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Molecular Geometries and Bonding Molecular Geometries and Bonding Chapter 9 Molecular Geometries and Bonding Theories Chemistry, The Central Science, 10th.

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Presentation on theme: "Molecular Geometries and Bonding Molecular Geometries and Bonding Chapter 9 Molecular Geometries and Bonding Theories Chemistry, The Central Science, 10th."— Presentation transcript:

1 Molecular Geometries and Bonding Molecular Geometries and Bonding Chapter 9 Molecular Geometries and Bonding Theories Chemistry, The Central Science, 10th edition Theodore L. Brown, H. Eugene LeMay, Jr., and Bruce E. Bursten John D. Bookstaver St. Charles Community College St. Peters, MO  2006, Prentice-Hall, Inc.

2 Molecular Geometries and Bonding Molecular Geometries and Bonding November 23 ONLINE PRACTICE QUESTIONS UNIT 8 – BY NEXT SUNDAY!!! WATCH PODCASTS 8-3 is this lesson 8-1 and 8-2 review of chapter 8 Molecular Shapes VSEPR theory Electron Domain Geometry Molecular Geometry Hw for section 9.1 and 9.2 : 1,2,3,11 to 23 odd

3 Molecular Geometries and Bonding Molecular Geometries and Bonding What Determines the Shape of a Molecule? The Lewis structure is drawn with the atoms all in the same plane. The overall shape of the molecule will be determined by its bond angles.

4 Molecular Geometries and Bonding Molecular Geometries and Bonding What Determines the Shape of a Molecule? Electron pairs, whether they be bonding or nonbonding, repel each other. By assuming the electron pairs are placed as far as possible from each other, we can predict the shape of the molecule.

5 Molecular Geometries and Bonding Molecular Geometries and Bonding There are five fundamental geometries for molecular shape

6 Molecular Geometries and Bonding Molecular Geometries and Bonding

7 Molecular Geometries and Bonding Molecular Geometries and Bonding Electron Domains We can refer to the electron pairs as electron domains. Each pair of electrons count as an electron domain, whether they are in a lone pair, in a single, double or triple bond. This molecule has four electron domains.

8 Molecular Geometries and Bonding Molecular Geometries and Bonding Electron-Domain Geometries All one must do is count the number of electron domains in the Lewis structure. The geometry will be that which corresponds to that number of electron domains.

9 Molecular Geometries and Bonding Molecular Geometries and Bonding In order to predict molecular shape, we assume the valence electrons repel each other. Therefore, the molecule adopts whichever 3D geometry minimizes this repulsion. We call this process Valence Shell Electron Pair Repulsion (VSEPR) theory. There are simple shapes for AB 2 to AB 6 molecules.There are simple shapes for AB 2 to AB 6 molecules

10 Molecular Geometries and Bonding Molecular Geometries and Bonding Electron domain geometry : When considering the geometry about the central atom, we consider all electrons (lone pairs and bonding pairs). When naming the molecular geometry, we focus only on the positions of the atoms.

11 Molecular Geometries and Bonding Molecular Geometries and Bonding To determine the shape of a molecule, we distinguish between lone pairs (or non-bonding pairs, those not in a bond) of electrons and bonding pairs (those found between two atoms). We define the electron domain geometry (or orbital geometry) by the positions in 3D space of ALL electron pairs (bonding or non-bonding). The electrons adopt an arrangement in space to minimize e  -e  repulsion. VSEPR Model

12 Molecular Geometries and Bonding Molecular Geometries and Bonding

13 Molecular Geometries and Bonding Molecular Geometries and Bonding

14 Molecular Geometries and Bonding Molecular Geometries and Bonding Examples – Draw the Lewis structures, and then determine the orbital geometry of each. Indicate the number of electron domains first: 1.H 2 S 2.CO 2 3.PCl 3 4.CH 4 5.SO 2

15 Molecular Geometries and Bonding Molecular Geometries and Bonding Examples – Draw the Lewis structures, and then determine the orbital geometry of each: e - domain orbital geometry or e - domain geom 1.H 2 S4 tetrahedral 2.CO 2 2 linear 3.PCl 3 4 tetrahedral 4.CH 4 4 tetrahedral 5.SO 2 3 trigonal planar

16 Molecular Geometries and Bonding Molecular Geometries and Bonding To determine the electron pair ( electron domain) geometry: draw the Lewis structure, count the total number of electron pairs around the central atom, arrange the electron pairs in one of the above geometries to minimize e  -e  repulsion, and count multiple bonds as one bonding pair. But then we have to account for the shape of the molecule

17 Molecular Geometries and Bonding Molecular Geometries and Bonding Molecular Geometries Within each electron domain, then, there might be more than one molecular geometry.

18 Molecular Geometries and Bonding Molecular Geometries and Bonding Linear Electron Domain In this domain, there is only one molecular geometry: linear. NOTE: If there are only two atoms in the molecule, the molecule will be linear no matter what the electron domain is.

19 Molecular Geometries and Bonding Molecular Geometries and Bonding Nonbonding Pairs and Bond Angle Nonbonding pairs are physically larger than bonding pairs. Therefore, their repulsions are greater; this tends to decrease bond angles in a molecule.

20 Molecular Geometries and Bonding Molecular Geometries and Bonding By experiment, the H-X-H bond angle decreases on moving from C to N to O: Since electrons in a bond are attracted by two nuclei, they do not repel as much as lone pairs. Therefore, the bond angle decreases as the number of lone pairs increase.

21 Molecular Geometries and Bonding Molecular Geometries and Bonding Similarly, electrons in multiple bonds repel more than electrons in single bonds.

22 Molecular Geometries and Bonding Molecular Geometries and Bonding Multiple Bonds and Bond Angles Double and triple bonds place greater electron density on one side of the central atom than do single bonds. Therefore, they also affect bond angles.

23 Molecular Geometries and Bonding Molecular Geometries and Bonding Trigonal Planar Electron Domain There are two molecular geometries:  Trigonal planar, if all the electron domains are bonding  Bent, if one of the domains is a nonbonding pair.

24 Molecular Geometries and Bonding Molecular Geometries and Bonding Tetrahedral Electron Domain There are three molecular geometries:  Tetrahedral, if all are bonding pairs  Trigonal pyramidal if one is a nonbonding pair  Bent if there are two nonbonding pairs

25 Molecular Geometries and Bonding Molecular Geometries and Bonding

26 Molecular Geometries and Bonding Molecular Geometries and Bonding Examples – Same examples as before, now determine the the molecular geometry of each, including shapes and bond angles: 1.H 2 S 2.CO 2 3.PCl 3 4.CH 4 5.SO 2

27 Molecular Geometries and Bonding Molecular Geometries and Bonding Examples – Determine the molecular geometry of each, including shapes and bond angles: Shape Angles 1.H 2 Sbent <109° 2.CO 2 linear180° 3.PCl 3 trigonal pyramid<109° 4.CH 4 tetrahedral109.5° 5.SO 2 bent <120°

28 Molecular Geometries and Bonding Molecular Geometries and Bonding Molecules with Expanded Valence Shells For elements of the 3 rd shell and below, some atoms can have expanded octets. AB 5 (trigonal bipyramidal) or AB 6 (octahedral) electron pair geometries.

29 Molecular Geometries and Bonding Molecular Geometries and Bonding Trigonal Bipyramidal Electron Domain (5 e domains) There are two distinct positions in this geometry:  Axial  Equatorial

30 Molecular Geometries and Bonding Molecular Geometries and Bonding Molecules with Expanded Valence Shells To minimize e   e  repulsion, lone pairs are always placed in equatorial positions.

31 Molecular Geometries and Bonding Molecular Geometries and Bonding Trigonal Bipyramidal (e - domain) There are four distinct molecular geometries in this domain: *Trigonal bipyramidal *Seesaw *T-shaped *Linear

32 Molecular Geometries and Bonding Molecular Geometries and Bonding

33 Molecular Geometries and Bonding Molecular Geometries and Bonding Trigonal Bipyramidal Electron Domain Lower-energy conformations result from having nonbonding electron pairs in equatorial, rather than axial, positions in this geometry.

34 Molecular Geometries and Bonding Molecular Geometries and Bonding Octahedral electron domain 6 electron pairs All positions are equivalent in the octahedral domain. There are three molecular geometries: *Octahedral *Square pyramidal *Square planar

35 Molecular Geometries and Bonding Molecular Geometries and Bonding

36 Molecular Geometries and Bonding Molecular Geometries and Bonding

37 Molecular Geometries and Bonding Molecular Geometries and Bonding Examples – Determine the Shape of each, indicate the electron domain, molecular geometry and angles. 1.PF 5 2.XeF 4 3.SF 6 4.SCl 4

38 Molecular Geometries and Bonding Molecular Geometries and Bonding Examples – Determine the Shape of each: 1.PF 5 trigonal bipyramid90°, 120° 2.XeF 4 square planar 90° 3.SF 6 octahedral 90° 4.SCl 4 see-saw <90°,< 120° See moving chart

39 Molecular Geometries and Bonding Molecular Geometries and Bonding November 28 Large molecules Molecular shape and Polarity Hybridization Multiple bonding HW 25, 26, 35, 47, 51, 55

40 Molecular Geometries and Bonding Molecular Geometries and Bonding Larger Molecules In larger molecules, it makes more sense to talk about the geometry about a particular atom rather than the geometry of the molecule as a whole.

41 Molecular Geometries and Bonding Molecular Geometries and Bonding Larger Molecules This approach makes sense, especially because larger molecules tend to react at a particular site in the molecule.

42 Molecular Geometries and Bonding Molecular Geometries and Bonding Shapes of Larger Molecules In acetic acid, CH 3 COOH, there are three central atoms. We assign the geometry about each central atom separately.

43 Molecular Geometries and Bonding Molecular Geometries and Bonding 1 2 Examples – Determine the shape and angles about each atom:

44 Molecular Geometries and Bonding Molecular Geometries and Bonding Examples – Determine the shape and angles about each atom: 1 2 Trigonal planarlinear Benttrigonal pyramid Trigonal planar

45 Molecular Geometries and Bonding Molecular Geometries and Bonding When there is a difference in electronegativity between two atoms, then the bond between them is polar. It is possible for a molecule to contain polar bonds, but not be polar. For example, the bond dipoles in CO 2 cancel each other because CO 2 is linear. Molecular Shape and Molecular Polarity

46 Molecular Geometries and Bonding Molecular Geometries and Bonding

47 Molecular Geometries and Bonding Molecular Geometries and Bonding In water, the molecule is not linear and the bond dipoles do not cancel each other. Therefore, water is a polar molecule. The overall polarity of a molecule depends on its molecular geometry.

48 Molecular Geometries and Bonding Molecular Geometries and Bonding

49 Molecular Geometries and Bonding Molecular Geometries and Bonding Polarity By adding the individual bond dipoles, one can determine the overall dipole moment for the molecule.

50 Molecular Geometries and Bonding Molecular Geometries and Bonding Polarity

51 Molecular Geometries and Bonding Molecular Geometries and Bonding To remember Trigonal Planar molecular shape if the 3 surroundings atoms are the same the molecule is non polar molecule because the dipoles cancel each other. Tetrahedral: if 4 surrounding atoms are the same molecule non-polar. This is important for carbon compounds with 4 single bonds.

52 Molecular Geometries and Bonding Molecular Geometries and Bonding Examples – Determine whether each is polar or nonpolar: 1.CCl 4 2.PCl 3 3.BF 3 4.BrClFCH 5.SO 3

53 Molecular Geometries and Bonding Molecular Geometries and Bonding Examples – Determine whether each is polar or nonpolar: 1.CCl 4 nonpolar 2.PCl 3 polar 3.BF 3 nonpolar 4.BrClFCHpolar 5.SO 3 nonpolar

54 Molecular Geometries and Bonding Molecular Geometries and Bonding Overlap and Bonding We think of covalent bonds forming through the sharing of electrons by adjacent atoms. In such an approach this can only occur when orbitals on the two atoms overlap.

55 Molecular Geometries and Bonding Molecular Geometries and Bonding OverlapOverlap and Bonding Increased overlap brings the electrons and nuclei closer together while simultaneously decreasing electron- electron repulsion. However, if atoms get too close, the internuclear repulsion greatly raises the energy.

56 Molecular Geometries and Bonding Molecular Geometries and Bonding HYBRIDIZATION VALENCE BOND THEORY (  and  bonds  BOND ORDER

57 Molecular Geometries and Bonding Molecular Geometries and Bonding Hybrid Orbitals It’s hard to imagine tetrahedral, trigonal bipyramidal, and other geometries arising from the atomic orbitals we recognize.

58 Molecular Geometries and Bonding Molecular Geometries and Bonding Atomic orbitals can mix or hybridize in order to adopt an appropriate geometry for bonding. Hybridization is determined by the electron domain geometry. sp Hybrid Orbitals Consider the BeF 2 molecule (experimentally known to exist): Hybrid Orbitals Hybrid Orbitals

59 Molecular Geometries and Bonding Molecular Geometries and Bonding Hybrid Orbitals Consider beryllium:  In its ground electronic state, it would not be able to form bonds because it has no singly-occupied orbitals.

60 Molecular Geometries and Bonding Molecular Geometries and Bonding Hybrid Orbitals But if it absorbs the small amount of energy needed to promote an electron from the 2s to the 2p orbital, it can form two bonds.

61 Molecular Geometries and Bonding Molecular Geometries and Bonding Hybrid Orbitals Mixing the s and p orbitals yields two degenerate orbitals that are hybrids of the two orbitals.  These sp hybrid orbitals have two lobes like a p orbital.  One of the lobes is larger and more rounded as is the s orbital.

62 Molecular Geometries and Bonding Molecular Geometries and Bonding Hybrid Orbitals (sp) These two degenerate orbitals would align themselves 180  from each other. This is consistent with the observed geometry of beryllium compounds: linear.

63 Molecular Geometries and Bonding Molecular Geometries and Bonding Hybrid Orbitals With hybrid orbitals the orbital diagram for beryllium would look like this. The sp orbitals are higher in energy than the 1s orbital but lower than the 2p.

64 Molecular Geometries and Bonding Molecular Geometries and Bonding

65 Molecular Geometries and Bonding Molecular Geometries and Bonding Since only one of the Be 2p orbitals has been used in hybridization, there are two unhybridized p orbitals remaining on Be.

66 Molecular Geometries and Bonding Molecular Geometries and Bonding sp 2 Hybrid Orbitals Important: when we mix n atomic orbitals we must get n hybrid orbitals. sp 2 hybrid orbitals are formed with one s and two p orbitals. (Therefore, there is one unhybridized p orbital remaining.) The large lobes of sp 2 hybrids lie in a trigonal plane. All molecules with trigonal planar electron pair geometries have sp 2 orbitals on the central atom.

67 Molecular Geometries and Bonding Molecular Geometries and Bonding Hybrid Orbitals (sp 2 ) For boron

68 Molecular Geometries and Bonding Molecular Geometries and Bonding Hybrid Orbitals …three degenerate sp 2 orbitals.

69 Molecular Geometries and Bonding Molecular Geometries and Bonding November 29 HYBRIDIZATION WITH MULTIPLE BONDS – VALENCE BOND THEORY ORDER OF A BOND PARAMAGNETIC DIAMAGNETIC MOLECULES PRACTICE PROBLEMS

70 Molecular Geometries and Bonding Molecular Geometries and Bonding Hybrid Orbitals With carbon we get…

71 Molecular Geometries and Bonding Molecular Geometries and Bonding Hybrid Orbitals …four degenerate sp 3 orbitals.

72 Molecular Geometries and Bonding Molecular Geometries and Bonding sp 2 and sp 3 Hybrid Orbitals sp 3 Hybrid orbitals are formed from one s and three p orbitals. Therefore, there are four large lobes. Each lobe points towards the vertex of a tetrahedron. The angle between the large lobs is . All molecules with tetrahedral electron pair geometries are sp 3 hybridized.

73 Molecular Geometries and Bonding Molecular Geometries and Bonding Hybridization Involving d Orbitals Since there are only three p-orbitals, trigonal bipyramidal and octahedral electron domain geometries must involve d-orbitals. Trigonal bipyramidal electron domain geometries require sp 3 d hybridization. Octahedral electron domain geometries require sp 3 d 2 hybridization. Note the electron domain geometry from VSEPR theory determines the hybridization.

74 Molecular Geometries and Bonding Molecular Geometries and Bonding Sp 3 d and sp 3 d 2 Hybrid Orbitals For geometries involving expanded octets on the central atom, we must use d orbitals in our hybrids.

75 Molecular Geometries and Bonding Molecular Geometries and Bonding Hybrid Orbitals This leads to five degenerate sp 3 d orbitals… …or six degenerate sp 3 d 2 orbitals.

76 Molecular Geometries and Bonding Molecular Geometries and Bonding Summary 1.Draw the Lewis structure. 2.Determine the electron domain geometry with VSEPR. 3.Specify the hybrid orbitals required for the electron pairs based on the electron domain geometry. Hybrid Orbitals

77 Molecular Geometries and Bonding Molecular Geometries and Bonding

78 Molecular Geometries and Bonding Molecular Geometries and Bonding

79 Molecular Geometries and Bonding Molecular Geometries and Bonding Hybrid Orbitals Once you know the electron-domain geometry, you know the hybridization state of the atom.

80 Molecular Geometries and Bonding Molecular Geometries and Bonding Examples – Determine the hybridization on the central atom of each: 1.NCl 3 2.CO 2 3.H 2 O 4.SF 4 5.BF 3 6.XeF 4

81 Molecular Geometries and Bonding Molecular Geometries and Bonding Examples – Determine the hybridization on the central atom of each: 1.NCl 3 sp 3 2.CO 2 sp 3.H 2 Osp 3 4.SF 4 sp 3 d 5.BF 3 sp 2 6.XeF 4 sp 3 d 2

82 Molecular Geometries and Bonding Molecular Geometries and Bonding Examples – Determine the hybridization on EACH atom:

83 Molecular Geometries and Bonding Molecular Geometries and Bonding Examples – Determine the hybridization on EACH atom: sp 2 sp sp 3 sp 2 sp 3

84 Molecular Geometries and Bonding Molecular Geometries and Bonding Valence Bond Theory Hybridization is a major player in this approach to bonding. There are two ways orbitals can overlap to form bonds between atoms.

85 Molecular Geometries and Bonding Molecular Geometries and Bonding Sigma (  ) Bonds Sigma bonds are characterized by  Head-to-head overlap.  Cylindrical symmetry of electron density about the internuclear axis.

86 Molecular Geometries and Bonding Molecular Geometries and Bonding Pi (  ) Bonds Pi bonds are characterized by  Side-to-side overlap.  Electron density above and below the internuclear axis.

87 Molecular Geometries and Bonding Molecular Geometries and Bonding Single Bonds Single bonds are always  bonds, because  overlap is greater, resulting in a stronger bond and more energy lowering.

88 Molecular Geometries and Bonding Molecular Geometries and Bonding Multiple Bonds In a multiple bond one of the bonds is a  bond and the rest are  bonds.

89 Molecular Geometries and Bonding Molecular Geometries and Bonding Multiple Bonds In a molecule like formaldehyde (shown at left) an sp 2 orbital on carbon overlaps in  fashion with the corresponding orbital on the oxygen. The unhybridized p orbitals overlap in  fashion.

90 Molecular Geometries and Bonding Molecular Geometries and Bonding Multiple Bonds In triple bonds, as in acetylene, two sp orbitals form a  bond between the carbons, and two pairs of p orbitals overlap in  fashion to form the two  bonds.

91 Molecular Geometries and Bonding Molecular Geometries and Bonding Delocalized Electrons: Resonance When writing Lewis structures for species like the nitrate ion, we draw resonance structures to more accurately reflect the structure of the molecule or ion.

92 Molecular Geometries and Bonding Molecular Geometries and Bonding Delocalized Electrons: Resonance In reality, each of the four atoms in the nitrate ion has a p orbital. The p orbitals on all three oxygens overlap with the p orbital on the central nitrogen.

93 Molecular Geometries and Bonding Molecular Geometries and Bonding Delocalized Electrons: Resonance This means the  electrons are not localized between the nitrogen and one of the oxygens, but rather are delocalized throughout the ion.

94 Molecular Geometries and Bonding Molecular Geometries and Bonding Resonance The organic molecule benzene has six  bonds and a p orbital on each carbon atom.

95 Molecular Geometries and Bonding Molecular Geometries and Bonding Resonance In reality the  electrons in benzene are not localized, but delocalized. The even distribution of the  electrons in benzene makes the molecule unusually stable.

96 Molecular Geometries and Bonding Molecular Geometries and Bonding General Conclusions Every two atoms share at least 2 electrons. Two electrons between atoms on the same axis as the nuclei are  bonds.  -Bonds are always localized. If two atoms share more than one pair of electrons, the second and third pair form  -bonds. When resonance structures are possible, delocalization is also possible.

97 Molecular Geometries and Bonding Molecular Geometries and Bonding Bond Order Bond Order = total number of covalent bonds between 2 atoms Bond order = 1 for single bond. Bond order = 2 for double bond. Bond order = 3 for triple bond. If there is resonance then divide the bond order by the number of atoms that share the resonance structure. Fractional bond orders are possible (with resonance)

98 Molecular Geometries and Bonding Molecular Geometries and Bonding Examples – Draw Lewis Structures (including resonance). Determine the total number of σ and π bonds in the molecule, and the bond order of each bond: 1.O 3 2.SO 3 3.CO 2

99 Molecular Geometries and Bonding Molecular Geometries and Bonding Examples – Draw Lewis Structures (including resonance). Determine the total number of σ and π bonds in the molecule, and the bond order of each bond: 1.O 3 2.SO 3 3.CO 2 2σ and 1π Each bond 1.5 3σ and 1πeach bond σ and 2πeach bond 2

100 Molecular Geometries and Bonding Molecular Geometries and Bonding Electron Configurations and Molecular Properties Two types of magnetic behavior: paramagnetism (unpaired electrons in atom or molecule): strong attraction between magnetic field and molecule; diamagnetism (no unpaired electrons in atom or molecule): weak repulsion between magnetic field and molecule. Magnetic behavior is detected by determining the mass of a sample in the presence and absence of magnetic field:

101 Molecular Geometries and Bonding Molecular Geometries and Bonding Second-Row Diatomic Molecules Electron Configurations and Molecular Properties large increase in mass indicates paramagnetism, small decrease in mass indicates diamagnetism.


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