Chemical Bonding II Molecular Geometry Valence Bond Theory Phys./Chem. Properties Quantum Mechanics Sigma & Pi bonds ybridization MO theory 1
Molecular Geometry 3-D arrangement of atoms 2
VSEPR Valence-shell electron-pair repulsion theory. All valence shell e - pairs (or e - domains ) repel each other. (Coulomb s law as applied to the repulsion of valence electrons.) Determines the geometry of e - domains around central atom. 3
VSEPR Theory: Rules 1.Single, double, triple bonds and lone pairs are treated as one e - domain (approximation) 2.Apply VSEPR to any one resonance structure 3.Geometry move e - domains as far apart as possible in 3-D space. 4
AB 2 (with no lone pairs) Beryllium chloride BeCl 2 e - Be 180 o atoms Cl Be Cl linear geometry 5
AB 3 (with no lone pairs) Boron trifluoride B 3 120 o B Trigonal planar 6
AB 4 (with no lone pairs) Methane C 4 C 109.5 o tetrahedron 7
AB 5 (with no lone pairs) Phosphorus Pentachloride PCl 5 90 o axial equatorial 120 o trigonal bipyramid 8
AB 6 (with no lone pairs) Sulfur hexafluoride S 6 S All angles 90 o & 180 o octahedral 9
Quiz: Name That Shape! AB 2 AB 3 AB 4 AB 5 AB 6 10
VSEPR: Lone Pairs Molecules in which the central atom has lone pair(s) AB x E y central atom lone pairs on A surrounding atoms 11
AB x E y Geometry is similar to AB, but x nonbonding pairs are treated like bonding pairs to determine geometry of the e - domains. 12
VSEPR Bonding e - take up less space than nonbonding e - Bonding e - are focused between the nuclei. Example: water 13
VSEPR Decreasing e- pair repulsion lone pair lone pair bonding pair vs vs vs lone pair bonding pair bonding pair repulsion repulsion repulsion 14
AB 2 E Sulfur dioxide SO 2 Draw Lewis structure O S O S 3 e - domains O O OSO angle < 120 o 15
Shape: e - Pairs vs. Atoms S S O O O O e - domains are trigonal planar molecule is bent linear (always state molecular shape) 16
AB 3 E Ammonia N e - tetrahedral trigonal pyramidal 17
AB 2 E 2 Water O e - tetrahedral molecule: bent linear 18
Comparison AB 4 AB 3 E AB 2 E 2 Methane ammonia water C N O 109.5 107.3 104.5 19
AB 4 E Sulfur tetrafluoride S 4 Draw Lewis structure S Predict: trigonal bipyramid for e - domains. 20
AB 4 E: S 4 trigonal bipyramid: 2 choices distorted tetrahedron or seesaw 21
AB 3 E 2 Cl 3 A little weird. molecule: T-shaped 22
AB 2 E 3 Three possibilities for I 3 - Want nonbonding e - domains farthest apart. 23
I 3 - AB 2 E 3 I - molecule: linear I Lone pairs always go on equatorial position(s) 24
AB 5 E Br 5 molecule: square pyramidal 25
AB 4 E 2 : 2 Possibilities Xe 4 1 Want nonbonding e- pairs farthest apart. 2 26
AB 4 E 2 Xe 4 molecule: square planar 27
You Try It Predict geometry and approximate bond angles for: AlCl 4 - Xe 2 XeO 2 Text: Tables 10.1 and 10.2. 28
Bond Polarity d+ d- is more electronegative than. Shift of e - density toward, Thus is polar. 29
Overall Molecular Polarity pull test O C O Even though CO 2 has polar bonds, it is nonpolar since the individual bond polarities add to zero. Dipoles are a vector quantities. Symmetrical = nonpolar (Recall the Regents rule about lone pairs on the central atom.) 30
Molecular Polarity Molecule Geometry Dipole Strength Linear 1.92 Br Linear 1.08 Water Bent 1.87 SO 2 Bent 1.60 31
Molecular Polarity Which has larger polarity? N 3 N 3 N N m = 1.46 D m =0.24 D 32
Dipole Moments Predict whether the following molecules are polar. IBr C Cl 2 2 AlCl 3 Remember to distinguish between bond polarity and molecular polarity. 33
Shortfall of VSEPR Both 2 & 2 have single bonds, but Bond length Bond energy 2 74 pm 436 kj 2 142 pm 151 kj 34
Quantum Mechanics to the rescue Valence Bond Theory e - in molecule occupy blended atomic orbitals Molecular Orbital Theory molecule has molecular orbitals 35
Valence Bond Theory Atomic orbitals (s, p, d ) of the valence electrons hybridize or mix to form new orbitals for the molecule. 36
Valence Bond Theory C Consider tetrahedral C 4 carbon atomic orbitals 2s 2p ow can carbon form four equal bonds with four hydrogen atoms using its atomic s and p orbitals? 37
C 4 : sp 3 ybridization C carbon atomic orbitals 2s 2p C carbon hybridized orbitals sp 3 (all the same) 38
ybridization Analogy s p p p 4 sp 3 hybrid orbitals 39
sp 3 ybridization It takes energy to form hybrid orbitals, but this energy is more than compensated by bond formation. 40
sp 3 Orbital Shapes One s & three p orbitals change to four sp 3 hybrid orbitals. tetrahedral Each orbital can hold 2 e - 41
C 4 Bonding The 4 sp 3 hybrid orbitals of C bond (overlap) with the 4 1s atomic orbitals of the atoms. C 42
N 3 sp 3 Bonding N nitrogen atomic orbitals 2s 2p N nitrogen hybridized orbitals sp 3 (all the same) 43
N 3 sp 3 Bonding sp 3 lone pair bonding orbitals 44
ybridization irst use VSEPR to predict the arrangement of e - pairs, then use hybridization to predict the type of bonding. 45
sp ybridization Be Be atomic 2s 2p orbitals Be Be hybrid orbitals sp 2p 46
sp ybridization: BeCl 2 Cl Be Cl one atomic p orbital of Cl two sp hybrid orbitals of Be 47
sp 2 ybridization B B atomic orbitals 2s 2p B B hybrid orbitals sp 2 2p 48
sp 2 ybridization: B 3 one p orbital of B three sp 2 hybrid orbitals of B 49
ybridization ybridization for 2 nd Period elements (C, N, O, ) explains the octet rule, since there are 4 hybrid orbitals formed from one s and three p atomic orbitals. s p sp 3 50
ybridization: Review 1.Not applied to isolated atoms 2.irst determine VSEPR geometry 3.Mix nonequivalent atomic orbitals of central atom to form hybrid orbitals 51
ybridization: Review 4.Requires energy, but energy is more than returned by bond formation 5.Covalent bonds formed by overlap of hybrid-hybrid and/or hybrid-unhybridized orbital 52
ybridization: Try It Determine hybridization in: AlBr 3 P 3 gcl 2 53
Let s Not orget d Orbitals or elements in the 3 rd Period and higher, hybridization can also include d orbitals. d-orbital hybridization is still being debated! 54
d ybridization S 6 VSEPR predicts octahedral geometry S 55
ybridization: S 6 S atomic orbitals 3s 3p 3d S hybridized sp 3 d 2 orbitals sp 3 d 2 3d 56
ybridization You Try It. What is the VSEPR geometry and the hybridization in PBr 5? 57
Summary: ybrid Orbitals 2 e - pairs (sp) linear e - domains (bonding & nonbonding pairs) 3 e - pairs (sp 2 ) trigonal planar 4 e - pairs (sp 3 ) tetrahedral 58
Summary: ybrid Orbitals 5 e - pairs (sp 3 d) trigonal bipyramid 6 e - pairs (sp 3 d 2 ) octahedral 59
Multiple Bonds In VSEPR, no distinction was made among single, double, triple bond or lone pair. All were counted as an e - domain. C 2 4 C C Each carbon is trigonal planar 60
Double Bond: C 2 4 C atomic orbitals 2s 2p C hybrid orbitals sp 2 + p z unhybridized p orbital 61
Double Bond: C 2 4 C hybrid orbitals sp 2 + p z trigonal planar dumb bell 62
Double Bonds: C 2 4 C C 63
Double Bond: C 2 4 Sigma (s) and Pi (p) Bonds (model) 64
Pi & Sigma Bonds Sigma (s) bond: covalent bond formed by e - overlap along the axis connecting atoms Pi (p) bond: covalent bond formed by sideways e - overlap above and below the plane connecting atoms (weaker than s) 65
Double Bond: C 2 4 C C The double bond is one sigma and one pi bond between the carbon atoms 66
Triple Bond: -C=C- C atomic orbitals 2s 2p C hybrid orbitals sp + p y p z unhybridized p orbitals 67
Triple Bond: C 2 2 unhybridized p orbitals hybridized sp orbitals ybridize s bonds only 68
ybridizing Shortcuts 1.Single bonds: sigma 2.Double bonds: one sigma, one pi 3.Triple bonds: one sigma, two pi 4.ybridize: add the number of sigma bonds plus lone pairs s p d 69
You Try It Describe the bonding and hybridization for each atom in: formaldehyde, C 2 O hydrogen cyanide, CN (assign formal charges) 70
Delocalized Molecular Orbitals An explanation of resonance. O O O O O O ozone Ozone is a blend of the two resonance structures. 71
Delocalized Molecular Orbitals The p component of the double bond is delocalized over the molecule. sp 2 sp 2 sp 2 72
Delocalized MOs e - in s bonds localized e - in p bonds-- delocalized benzene 73
Sigma bonds (localized) Benzene sp 2 C C sp 2 C C C C 74
Benzene sp 2 + p z p bonds (delocalized) 75
Benzene Bond order 1.5 Conjugated double bonds are more stable (less reactive) 76
Carbonate Ion O C O O 2- + resonance structures C: sp 2 for s bonds (localized) plus a 2p z for p bond (delocalized) O: each has a 2p z orbital (delocalized) 77
Carbonate Ion o Additional stability due to delocalization of the pi bond. o C o C o o o 78
You Try It Would you predict the NO 3 - anion to have additional stability due to delocalization of the p electrons? 79
Polyatomic Ions Many common polyatomic ions have delocalized pi bonds, partially accounting for their additional stability in chemical reactions. CO 3-2, NO 3 -, ClO 3 - etc. 80
CaCO 3 Ionic & Covalent Ionic compounds containing polyatomic ions have both ionic and covalent bonds! O 2- Ca 2+ O C O Trigonal planar, sp 2, 12081 o