1. Chapter 9 Chemical Bonding I: Lewis Theory 2008, Prentice Hall Chemistry: A Molecular Approach, 1 st Ed. Nivaldo Tro Roy Kennedy Massachusetts Bay Community College Wellesley Hills, MA

2. Tro, Chemistry: A Molecular Approach2 Bonding Theories explain how and why atoms attach together explain why some combinations of atoms are stable and others are not why is water H 2 O, not HO or H 3 O one of the simplest bonding theories was developed by G.N. Lewis and is called Lewis Theory Lewis Theory emphasizes valence electrons to explain bonding using Lewis Theory, we can draw models – called Lewis structures – that allow us to predict many properties of molecules aka Electron Dot Structures such as molecular shape, size, polarity

3. Tro, Chemistry: A Molecular Approach3 Why Do Atoms Bond? processes are spontaneous if they result in a system with lower potential energy chemical bonds form because they lower the potential energy between the charged particles that compose atoms the potential energy between charged particles is directly proportional to the product of the charges the potential energy between charged particles is inversely proportional to the distance between the charges

4. Tro, Chemistry: A Molecular Approach4 Potential Energy Between Charged Particles  0 is a constant = 8.85 x 10 -12 C 2 /J∙m for charges with the same sign, E potential is + and the magnitude gets less positive as the particles get farther apart for charges with the opposite signs, E potential is  and the magnitude gets more negative as the particles get closer together remember: the more negative the potential energy, the more stable the system becomes

5. Tro, Chemistry: A Molecular Approach5 Potential Energy Between Charged Particles The repulsion between like-charged particles increases as the particles get closer together. To bring them closer requires the addition of more energy. The attraction between opposite-charged particles increases as the particles get closer together. Bringing them closer lowers the potential energy of the system.

6. Tro, Chemistry: A Molecular Approach6 Bonding a chemical bond forms when the potential energy of the bonded atoms is less than the potential energy of the separate atoms have to consider following interactions: nucleus-to-nucleus repulsion electron-to-electron repulsion nucleus-to-electron attraction

7. Tro, Chemistry: A Molecular Approach7 Types of Bonds Types of AtomsType of Bond Bond Characteristic metals to nonmetals Ionic electrons transferred nonmetals to nonmetals Covalent electrons shared metal to metal Metallic electrons pooled

8. 8 Types of Bonding

9. Tro, Chemistry: A Molecular Approach9 Ionic Bonds when metals bond to nonmetals, some electrons from the metal atoms are transferred to the nonmetal atoms metals have low ionization energy, relatively easy to remove an electron from nonmetals have high electron affinities, relatively good to add electrons to

10. Tro, Chemistry: A Molecular Approach10 Covalent Bonds nonmetals have relatively high ionization energies, so it is difficult to remove electrons from them when nonmetals bond together, it is better in terms of potential energy for the atoms to share valence electrons potential energy lowest when the electrons are between the nuclei shared electrons hold the atoms together by attracting nuclei of both atoms

11. Tro, Chemistry: A Molecular Approach11 Determining the Number of Valence Electrons in an Atom the column number on the Periodic Table will tell you how many valence electrons a main group atom has Transition Elements all have 2 valence electrons; Why? 1A2A3A4A5A6A7A8A LiBeBCNOFNe 1 e -1 2 e -1 3 e -1 4 e -1 5 e -1 6 e -1 7 e -1 8 e -1

12. Tro, Chemistry: A Molecular Approach12 Lewis Symbols of Atoms aka electron dot symbols use symbol of element to represent nucleus and inner electrons use dots around the symbol to represent valence electrons pair first two electrons for the s orbital put one electron on each open side for p electrons then pair rest of the p electrons

13. Tro, Chemistry: A Molecular Approach13 Lewis Symbols of Ions Cations have Lewis symbols without valence electrons Lost in the cation formation Anions have Lewis symbols with 8 valence electrons Electrons gained in the formation of the anion Li Li +1

14. Tro, Chemistry: A Molecular Approach14 What We Know the noble gases are the least reactive group of elements the alkali metals are the most reactive metals and their atoms almost always lose 1 electron when they react the halogens are the most reactive group of nonmetals and in a lot of reactions they gain 1 electron

15. Tro, Chemistry: A Molecular Approach15 Stable Electron Arrangements And Ion Charge Metals form cations by losing enough electrons to get the same electron configuration as the previous noble gas Nonmetals form anions by gaining enough electrons to get the same electron configuration as the next noble gas The noble gas electron configuration must be very stable

16. Tro, Chemistry: A Molecular Approach16 Octet Rule when atoms bond, they tend to gain, lose, or share electrons to result in 8 valence electrons ns 2 np 6 noble gas configuration many exceptions H, Li, Be, B attain an electron configuration like He  He = 2 valence electrons  Li loses its one valence electron  H shares or gains one electron  though it commonly loses its one electron to become H +  Be loses 2 electrons to become Be 2+  though it commonly shares its two electrons in covalent bonds, resulting in 4 valence electrons  B loses 3 electrons to become B 3+  though it commonly shares its three electrons in covalent bonds, resulting in 6 valence electrons expanded octets for elements in Period 3 or below  using empty valence d orbitals

17. Tro, Chemistry: A Molecular Approach17 Lewis Theory the basis of Lewis Theory is that there are certain electron arrangements in the atom that are more stable octet rule bonding occurs so atoms attain a more stable electron configuration more stable = lower potential energy no attempt to quantify the energy as the calculation is extremely complex

18. Tro, Chemistry: A Molecular Approach18 Properties of Ionic Compounds hard and brittle crystalline solids all are solids at room temperature melting points generally > 300  C the liquid state conducts electricity the solid state does not conduct electricity many are soluble in water the solution conducts electricity well Melting an Ionic Solid

19. Tro, Chemistry: A Molecular Approach19 Conductivity of NaCl in NaCl(s), the ions are stuck in position and not allowed to move to the charged rods in NaCl(aq), the ions are separated and allowed to move to the charged rods

20. Tro, Chemistry: A Molecular Approach20 Lewis Theory and Ionic Bonding Lewis symbols can be used to represent the transfer of electrons from metal atom to nonmetal atom, resulting in ions that are attracted to each other and therefore bond + Li +

21. Tro, Chemistry: A Molecular Approach21 Predicting Ionic Formulas Using Lewis Symbols electrons are transferred until the metal loses all its valence electrons and the nonmetal has an octet numbers of atoms are adjusted so the electron transfer comes out even 2 Li + Li 2 O

22. Tro, Chemistry: A Molecular Approach22 Energetics of Ionic Bond Formation the ionization energy of the metal is endothermic Na(s) → Na + (g) + 1 e ─  H° = +603 kJ/mol the electron affinity of the nonmetal is exothermic ½Cl 2 (g) + 1 e ─ → Cl ─ (g)  H° = ─ 227 kJ/mol generally, the ionization energy of the metal is larger than the electron affinity of the nonmetal, therefore the formation of the ionic compound should be endothermic but the heat of formation of most ionic compounds is exothermic and generally large; Why? Na(s) + ½Cl 2 (g) → NaCl(s)  H° f = -410 kJ/mol

23. Tro, Chemistry: A Molecular Approach23 Ionic Bonds electrostatic attraction is nondirectional!! no direct anion-cation pair no ionic molecule chemical formula is an empirical formula, simply giving the ratio of ions based on charge balance ions arranged in a pattern called a crystal lattice every cation surrounded by anions; and every anion surrounded by cations maximizes attractions between + and - ions

24. Tro, Chemistry: A Molecular Approach24 Lattice Energy the lattice energy is the energy released when the solid crystal forms from separate ions in the gas state always exothermic hard to measure directly, but can be calculated from knowledge of other processes lattice energy depends directly on size of charges and inversely on distance between ions

25. Tro, Chemistry: A Molecular Approach25 Born-Haber Cycle method for determining the lattice energy of an ionic substance by using other reactions use Hess’s Law to add up heats of other processes  H° f ( salt ) =  H° f ( metal atoms, g ) +  H° f ( nonmetal atoms, g ) +  H° f ( cations, g ) +  H° f ( anions, g ) +  H° f ( crystal lattice )  H° f ( crystal lattice ) = Lattice Energy metal atoms (g)  cations (g),  H° f = ionization energy  don’t forget to add together all the ionization energies to get to the desired cation  M 2+ = 1 st IE + 2 nd IE nonmetal atoms (g)  anions (g),  H° f = electron affinity

26. Tro, Chemistry: A Molecular Approach26 Born-Haber Cycle for NaCl

27. Tro, Chemistry: A Molecular Approach27 Practice - Given the Information Below, Determine the Lattice Energy of MgCl 2 Mg(s)  Mg(g)  H 1 ° f = +147.1 kJ/mol ½ Cl 2 (g)  Cl(g)  H 2 ° f = +121.3 kJ/mol Mg(g)  Mg +1 (g)  H 3 ° f = +738 kJ/mol Mg +1 (g)  Mg +2 (g)  H 4 ° f = +1450 kJ/mol Cl(g)  Cl -1 (g)  H 5 ° f = -349 kJ/mol Mg(s) + Cl 2 (g)  MgCl 2 (s)  H 6 ° f = -641.3 kJ/mol

28. Tro, Chemistry: A Molecular Approach28 Practice - Given the Information Below, Determine the Lattice Energy of MgCl 2 Mg(s)  Mg(g)  H 1 ° f = +147.1 kJ/mol ½ Cl 2 (g)  Cl(g)  H 2 ° f = +121.3 kJ/mol Mg(g)  Mg +1 (g)  H 3 ° f = +738 kJ/mol Mg +1 (g)  Mg +2 (g)  H 4 ° f = +1450 kJ/mol Cl(g)  Cl -1 (g)  H 5 ° f = -349 kJ/mol Mg(s) + Cl 2 (g)  MgCl 2 (s)  H 6 ° f = -641.3 kJ/mol

29. Tro, Chemistry: A Molecular Approach29 Trends in Lattice Energy Ion Size the force of attraction between charged particles is inversely proportional to the distance between them larger ions mean the center of positive charge (nucleus of the cation) is farther away from negative charge (electrons of the anion) larger ion = weaker attraction = smaller lattice energy

30. Tro, Chemistry: A Molecular Approach30 Lattice Energy vs. Ion Size Metal Chloride Lattice Energy (kJ/mol) LiCl-834 NaCl-787 KCl-701 CsCl-657

31. Tro, Chemistry: A Molecular Approach31 Trends in Lattice Energy Ion Charge the force of attraction between oppositely charged particles is directly proportional to the product of the charges larger charge means the ions are more strongly attracted larger charge = stronger attraction = larger lattice energy of the two factors, ion charge generally more important Lattice Energy = -910 kJ/mol Lattice Energy = -3414 kJ/mol

32. Tro, Chemistry: A Molecular Approach32 Example 9.2 – Order the following ionic compounds in order of increasing magnitude of lattice energy. CaO, KBr, KCl, SrO First examine the ion charges and order by product of the charges Ca 2+ & O 2-, K + & Br ─, K + & Cl ─, Sr 2+ & O 2─ (KBr, KCl) < (CaO, SrO) Then examine the ion sizes of each group and order by radius; larger < smaller (KBr, KCl) same cation, Br ─ > Cl ─ (same Group) KBr < KCl < (CaO, SrO) (CaO, SrO) same anion, Sr 2+ > Ca 2+ (same Group) KBr < KCl < SrO < CaO

33. Tro, Chemistry: A Molecular Approach33 Ionic Bonding Model vs. Reality ionic compounds have high melting points and boiling points MP generally > 300°C all ionic compounds are solids at room temperature because the attractions between ions are strong, breaking down the crystal requires a lot of energy the stronger the attraction (larger the lattice energy), the higher the melting point

34. Tro, Chemistry: A Molecular Approach34 Ionic Bonding Model vs. Reality ionic solids are brittle and hard the position of the ion in the crystal is critical to establishing maximum attractive forces – displacing the ions from their positions results in like charges close to each other and the repulsive forces take over + - ++++ ++++ -- - - - - - - + - ++++ ++++ -- - - - - - - + - ++++ ++++ -- - - - - - -

35. Tro, Chemistry: A Molecular Approach35 Ionic Bonding Model vs. Reality ionic compounds conduct electricity in the liquid state or when dissolved in water, but not in the solid state to conduct electricity, a material must have charged particles that are able to flow through the material in the ionic solid, the charged particles are locked in position and cannot move around to conduct in the liquid state, or when dissolved in water, the ions have the ability to move through the structure and therefore conduct electricity

36. Tro, Chemistry: A Molecular Approach36 Covalent Bonding: Bonding and Lone Pair Electrons Covalent bonding results when atoms share pairs of electrons to achieve an “octet” Electrons that are shared by atoms are called bonding pairs Electrons that are not shared by atoms but belong to a particular atom are called lone pairs aka nonbonding pairs O S O Lone PairsBonding Pairs

37. Tro, Chemistry: A Molecular Approach37 Single Covalent Bonds two atoms share a pair of electrons 2 electrons one atom may have more than one single bond F F F F H H O H H O F F

38. Tro, Chemistry: A Molecular Approach38 Double Covalent Bond two atoms sharing two pairs of electrons 4 electrons O O O O

39. Tro, Chemistry: A Molecular Approach39 Triple Covalent Bond two atoms sharing 3 pairs of electrons 6 electrons N N N N

40. Tro, Chemistry: A Molecular Approach40 Covalent Bonding Predictions from Lewis Theory Lewis theory allows us to predict the formulas of molecules Lewis theory predicts that some combinations should be stable, while others should not because the stable combinations result in “octets” Lewis theory predicts in covalent bonding that the attractions between atoms are directional the shared electrons are most stable between the bonding atoms resulting in molecules rather than an array

41. Tro, Chemistry: A Molecular Approach41 Covalent Bonding Model vs. Reality molecular compounds have low melting points and boiling points MP generally < 300°C molecular compounds are found in all 3 states at room temperature melting and boiling involve breaking the attractions between the molecules, but not the bonds between the atoms the covalent bonds are strong the attractions between the molecules are generally weak the polarity of the covalent bonds influences the strength of the intermolecular attractions

42. Tro, Chemistry: A Molecular Approach42 Intermolecular Attractions vs. Bonding

43. Tro, Chemistry: A Molecular Approach43 Ionic Bonding Model vs. Reality some molecular solids are brittle and hard, but many are soft and waxy the kind and strength of the intermolecular attractions varies based on many factors the covalent bonds are not broken, however, the polarity of the bonds has influence on these attractive forces

44. Tro, Chemistry: A Molecular Approach44 Ionic Bonding Model vs. Reality molecular compounds do not conduct electricity in the liquid state molecular acids conduct electricity when dissolved in water, but not in the solid state in molecular solids, there are no charged particles around to allow the material to conduct when dissolved in water, molecular acids are ionized, and have the ability to move through the structure and therefore conduct electricity

45. Tro, Chemistry: A Molecular Approach45 Bond Polarity covalent bonding between unlike atoms results in unequal sharing of the electrons one atom pulls the electrons in the bond closer to its side one end of the bond has larger electron density than the other the result is a polar covalent bond bond polarity the end with the larger electron density gets a partial negative charge the end that is electron deficient gets a partial positive charge

46. Tro, Chemistry: A Molecular Approach46 HF  EN 2.1 EN 4.0

47. Tro, Chemistry: A Molecular Approach47 Electronegativity measure of the pull an atom has on bonding electrons increases across period (left to right) and decreases down group (top to bottom) fluorine is the most electronegative element francium is the least electronegative element the larger the difference in electronegativity, the more polar the bond negative end toward more electronegative atom

48. Tro, Chemistry: A Molecular Approach48 Electronegativity Scale

49. 49 Electronegativity and Bond Polarity If difference in electronegativity between bonded atoms is 0, the bond is pure covalent equal sharing If difference in electronegativity between bonded atoms is 0.1 to 0.4, the bond is nonpolar covalent If difference in electronegativity between bonded atoms 0.5 to 1.9, the bond is polar covalent If difference in electronegativity between bonded atoms larger than or equal to 2.0, the bond is ionic “100%” 00.42.04.0 4%51% Percent Ionic Character Electronegativity Difference

50. Tro, Chemistry: A Molecular Approach50 Bond Polarity EN Cl = 3.0 3.0 - 3.0 = 0 Pure Covalent EN Cl = 3.0 EN H = 2.1 3.0 – 2.1 = 0.9 Polar Covalent EN Cl = 3.0 EN Na = 1.0 3.0 – 0.9 = 2.1 Ionic

51. Tro, Chemistry: A Molecular Approach51

52. Tro, Chemistry: A Molecular Approach52 Bond Dipole Moments the dipole moment is a quantitative way of describing the polarity of a bond a dipole is a material with positively and negatively charged ends measured dipole moment, , is a measure of bond polarity it is directly proportional to the size of the partial charges and directly proportional to the distance between them   = (q)(r)  not Coulomb’s Law  measured in Debyes, D the percent ionic character is the percentage of a bond’s measured dipole moment to what it would be if full ions

53. Tro, Chemistry: A Molecular Approach53 Dipole Moments

54. Tro, Chemistry: A Molecular Approach54 Water – a Polar Molecule stream of water attracted to a charged glass rod stream of hexane not attracted to a charged glass rod

55. Tro, Chemistry: A Molecular Approach55 Example 9.3(c) - Determine whether an N-O bond is ionic, covalent, or polar covalent. Determine the electronegativity of each element N = 3.0; O = 3.5 Subtract the electronegativities, large minus small (3.5) - (3.0) = 0.5 If the difference is 2.0 or larger, then the bond is ionic; otherwise it’s covalent difference (0.5) is less than 2.0, therefore covalent If the difference is 0.5 to 1.9, then the bond is polar covalent; otherwise it’s covalent difference (0.5) is 0.5 to 1.9, therefore polar covalent

56. Tro, Chemistry: A Molecular Approach56 Lewis Structures of Molecules shows pattern of valence electron distribution in the molecule useful for understanding the bonding in many compounds allows us to predict shapes of molecules allows us to predict properties of molecules and how they will interact together

57. Tro, Chemistry: A Molecular Approach57 Lewis Structures use common bonding patterns C = 4 bonds & 0 lone pairs, N = 3 bonds & 1 lone pair, O= 2 bonds & 2 lone pairs, H and halogen = 1 bond, Be = 2 bonds & 0 lone pairs, B = 3 bonds & 0 lone pairs often Lewis structures with line bonds have the lone pairs left off  their presence is assumed from common bonding patterns structures which result in bonding patterns different from common have formal charges B C NOF

58. Tro, Chemistry: A Molecular Approach58 Writing Lewis Structures of Molecules HNO 3 1) Write skeletal structure H always terminal  in oxyacid, H outside attached to O’s make least electronegative atom central  N is central 2)Count valence electrons sum the valence electrons for each atom add 1 electron for each − charge subtract 1 electron for each + charge N = 5 H = 1 O 3 = 3∙6 = 18 Total = 24 e -

59. Tro, Chemistry: A Molecular Approach59 Writing Lewis Structures of Molecules HNO 3 3) Attach central atom to the surrounding atoms with pairs of electrons and subtract from the total Electrons Start24 Used8 Left16

60. Tro, Chemistry: A Molecular Approach60 Writing Lewis Structures of Molecules HNO 3 4) Complete octets, outside-in H is already complete with 2  1 bond and re-count electrons N = 5 H = 1 O 3 = 3∙6 = 18 Total = 24 e - Electrons Start24 Used8 Left16 Electrons Start16 Used16 Left0

61. Tro, Chemistry: A Molecular Approach61 Writing Lewis Structures of Molecules HNO 3 5) If all octets complete, give extra electrons to central atom. elements with d orbitals can have more than 8 electrons  Period 3 and below 6) If central atom does not have octet, bring in electrons from outside atoms to share follow common bonding patterns if possible

62. Tro, Chemistry: A Molecular Approach62 Practice - Lewis Structures CO 2 SeOF 2 NO 2 -1 H 3 PO 4 SO 3 -2 P 2 H 4

63. Tro, Chemistry: A Molecular Approach63 Practice - Lewis Structures CO 2 SeOF 2 NO 2 -1 H 3 PO 4 SO 3 -2 P 2 H 4 : O :: C :: O : :: 16 e - 26 e - 18 e - 26 e - 32 e - 14 e -

64. Tro, Chemistry: A Molecular Approach64 Formal Charge during bonding, atoms may wind up with more or less electrons in order to fulfill octets - this results in atoms having a formal charge FC = valence e - - nonbonding e - - ½ bonding e - left OFC = 6 - 4 - ½ (4) = 0 SFC = 6 - 2 - ½ (6) = +1 right OFC = 6 - 6 - ½ (2) = -1 sum of all the formal charges in a molecule = 0 in an ion, total equals the charge O S O

65. Tro, Chemistry: A Molecular Approach65 Writing Lewis Formulas of Molecules (cont’d) 7) Assign formal charges to the atoms a)formal charge = valence e - - lone pair e - - ½ bonding e - b)follow the common bonding patterns 0 +1 all 0

66. Tro, Chemistry: A Molecular Approach66 Common Bonding Patterns B C N O C + N + O + C - N - O - B - F F + - F

67. Tro, Chemistry: A Molecular Approach67 Practice - Assign Formal Charges CO 2 SeOF 2 NO 2 -1 H 3 PO 4 SO 3 -2 P 2 H 4

68. Tro, Chemistry: A Molecular Approach68 Practice - Assign Formal Charges CO 2 SeOF 2 NO 2 -1 H 3 PO 4 SO 3 -2 P 2 H 4 all 0 P = +1 rest 0 S = +1 Se = +1 all 0

69. Tro, Chemistry: A Molecular Approach69 Resonance when there is more than one Lewis structure for a molecule that differ only in the position of the electrons, they are called resonance structures the actual molecule is a combination of the resonance forms – a resonance hybrid it does not resonate between the two forms, though we often draw it that way look for multiple bonds or lone pairs O S O

70. Tro, Chemistry: A Molecular Approach70 Resonance

71. Tro, Chemistry: A Molecular Approach71 Ozone Layer

72. Tro, Chemistry: A Molecular Approach72 Rules of Resonance Structures Resonance structures must have the same connectivity only electron positions can change Resonance structures must have the same number of electrons Second row elements have a maximum of 8 electrons bonding and nonbonding third row can have expanded octet Formal charges must total same Better structures have fewer formal charges Better structures have smaller formal charges Better structures have − formal charge on more electronegative atom

73. Tro, Chemistry: A Molecular Approach73 Drawing Resonance Structures 1.draw first Lewis structure that maximizes octets 2.assign formal charges 3.move electron pairs from atoms with (-) formal charge toward atoms with (+) formal charge 4.if (+) fc atom 2 nd row, only move in electrons if you can move out electron pairs from multiple bond 5.if (+) fc atom 3 rd row or below, keep bringing in electron pairs to reduce the formal charge, even if get expanded octet. +1 +1

74. Tro, Chemistry: A Molecular Approach74 Exceptions to the Octet Rule expanded octets elements with empty d orbitals can have more than 8 electrons odd number electron species e.g., NO will have 1 unpaired electron free-radical very reactive incomplete octets B, Al

75. Tro, Chemistry: A Molecular Approach75 Drawing Resonance Structures 1.draw first Lewis structure that maximizes octets 2.assign formal charges 3.move electron pairs from atoms with (-) formal charge toward atoms with (+) formal charge 4.if (+) fc atom 2 nd row, only move in electrons if you can move out electron pairs from multiple bond 5.if (+) fc atom 3 rd row or below, keep bringing in electron pairs to reduce the formal charge, even if get expanded octet. +2 0 0 0

76. Tro, Chemistry: A Molecular Approach76 Practice - Identify Structures with Better or Equal Resonance Forms and Draw Them CO 2 SeOF 2 NO 2 -1 H 3 PO 4 SO 3 -2 P 2 H 4 all 0 P = +1 S = +1 Se = +1 all 0

77. Tro, Chemistry: A Molecular Approach77 Practice - Identify Structures with Better or Equal Resonance Forms and Draw Them CO 2 SeOF 2 NO 2 -1 H 3 PO 4 SO 3 -2 P 2 H 4 none +1 all 0 +1 all 0 none S = 0 in all res. forms

78. Tro, Chemistry: A Molecular Approach78 Bond Energies chemical reactions involve breaking bonds in reactant molecules and making new bond to create the products the  H° reaction can be calculated by comparing the cost of breaking old bonds to the profit from making new bonds the amount of energy it takes to break one mole of a bond in a compound is called the bond energy in the gas state homolytically – each atom gets ½ bonding electrons

79. Tro, Chemistry: A Molecular Approach79 Trends in Bond Energies the more electrons two atoms share, the stronger the covalent bond C≡C (837 kJ) > C=C (611 kJ) > C−C (347 kJ) C≡N (891 kJ) > C=N (615 kJ) > C−N (305 kJ) the shorter the covalent bond, the stronger the bond Br−F (237 kJ) > Br−Cl (218 kJ) > Br−Br (193 kJ) bonds get weaker down the column

80. Tro, Chemistry: A Molecular Approach80 Using Bond Energies to Estimate  H° rxn the actual bond energy depends on the surrounding atoms and other factors we often use average bond energies to estimate the  H rxn works best when all reactants and products in gas state bond breaking is endothermic,  H(breaking) = + bond making is exothermic,  H(making) = −  H rxn = ∑ (  H(bonds broken)) + ∑ (  H(bonds formed))

81. 81

82. 82 Estimate the Enthalpy of the Following Reaction

83. Tro, Chemistry: A Molecular Approach83 Estimate the Enthalpy of the Following Reaction H 2 (g) + O 2 (g)  H 2 O 2 (g) reaction involves breaking 1mol H-H and 1 mol O=O and making 2 mol H-O and 1 mol O-O bonds broken (energy cost) (+436 kJ) + (+498 kJ) = +934 kJ bonds made (energy release) 2(464 kJ) + (142 kJ) = -1070  H rxn = (+934 kJ) + (-1070. kJ) = -136 kJ (Appendix  H° f = -136.3 kJ/mol)

84. Tro, Chemistry: A Molecular Approach84 Bond Lengths the distance between the nuclei of bonded atoms is called the bond length because the actual bond length depends on the other atoms around the bond we often use the average bond length averaged for similar bonds from many compounds

85. Tro, Chemistry: A Molecular Approach85 Trends in Bond Lengths the more electrons two atoms share, the shorter the covalent bond C≡C (120 pm) < C=C (134 pm) < C−C (154 pm) C≡N (116 pm) < C=N (128 pm) < C−N (147 pm) decreases from left to right across period C−C (154 pm) > C−N (147 pm) > C−O (143 pm) increases down the column F−F (144 pm) > Cl−Cl (198 pm) > Br−Br (228 pm) in general, as bonds get longer, they also get weaker

86. Tro, Chemistry: A Molecular Approach86 Bond Lengths

87. Tro, Chemistry: A Molecular Approach87 Metallic Bonds low ionization energy of metals allows them to lose electrons easily the simplest theory of metallic bonding involves the metals atoms releasing their valence electrons to be shared by all to atoms/ions in the metal an organization of metal cation islands in a sea of electrons electrons delocalized throughout the metal structure bonding results from attraction of cation for the delocalized electrons

88. Tro, Chemistry: A Molecular Approach88 Metallic Bonding

89. Tro, Chemistry: A Molecular Approach89 Metallic Bonding Model vs. Reality metallic solids conduct electricity because the free electrons are mobile, it allows the electrons to move through the metallic crystal and conduct electricity as temperature increases, electrical conductivity decreases heating causes the metal ions to vibrate faster, making it harder for electrons to make their way through the crystal

90. Tro, Chemistry: A Molecular Approach90 Metallic Bonding Model vs. Reality metallic solids conduct heat the movement of the small, light electrons through the solid can transfer kinetic energy quicker than larger particles metallic solids reflect light the mobile electrons on the surface absorb the outside light and then emit it at the same frequency

91. Tro, Chemistry: A Molecular Approach91 Metallic Bonding Model vs. Reality metallic solids are malleable and ductile because the free electrons are mobile, the direction of the attractive force between the metal cation and free electrons is adjustable this allows the position of the metal cation islands to move around in the sea of electrons without breaking the attractions and the crystal structure

92. Tro, Chemistry: A Molecular Approach92 Metallic Bonding Model vs. Reality metals generally have high melting points and boiling points all but Hg are solids at room temperature the attractions of the metal cations for the free electrons is strong and hard to overcome melting points generally increase to right across period the charge on the metal cation increases across the period, causing stronger attractions melting points generally decrease down column the cations get larger down the column, resulting in a larger distance from the nucleus to the free electrons