is one with noformal charges. When formal charges are required, they should be as smallas possible. Negative formal charges should appear on the mostelectronegative atoms. Adjacent atoms in a structure should not carry formalcharges of the same sign.Prentice Hall 2005General Chemistry 4th edition, Hill, Petrucci, McCreary, PerryChapter Nine5
31Formal Charge Illustrated32Example 9.9In Example 9.7, we wrote a Lewis structure for themolecule COCl2, shown here as structure (a). Showthat structure (a) is more plausible than (b) or (c).Prentice Hall 2005General Chemistry 4th edition, Hill, Petrucci, McCreary, PerryChapter NinePrentice Hall 2005General Chemistry 4th edition, Hill, Petrucci, McCreary, PerryChapter Nine33Resonance: Delocalized Bonding34Example 9.10Write three equivalent Lewis structures for the SO3molecule that conform to the octet rule, and describehow the resonance hybrid is related to the threestructures. When a molecule or ion can be represented by two or moreplausible Lewis structures that differ only in the distribution ofelectrons, the true structure is a composite, or hybrid, of them. The different plausible structures are called resonancestructures. The actual molecule or ion that is a hybrid of the resonancestructures is called a resonance hybrid. Electrons that are part of the resonance hybrid are spread outover several atoms and are referred to as being delocalized.Three pairs ofelectrons aredistributed amongtwo bonds.Prentice Hall 2005General Chemistry 4th edition, Hill, Petrucci, McCreary, PerryChapter NinePrentice Hall 2005General Chemistry 4th edition, Hill, Petrucci, McCreary, PerryChapter Nine35Molecules that Don’t Followthe Octet RuleExample 9.11 Molecules with an odd number of valence electrons haveat least one of them unpaired and are called free radicals. Some molecules have incomplete octets. These are usuallycompounds of Be, B, or Al; they generally have someunusual bonding characteristics, and are often quitereactive. Some compounds have expanded valence shells, whichmeans that the central atom has more than eight electronsaround it. A central atom can have expanded valence if it is in thethird period or lower (i.e., S, Cl, P).Prentice Hall 2005General Chemistry 4th edition, Hill, Petrucci, McCreary, Perry36Chapter NineWrite the Lewis structure for bromine pentafluoride,BrF5.Example 9.12A Conceptual ExampleIndicate the error in each of the following Lewisstructures. Replace each by a more acceptablestructure(s).Prentice Hall 2005General Chemistry 4th edition, Hill, Petrucci, McCreary, PerryChapter Nine6
37Bond Order and Bond LengthExample 9.13Estimate the length of (a) the nitrogen-to-nitrogen bondin N2H4 and (b) the bond in BrCl. Bond order is the number ofshared electron pairs in a bond. A single bond has BO 1, adouble bond has BO 2, etc. Bond length is the distancebetween the nuclei of two atomsjoined by a covalent bond. Bond length depends on theparticular atoms in the bond andon the bond order.Prentice Hall 2005General Chemistry 4th edition, Hill, Petrucci, McCreary, Perry38Chapter NinePrentice Hall 2005Chapter NineGeneral Chemistry 4th edition, Hill, Petrucci, McCreary, Perry3940Bond Energy Bond-dissociation energy (D) is the energy required tobreak one mole of a particular type of covalent bond in agas-phase compound. Energies of some bonds can differ from compound tocompound, so we use an average bond energy.The H—H bondenergy is preciselyknown while the O—H bondenergies for the two bondsin H2O are different.Prentice Hall 2005General Chemistry 4th edition, Hill, Petrucci, McCreary, PerryChapter NinePrentice Hall 2005Chapter NineGeneral Chemistry 4th edition, Hill, Petrucci, McCreary, Perry41Trends in Bond Lengths and Energies The higher the order (for a particular type ofbond), the shorter and the stronger (higherenergy) the bond. A N N double bond is shorter and stronger than aN–N single bond. There are four electrons between the two positivenuclei in N N. This produces more electrostaticattraction than the two electrons between thenuclei in N–N.Prentice Hall 2005General Chemistry4thedition, Hill, Petrucci, McCreary, PerryChapter Nine42Calculations Involving Bond EnergiesFor the reactionN2(g) 2 H2(g) Æ N2H4(g) plus 2(436 kJ),to break bonds ofthe reactants.Prentice Hall 2005to occur When the bonds of theproduct form, 163 kJ plus4(389 kJ) of energy isliberated. we mustsupply 946 kJ ΔH ( 946 kJ) 2( 436 kJ) (–163 kJ) 4(–389 kJ)General Chemistry 4th edition, Hill, Petrucci, McCreary, PerryChapter Nine7
4344Alkenes and AlkynesExample 9.14Use bond energies from Table 9.1 to estimate theenthalpy of formation of gaseous hydrazine. Comparethe result with the value of ΔH f [N2H4(g)] fromAppendix C.Prentice Hall 2005 Hydrocarbons with double or triple bonds between carbonatoms are called unsaturated hydrocarbons. Alkenes are hydrocarbons with one or more C C doublebonds. Simple alkenes have just one double bond in theirmolecules. The simplest alkene is C2H4, ethene (ethylene). Alkynes are hydrocarbons that have one or more carbon–carbon triple bonds. The simplest alkyne is C2H2, ethyne (acetylene).Chapter NineGeneral Chemistry 4th edition, Hill, Petrucci, McCreary, PerryPrentice Hall 2005General Chemistry 4th edition, Hill, Petrucci, McCreary, PerryChapter Nine4546PolymersMolecular Models of Ethene and Ethyne Polymers are compounds in which many identicalmolecules have been joined together. Monomers are the simple molecules which join together toform polymers. Often, the monomers have double or triple bonds. The process of these molecules joining together is calledpolymerization. Many everyday products and many biological compoundsare polymers.Prentice Hall 2005Chapter NineGeneral Chemistry 4th edition, Hill, Petrucci, McCreary, PerryPrentice Hall 2005General Chemistry 4th edition, Hill, Petrucci, McCreary, Perry47Chapter Nine48Formation of PolyethyleneAnother ethylenemolecule adds to a longchain formed frommore ethylenemolecules.Prentice Hall 2005General Chemistry 4th edition, Hill, Petrucci, McCreary, PerryChapter NinePrentice Hall 2005General Chemistry 4th edition, Hill, Petrucci, McCreary, PerryChapter Nine8
49Cumulative ExampleUse data from this chapter and elsewhere in the text toestimate a value for the standard enthalpy of formationat 25 C of HNCO(g). Assess the most likely source oferror in this estimation.Prentice Hall 2005General Chemistry 4th edition, Hill, Petrucci, McCreary, PerryChapter Nine9
Chapter Nine Chemical Bonds: A Preview Chemical Bonds . a) (to .
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Part One: Heir of Ash Chapter 1 Chapter 2 Chapter 3 Chapter 4 Chapter 5 Chapter 6 Chapter 7 Chapter 8 Chapter 9 Chapter 10 Chapter 11 Chapter 12 Chapter 13 Chapter 14 Chapter 15 Chapter 16 Chapter 17 Chapter 18 Chapter 19 Chapter 20 Chapter 21 Chapter 22 Chapter 23 Chapter 24 Chapter 25 Chapter 26 Chapter 27 Chapter 28 Chapter 29 Chapter 30 .
TO KILL A MOCKINGBIRD. Contents Dedication Epigraph Part One Chapter 1 Chapter 2 Chapter 3 Chapter 4 Chapter 5 Chapter 6 Chapter 7 Chapter 8 Chapter 9 Chapter 10 Chapter 11 Part Two Chapter 12 Chapter 13 Chapter 14 Chapter 15 Chapter 16 Chapter 17 Chapter 18. Chapter 19 Chapter 20 Chapter 21 Chapter 22 Chapter 23 Chapter 24 Chapter 25 Chapter 26
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DEDICATION PART ONE Chapter 1 Chapter 2 Chapter 3 Chapter 4 Chapter 5 Chapter 6 Chapter 7 Chapter 8 Chapter 9 Chapter 10 Chapter 11 PART TWO Chapter 12 Chapter 13 Chapter 14 Chapter 15 Chapter 16 Chapter 17 Chapter 18 Chapter 19 Chapter 20 Chapter 21 Chapter 22 Chapter 23 .
of bonds in a compound determines many of its properties. Three types of bonds are ionic, covalent, and metallic bonds. An ionic bond is the force of attraction that holds together oppositely charged ions. Ionic bonds form crystals. Table salt contains ionic bonds. A covalent bond is the force of
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