Lecture outline: Section 9. theory 2. Valence bond theory 3. Molecular orbital theory. S. Ensign, Chem. 1210

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1 Lecture outline: Section 9 Molecular l geometry and bonding theories 1. Valence shell electron pair repulsion theory 2. Valence bond theory 3. Molecular orbital theory 1

2 Ionic bonding Covalent bonding Quick concept review Electronegativity Polar covalent bond Nonpolar covalent bond Lewis structures Octet rule Octet violations Resonance Formal charge Bond strength Oxidation number 2

3 What we learn from Lewis structures Atom connections Number of bonds Types of bonds (single, double, triple) Nonbonding electrons Nothing about geometry/shape 3

4 Bond angles: the angles made by lines joining nuclei of atoms in a molecule. Three atoms are required to define a bond angle 4

5 What are the possible geometries where A is the central atom? AB B A Only one possible geometry AB 2 B A B B A B 2-dimensional AB 3 AB 4 B A B B A B B B B A B B B B B A B B 2- or 3-dimensional 2- or 3-dimensional AB 5, AB 6, And so,on.. 5

6 Some possible shapes for binary molecular compounds with the formulas AB 2, AB 3, and AB 4 2-dimensional 2-dimensional 2-dimensional 2-dimensional 3-dimensional 2-dimensional 3-dimensional 3-dimensional 6

7 Predicting molecular geometry from Lewis dot structures Valence Shell Electron Pair Repulsion VSEPR The shape of a molecule is related to the number of electron domains in the valence shell of the central atom An electron domain about an atom can be a: 1. nonbonded e - pair (lone pair) 2. bond location (single, double, or triple bond) 7

8 VSEPR An electron domain about an atom can be a: 1. A nonbonded e - pair (lone pair) 2. A bond location (a single, double, or triple bond counts as one domain) = = = How many e - domains on the (central) atom? 8

9 VSEPR balloon model: e - domains repel each other, and hence want to get as far away from each other as possible = A = A 9

10 Two electron domains about an atom result in a linear electron domain geometry Bonded atoms ( X ) at termini of electron pairs: 180 = = X A X Lone e - pairs not bonded to atoms( E ): 180 = = A 180 A 10

11 Three electron domains attached to a central atom Trigonal planar e - domain geometry 11

12 Trigonal planar e - domain geometry Bonded atoms ( X ) at termini of electron ect pairs: X = = A X Lone e - pairs not bonded to atoms( E ): X = = A 12

13 Four electron domains attached to a central atom Constrain to two dimensions: 4 x 90 angles Use three dimensions: tetrahedral geometry 13

Created by en:user:cyp, licensed under the

14 A tetrahedron: a three-dimensional polyhedron composed of four triangular faces meeting at four vertices A rotating Tetrahedron. Animated GIF image. Created by en:user:cyp, licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license. 14

15 Tetrahedral electron domain geometry: The central atom is at the center of a tetrahedron and the electron pairs are pointing towards the four vertices 15

16 Tetrahedral e - domain geometry Bonded atoms ( X ) at termini of electron ect pairs: X = = X X A X Lone e - pairs not bonded to atoms( E ): = = A 16

17 Electron domain geometry about a central atom Two e - domains: linear, two dimensional, 180 bond angles Three e - domains: trigonal planar, two dimensional, 120 bond angles Four e - domains: tetrahedral, three dimensional, bond angles

18 Going beyond octets: central atoms surrounded by 5 or 6 electron domains 18

19 Five electron domains attached to a central atom: trigonal bipyramidal geometry 19

20 Trigonal bipyramidal electron domain geometry 90 axial equatorial 120 axial 20

21 Trigonal bipyramidal e - domain geometry Bonded atoms ( X ) at termini of electron ect pairs: X = = X A X X Lone e - pairs not bonded to atoms( E ): X = = A 21

22 Six electron domains attached to a central atom: octahedral geometry 22

23 Octahedral e - domain geometry Bonded atoms ( X ) at termini of electron ect pairs: X X = = X A X X Lone e - pairs not bonded to atoms( E ): X = = A 23

24 # of e - domains e - domain geometry Bond angles Structure 2 linear Trigonal planar Tetrahedral Trigonal bipyramidal 90, 120 and Octahedral 90 and

25 Electron domain geometry: the arrangement of electron domains in space about a central atom Molecular l geometry: the arrangement of bonded atoms in space about a central atom 25

26 Predicting molecular geometry (1) Draw a good Lewis structure (2) Count the number of electron domains around the central atom. Double and triple bonds count as one e- domain (3) Determine # of bonded d electron domains and number of nonbonded electron domains (nonbonded domains occupy positions that best minimize electron pair repulsion) (4) Molecular shape is described in terms of the arrangement of the bonded domains 26

27 Tetrahedral electron domain geometry 27

28 Tetrahedral electron domain geometry with four bonded atoms (AX 4 4) Tetrahedral molecular geometry (shape) 28

29 Tetrahedral electron domain geometry with three bonded atoms and one lone pair (AX 3 E) Trigonal pyramidal molecular geometry (shape) 29

30 Tetrahedral electron domain geometry with three bonded atoms and one lone pair, showing only the atoms Trigonal pyramidal molecular geometry (shape) 30

31 Tetrahedral electron pair geometry with two bonded atoms and two lone pairs (AX 2 E 2 ) Bent molecular geometry (shape) 31

32 Tetrahedral electron pair geometry with two bonded atoms and two lone pairs, showing only the atoms Bent molecular geometry (shape) 32

33 Molecular geometries of molecules with tetrahedral e - domain geometries Tetrahedral Trigonal pyramidal Bent 33

34 Electron pair (domain) geometry: the arrangement of electron pairs in space about a central atom Molecular geometry: the arrangement of bonded atoms in space about a central atom when a central atom has no lone pairs (no nonbonded e - pairs), then the e - domain geometry = the molecular geometry when a central atom has one or more lone pairs, these lone pairs will influence the molecular geometry (shape of the molecule) due to VSEPR 34

35 Predicting molecular geometry (1) Draw a good Lewis structure t (2) Count the number of electron pairs around the central atom. Double and triple bonds count as one e- pair (3) Determine # of bonding electron pairs and number of nonbonding electron pairs (nonbonding pairs occupy positions that best minimize electron pair repulsion) (4) Molecular shape is described in terms of the arrangement of the bonding pairs of electrons Example: Compare the e- pair geometries and molecular shapes of H 2 O, NH 3, and CH 4 35

36 Compare the e- pair geometries and molecular shapes of H 2 O, NH 3, and CH 4 (1) Draw a good Lewis structure (2) Count the number of electron pairs around the central atom. Double and triple bonds count as one e- pair (3) Determine # of bonding electron pairs and number of nonbonding electron pairs (nonbonding pairs occupy positions that best minimize electron pair repulsion) (4) Molecular shape is described in terms of the arrangement of the bonding pairs of electrons 36

37 Different representations of ammonia and water NH 3 Formula Lewis Electron domain Molecular Structure geometry geometry H 2 O Formula Lewis Electron domain Molecular Structure geometry geometry 37

38 Molecular geometries for molecules with tetrahedral electron domain geometries bonded domains (X) nonbonded domains (E) Molecular geometry formula Bond angles Example 4 0 tetrahedral AX CH Trigonal AX 3 E ~107 NH3 pyramidal 2 2 Bent AX 2 E 2 ~104 H2O 38

39 Molecular geometries for molecules with trigonal planar electron domain geometries bonded domains (X) nonbonded domains (E) Molecular geometry formula Bond angles Example 3 0 Ti Trigonal AX BH 3 planar 2 1 Bent AX 2 E ~119 SO 2 39

Nonbonding (lone) electron pairs spread out more in space than do

5 B S 120 120 H Trigonal planar e - Trigonal planar e - Trigonal

geometry with two bonded no bonded pairs three bonded pairs pairs

40 Nonbonding (lone) electron pairs spread out more in space than do bonding electron pairs H B S H Trigonal planar e - Trigonal planar e - Trigonal planar e - domain domain geometry with domain geometry with geometry with two bonded no bonded pairs three bonded pairs pairs and one lone pair H O 119 O See 40

41 Trigonal planar e - domain geometry with no bonded pairs

42 Trigonal planar e - domain geometry with three bonded pairs H B H 120 H 42

43 Trigonal planar e - domain geometry with two bonded pairs and one lone pair S O 119 O 43

44 Electron domains containing multiple bonds spread out more in space than domains with a single electron pair

45 Octahedral electron pair geometry with six bonded atoms Octahedral molecular geometry (shape) 45

46 Octahedral electron pair geometry with five bonded atoms and one lone pair Square pyramidal molecular geometry (shape) 46

47 Octahedral electron pair geometry with five bonded atoms and one lone pair, showing only the atoms Square pyramidal molecular geometry (shape) 47

48 Octahedral electron pair geometry with four bonded atoms and two lone pairs Square planar molecular geometry (shape) 48

49 Octahedral electron pair geometry with four bonded atoms and two lone pairs, showing only the atoms Square planar molecular geometry (shape) 49

geometry formula Bond angles Example 6 0 octahedral AX 6 90 SF 6 and

50 Molecular geometries for molecules with octahedral electron domain geometries bonded domains (X) nonbonded domains (E) Molecular geometry formula Bond angles Example 6 0 octahedral AX 6 90 SF 6 and Square AX 5 E ~90 IF 5 pyramidal 4 2 Square planar AX 4 E 2 90 XeF 4 50

51 Trigonal bipyramidal electron pair geometry with five bonded atoms and no lone pairs Trigonal bipyramidal molecular geometry (shape) 51

52 Trigonal bipyramidal electron pair geometry with four bonded atoms and one lone pair NO YES nonbonding pairs occupy positions that best minimize electron pair repulsion 52

53 Trigonal bipyramidal electron pair geometry with four bonded atoms and one lone pair See: = Lone pairs prefer to occupy equatorial positions See-saw molecular geometry (shape) 53

54 Trigonal bipyramidal electron pair geometry with four bonded atoms and one lone pair showing only the atoms = Lone pairs prefer to occupy equatorial positions See-saw molecular geometry (shape) 54

55 Trigonal bipyramidal electron pair geometry with three bonded atoms and two lone pairs See: When present, lone pairs prefer to occupy equatorial positions T molecular geometry (shape) 55

56 Trigonal bipyramidal electron pair geometry with three bonded atoms and two lone pairs showing only the atoms When present, lone pairs prefer to occupy equatorial positions T molecular geometry (shape) 56

57 Trigonal bipyramidal electron pair geometry with two bonded atoms and three lone pairs When present, lone pairs prefer to occupy equatorial positions linear molecular geometry (shape) 57

58 Trigonal bipyramidal electron pair geometry with two bonded atoms and three lone pairs showing only the atoms When present, lone pairs prefer to occupy equatorial positions linear molecular geometry (shape) 58

Trigonal bipyramidal Bond angles AX 5 120, 90, and 180 Example PF 4 4 1 See saw AX 4

59 Molecular geometries for molecules with trigonal bipyramidal electron domain geometries bonded domains (X) nonbonded domains (E) Molecular geometry formula 5 0 Trigonal bipyramidal Bond angles AX 5 120, 90, and 180 Example PF See saw AX 4 E ~120, ~90, and T shaped AX 3 E 2 ~90 and 180 SF 4 H2O 2 3 Linear AX 2 E XeF 2 59

60 Visit the following webpage for an interactive version of the VSEPR shapes table, in which molecules can be viewed rotated, and manipulated using the open source web applet Jmol : VSEPR%20table.htm 60

61 Predict electron pair and molecule geometries ti for the following molecules l O 3 NH 4 + XeOF 4 (Xe is central) PF - 6 I 3-61

62 Predict electron pair and molecule geometries ti for the following molecules l O 3 NH 4 + XeOF 4 (Xe is central) PF - 6 I 3-62

63 Ball and stick vs. space filling models for CH4 63

64 Ball and stick vs. space filling models for H 2 O 64

65 What is the molecular shape of IF 5? A. trigonal biyramidal B. octahedral C. square pyramidal F D. see-saw F F I F F 65

66 Molecules with no central atom Tetrahedral mol. geom. Trigonal planar mol. geom. Bent mol. geom. 66

67 Differences in electronegativity of atoms dictate what kind of bond forms between the atoms ΔEN = 0, a nonpolar covalent bond ΔEN = 0 2, a polar covalent bond ΔEN > 2, an ionic bond 67

68 Polarity of molecules and dipoles 68

69 Bond dipoles and overall dipoles Compare CO 2, CO, CS 2, COS 69

70 net dipole δ Bond dipole Bond dipole δ + δ + 70

71 Shape and symmetry determine whether a molecule has a net dipole moment 71

72 Determine whether the following molecules are polar or nonpolar SiCl 4 SF 4 SF 6 CFCl 3 72

73 Determine whether the following molecules are polar or nonpolar SiCl 4 SF 4 SF 6 CFCl 3 73

74 Summary of VSEPR The Lewis structure for a molecule shows the number of bonded and nonbonded electron domains about the central atom Electron domain geometry is described in terms of the total number of e - domains Molecular geometry can be deduced from the number of bonding and nonbonding e - domains on the central atom The next step: rationalize VSEPR with the concept of orbitals, which we have only discussed for nonbonded atoms so far 74

75 Rationalizing VSEPR with the concept of orbitals: review some concepts/terms For atoms: Electrons reside in areas of space called orbitals Orbitals have defined energies (n), shapes (s, p, d, f), and orientations (p x, p y, p z ) in space Lewis symbols are used to represent the valence electrons an atom has For molecules: A covalent bond results from the sharing of a pair of electrons between two atoms The shared electrons originate in valence atomic orbitals The atomic orbitals may have to be changed to molecular orbitals in order for bond formation to occur in such a way that the geometries predicted from VSEPR can be obtained 75

76 Requirements for bond formation between two atoms Electrons in valence orbitals of two atoms must overlap A maximum of two electrons, of opposite spins, are present in overlapping orbitals for each bond. Different types of orbitals are used for single and multiple l bonds 76

Overlap of atomic orbitals works well in

like H 2 Electrons in valence orbitals of

electrons, of opposite spins, are present in

atoms approach each other, the proton and

feel attraction Energy is released as the

electrostatic attractions between electrons

overlap to form a covalent bond The distance

77 Overlap of atomic orbitals works well in describing the bonding in a simple molecule like H 2 Electrons in valence orbitals of two atoms must overlap A maximum of two electrons, of opposite spins, are present in overlapping orbitals for each bond As two H atoms approach each other, the proton and electron of the different atoms begin to feel attraction Energy is released as the two atoms move closer to maximize electrostatic attractions between electrons and protons The 1s orbitals from each atom overlap to form a covalent bond The distance between the bonded atoms represents the lowest energy state 1s 1 1s 1 1s 1 1s 1 77

78 Overlap of atomic orbitals works well in describing the bonding in a simple molecule like H 2 1s 1 1s 1 1s 1 1s 1 78

79 Potential energy diagram for H-H bond formation 400 Ene ergy (kj/m mol) radius (angstroms) 79

80 The dilemna The geometry of electron pairs in a molecule l is predicted fromvsepr 2 e - pairs: linear 3 e - pairs: trigonal planar 4 e - pairs: tetrahedral 5 e - pairs: trigonal bipyramidal 6 e - pairs: octahedral For most molecules, overlap of atomic orbitals from bonded atoms will not produce molecules with these geometries 80

81 The solution: Valence bond theory A way to reconcile VSEPR with the quantum mechanical model for atomic orbitals Appropriate s, p, and d orbitals on central atom are mixed to create a new set of hybrid orbitals directed towards bonded atoms and lone pairs Consider the bonding and geometry present in CH 4 to illustrate this 81

82 Bonding of C to H in methane, CH 4 C: Group 4A, 1s 2 2s 2 2p 2, 4 bonds needed to satisfy octet s 2s 2p Individual valence atomic orbitals of C in the ground state: z z z z y y y y x x x x s All four valence valence atomic orbitals shown together: p z y p y z p x x 82

83 Bonding of C to H in methane, CH H: 1s 1 1s To get four bonds to H, electrons in the valence orbitals could be redistributed as follows: 2s 2p 2s 2p Note the bond angles that would result from overlap of atomic orbitals with the 1s orbitals of H: z C y x + 4 H z H y z H y C H x C H H H H H 90 x 83

84 Orbital hybridization Appropriate s, p, and d orbitals on central atom are mixed to create a new set of hybrid orbitals directed towards bonded atoms and lone pairs To get four equivalent orbitals for a molecule predicted to have tetrahedral electron pair geometry, mix the one valence s and all three of the valence p orbitals to make four equivalent sp 3 orbitals 84

85 sp 3 orbital hybridization for C z y x p x p y p z s, p y, p z, p x s 85

86 sp 3 orbital hybridization for C z y x p x p y p z equivalent sp 3 orbitals s 86

87 View the four sp 3 orbitals individually z y x 87

88 Bonding of H atoms to the sp 3 hybrid orbitals H H H H 88

89 Bonding of H atoms to the sp 3 hybrid orbitals H H H H 89

90 Use the principle of sp 3 orbital hybridization to describe the molecular bonding in methane, ammonia and water Account for the bonding by writing out the orbital boxes and hybridizing appropriate p orbitals 90

91 Use the principle of sp 3 orbital hybridization to describe the molecular bonding in NH 3 and H 2O. Account for the bonding by writing out the orbital boxes and hybridizing appropriate orbitals 91

92 Use the principle of sp 3 orbital hybridization to describe the molecular bonding in NH 3 and H 2O. Account for the bonding by writing out the orbital boxes and hybridizing appropriate orbitals 92

93 Orbital hybridization Appropriate s, p, and d orbitals on central atom are mixed to create a new set of hybrid orbitals directed towards bonded atoms and lone pairs To get three equivalent orbitals for a molecule predicted to have trigonal planar electron pair geometry, mix the one valence s and two of the three valence p orbitals to make three equivalent sp 2 orbitals 93

94 z y sp 2 orbital hybridization z y z y z y x x x x s p y p z p x z y z y 120 x x p y 120 Three sp 2 orbitals 94

95 sp 2 orbital hybridization z y x p x p y p z nonhybridized p orbitals p y 3 equivalent sp 2 orbitals s 95

96 View the three sp 2 orbitals individually z y x With the three sp 2 orbitals in the xz plane, the nonhybridized p orbital is left in the y plane, which is perpendicular to this plane z y z y z y x x x 96

97 Orbital hybridization Appropriate s, p, and d orbitals on central atom are mixed to create a new set of hybrid orbitals directed towards bonded atoms and lone pairs To get two equivalent orbitals for a molecule predicted to have linear electron pair geometry, mix the one valence s and one of the three valence p orbitals to make two equivalent sp orbitals 97

98 z y sp orbital hybridization z y z y z y x x x x s p y p z p x z y z y z y 180 x x + x + x p y p z Two sp orbitals 98

99 sp orbital hybridization z y z y x x p x p y p z nonhybridized p orbitals p y p z s 2 equivalent sp 2 orbitals x x 99

100 Hybridization in a molecule with trigonal bipyramidal electron pair geometry 100

101 Hybridization in a molecule with octahedral electron pair geometry 101

102 Orbital hybridization Mixing atomic orbitals to form hybrid orbitals allows new orbitals to be formed with the correct gemoetries to satisfy VSEPR theory Appropriate s, p, and d orbitals on central atom are mixed to create a new set of hybrid orbitals directed towards bonded atoms and lone pairs The number of hybrid orbitals required is equal to the number of electron domains around the central atom n e - domains: n hybrid orbitals 102

103 Predicting orbital hybridization 1. Draw Lewis structure 2. Determine e - pair geometry (VSEPR) 3. Determine hybrid orbitals needed to give correct e - pair geometry # of e - pairs e - pair geometry 2 linear sp 3 ti trig. planar sp 2 4 tetrahedral sp 3 hybridization 5 trig. bipyr. sp 3 d 6 octahedral sp 3 d 2 103

104 SCl 2 BI 3 NCl 3 AlCl - 4 PF 5 SF 6 I - 3 Predict the hybrid orbitals formed on the central atom in the following molecules 104

105 SCl 2 BI 3 NCl 3 AlCl - 4 PF 5 SF 6 I - 3 Predict the hybrid orbitals formed on the central atom in the following molecules 105

106 Predicting orbital hybridization 1. Draw Lewis structure 2. Determine e - pair geometry (VSEPR) 3. Determine hybrid orbitals needed to give correct e - pair geometry # of e - pairs e - pair geometry 2 linear sp 3 ti trig. planar sp 2 4 tetrahedral sp 3 hybridization 5 trig. bipyr. sp 3 d 6 octahedral sp 3 d 2 106

107 107

108 108

109 Sigma (σ) bonds and pi (π) bonds 109

110 Single bonds; sigma (σ) bonds bonding electrons are centered on the internuclear axis, the line connecting the two nuclei of the bonded atoms H H H H 110

111 Multiple bonds; pi (π) bonds bonding electrons are in nonhybridizied p orbitals perpendicular to the internuclear axis (the line joining the two nuclei) z y z y z y x x x p y p z p x The regions of overlapping e - density are above and below the internuclear axis A double bond = one sigma and one pi bond At triple bond = one sigma and two pi bonds 111

112 Bonding in ethylene, C 2 H 2. There is trigonal planar molecular geometry about each C. Sigma bonds are shown as sticks 112

113 Bonding in ethylene, C 2 H 2. The C-C and C-H sigma bonds are represented as blue spheres 113

114 Bonding in ethylene, C 2 H 2. The pi bond is formed from overlap of electrons in the nonhybridized p orbital 114

115 Bonding in ethylene, C 2 H 2. The pi bond is formed from overlap of electrons in the nonhybridized p orbital 115

116 Bonding in ethylene, C 2 H 2. The pi bond is formed from overlap of electrons in the nonhybridized p orbital 116

117 Bonding in ethylene, C 2 H 2. The pi bond is formed from overlap of electrons in the nonhybridized p orbital 117

118 Account for the bonding in C 2 H 4 by writing out the orbital boxes for C and H, and hybridizing appropriate orbitals 1. Draw Lewis structure t Types and numbers of bonds Lone pairs 2. Determine e - pair geometry (VSEPR) 3. Determine hybrid orbitals needed to give correct e - pair geometry (sigma bonds) 4. Account for presence of double and triple bonds using available nonhybridized p orbitals (pi bonds) 118

119 Predict the types and numbers of bonds in acetylene, C 2 H 2 1. Draw Lewis structure Types and numbers of bonds Lone pairs 2. Determine e - pair geometry (VSEPR) 3. Determine hybrid orbitals needed to give correct e - pair geometry (sigma bonds) 4. Account for presence of double and triple bonds using available nonhybridized p orbitals (pi bonds) 119

120 Bonding in acetylene, C 2 H 2 Orbitals on an sp hybridized C atom: sp sp = sp C p p y z p z p y sp Orbital on a H atom: H Orbital overlap between 2C and 2H: H sp p z C p y sp sp p z C p y sp H 120

121 Bonding in acetylene, C 2 H 2 Orbitals on an sp hybridized C atom: sp sp = sp C p p y z p z p y sp Orbital on a H atom: H Orbital overlap between 2C and 2H: H sp p z C p y sp p z C p y sp H 121

122 Predict the types and numbers of bonds in formaldehyde, CH 2 O 1. Draw Lewis structure Types and numbers of bonds Lone pairs 2. Determine e - pair geometry (VSEPR) 3. Determine hybrid orbitals needed to give correct e - pair geometry (sigma bonds) 4. Account for presence of double and triple bonds using available nonhybridized p orbitals (pi bonds) 122

123 You don t have to prove the bonding by drawing the orbital boxes like I have I did this to show you what is happening at the molecular level All you have to do is: draw the Lewis structure count tthe electron domains about an atom predict hybridization from number of domains single bonds are sigma bonds double and triple bonds are pi bonds 123

124 Predict the types and numbers of bonds in carbon monoxide 1. Draw Lewis structure Types and numbers of bonds Lone pairs 2. Determine e - pair geometry (VSEPR) 3. Determine hybrid orbitals needed to give correct e - pair geometry (sigma bonds) 4. Account for presence of double and triple bonds using available nonhybridized p orbitals (pi bonds) 124

125 Predict the types and numbers of bonds in formaldehyde, CH 2 O 1. Draw Lewis structure Types and numbers of bonds Lone pairs 2. Determine e - pair geometry (VSEPR) 3. Determine hybrid orbitals needed to give correct e - pair geometry (sigma bonds) 4. Account for presence of double and triple bonds using available nonhybridized p orbitals (pi bonds) 125

126 Molecular Orbital Theory 126

Lewis Dot Structures for Methane, CH 4 The central C atom is bonded by single bonds (-) to 4 individual H atoms

Lewis Dot Structures for Methane, CH 4 The central C atom is bonded by single bonds (-) to 4 individual H atoms Chapter 10 (Hill/Petrucci/McCreary/Perry Bonding Theory and Molecular Structure This chapter deals with two additional approaches chemists use to describe chemical bonding: valence-shell electron pair