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1
Chemical Bonding II
Molecular Geometry
Valence Bond Theory
Phys./Chem. Properties
Quantum Mechanics
Sigma & Pi bonds
Hybridization
MO theory
2
Molecular Geometry
3-D arrangement of atoms
3
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.
4
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.
5
AB2 (with no lone pairs)
Beryllium chloride BeCl2
— Be—
Cl—Be—Cl
180o
linear geometry
e-
atoms
6
AB3 (with no lone pairs)
Boron trifluoride BF3
B
F
FF
Trigonal planar
120o
7
C
AB4 (with no lone pairs)
Methane CH4
tetrahedron
109.5o
8
AB5 (with no lone pairs)
Phosphorus Pentachloride PCl5
trigonal bipyramid
equatorial
axial90o
120o
9
SF
F
F
F
F
F
AB6 (with no lone pairs)
Sulfur hexafluoride SF6
octahedral
All angles
90o & 180o
10
Quiz: Name That Shape!
AB2
AB3
AB4
AB5
AB6
11
VSEPR: Lone Pairs
Molecules in which the
central atom has lone pair(s)
ABxEy
central atom
surrounding atoms
lone pairs on A
12
ABxEy
Geometry is similar to ABx, but
nonbonding pairs are treated like
bonding pairs to determine
geometry of the e- domains.
VSEPR
13
Bonding e- take up less space
than nonbonding e-
Example: water
Bonding e- are
“focused” between
the nuclei.
14
VSEPR
lone pair lone pair bonding pair
vs vs vs
lone pair bonding pair bonding pair
repulsion repulsion repulsion
Decreasing e- pair repulsion
15
AB2E
Sulfur dioxide SO2
O S—O
OSO angle < 120o
Draw Lewis structure
S
O O3 e- domains
16
Shape: e- Pairs vs. Atoms
S
O O
e- domains are trigonal planar
S
O O
molecule is bent linear
(always state molecular shape)
17e- tetrahedral trigonal pyramidal
AB3E
Ammonia H—N—H
H
18
AB2E2
Water H—O—H
e- tetrahedral molecule: bent linear
19
Comparison
AB4 AB3E AB2E2
Methane ammonia water
109.5 107.3 104.5
CH
H
H
H
NH
H
HO
H
H
20
AB4E
Sulfur tetrafluoride SF4
S
F F
F FPredict: trigonal bipyramid
for e- domains.
Draw Lewis structure
21
AB4E: SF4
trigonal bipyramid: 2 choices
F
F
F
F
F
F
F
F
distorted tetrahedron or seesaw
22
AB3E2
ClF3 F
F
F
molecule: T-shaped
A little weird.
23
AB2E3Three possibilities for I3
-
Want nonbonding e-
domains farthest apart.
24
AB2E3
I3-
molecule: linear
I
I
-
Lone pairs always go on
equatorial position(s)
25
AB5E
BrF5
molecule: square pyramidal
FF
FF
F
26
AB4E2: 2 Possibilities
Want
nonbonding
e- pairs
farthest
apart.
1
2
XeF4
27
AB4E2
XeF4
molecule: square planar
F
FF
F
28
You Try It
Predict geometry and
approximate bond angles for:
AlCl4-
XeF2
XeOF2
Text: Tables 10.1 and 10.2.
29
Bond Polarity
FH
Shift of e- density toward F,
Thus HF is polar.
d+ d-
F is more
electronegative
than H.
30
Overall Molecular Polarity
O C O
Even though CO2 has polar bonds,
it is nonpolar since the individual
bond polarities add to zero.“Dipoles” are a vector quantities.
Symmetrical = nonpolar
“pull test”
(Recall the “Regents” rule about
lone pairs on the central atom.)
31
Molecular Polarity
Molecule GeometryDipole
Strength
HF Linear 1.92
HBr Linear 1.08
Water Bent 1.87
SO2 Bent 1.60
32
Molecular Polarity
NH
H
H
Which has larger polarity?
NH3 NF3
NF
F
F
m =0.24 Dm = 1.46 D
33
Dipole Moments
Predict whether the following
molecules are polar.
IBr
CH2Cl2AlCl3
Remember to distinguish between
bond polarity and molecular polarity.
34
Both H2 & F2 have single bonds,
but…
Bond
length
Bond
energy
H2 74 pm 436 kJ
F2 142 pm 151 kJ
Shortfall of VSEPR
35
Quantum Mechanics…
Valence Bond Theory
e- in molecule occupy
blended atomic orbitals
…to the rescue
Molecular Orbital Theory
molecule has
“molecular orbitals”
36
Valence Bond Theory
Atomic orbitals (s, p, d …)
of the valence electrons
“hybridize” or mix to form new
orbitals for the molecule.
37
Valence Bond Theory
2s 2pcarbon
atomic
orbitalsC
How can carbon form four equal
bonds with four hydrogen atoms
using its atomic ‘s’ and ‘p’ orbitals?
Consider tetrahedral CH4
38
CH4: sp3 Hybridization
2s 2p
carbon
hybridized
orbitals
carbon
atomic
orbitalsC
Csp3 (all the same)
39
Hybridization Analogy
s p p p
4 sp3 hybrid orbitals
40
sp3 Hybridization
It takes energy to form hybrid
orbitals, but this energy is
more than compensated by
bond formation.
41
sp3 Orbital Shapes
Each
orbital
can hold
2 e-
tetrahedral
One s & three p orbitals change
to four sp3 hybrid orbitals.
42
CH4 Bonding
The 4 sp3 hybrid
orbitals of C bond
(overlap) with the
4 1s atomic orbitals
of the H atoms.
H
HHH
C
43
NH3 sp3 Bonding
nitrogen
hybridized
orbitals
nitrogen
atomic
orbitals
2s 2pN
sp3 (all the same)
N
44
sp3
bonding orbitalslone pair
NH3 sp3 Bonding
45
Hybridization
First use VSEPR to predict the
arrangement of e- pairs, then
use hybridization to predict the
type of bonding.
46
sp Hybridization
Be atomic
orbitals
2s 2pBe
Be hybrid
orbitals
sp 2p
Be
47
sp Hybridization: BeCl2
BeCl Cl
one atomic p
orbital of Cl
two sp hybrid
orbitals of Be
48
sp2 Hybridization
B atomic
orbitals
2s 2p
B
B hybrid
orbitals
sp2 2p
B
49
sp2 Hybridization: BF3
B
F F
F
one p orbital
of F
three sp2
hybrid orbitals
of B
50
Hybridization
Hybridization for 2nd Period
elements (C, N, O, F) explains the
“octet” rule, since there are 4 hybrid
orbitals formed from one s and three
p atomic orbitals.
s p sp3
51
Hybridization: Review
1.Not applied to isolated atoms
2.First determine VSEPR geometry
3.Mix nonequivalent atomic orbitals
of central atom to form hybrid
orbitals
52
Hybridization: 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
53
Hybridization: Try It
Determine hybridization in:
AlBr3PF3
HgCl2
54
Let’s Not Forget d Orbitals
For elements in the 3rd Period and
higher, hybridization can also
include ‘d’ orbitals.
d-orbital hybridization is still
being debated!
55
SF
F
F
F
F
F
d Hybridization
SF6 VSEPR predicts
octahedral geometry
56
Hybridization: SF6
S atomic orbitals
S hybridized sp3d2 orbitals
3s 3p 3d
sp3d2 3d
57
Hybridization
You Try It .
What is the VSEPR geometry
and the hybridization in PBr5?
58
Summary: Hybrid Orbitals
2 e- pairs (sp) linear
3 e- pairs (sp2)
trigonal planar
4 e- pairs (sp3)
tetrahedral
e- domains (bonding & nonbonding pairs)
59
Summary: Hybrid Orbitals
5 e- pairs (sp3d)
trigonal bipyramid
6 e- pairs (sp3d2)
octahedral
60
Multiple Bonds
In VSEPR, no distinction was made
among single, double, triple bond or lone
pair. All were counted as an “e- domain”.
Each carbon is trigonal planar
C2H4 C CH
H
H
H
61
Double Bond: C2H4
2s 2pC atomic
orbitals
C hybrid
orbitals
sp2 pz
+
unhybridized
p orbital
62
Double Bond: C2H4
C hybrid
orbitals
sp2 pz
+
trigonal planar dumb bell
63
Double Bonds: C2H4
C C
H
HH
H
64
Double Bond: C2H4
Sigma (s) and Pi (p) Bonds
(model)
65
Pi & Sigma Bonds
Pi (p) bond: covalent bond formed
by sideways e- overlap above and
below the plane connecting atoms
(weaker than s)
Sigma (s) bond: covalent bond
formed by e- overlap along the
axis connecting atoms
66
Double Bond: C2H4
The double bond is one sigma and one
pi bond between the carbon atoms
C CH
H
H
H
67
Triple Bond: H-C=C-H
2s 2pC atomic
orbitals
C hybrid
orbitals
sp py pz
+unhybridized
p orbitals
68
Triple Bond: C2H2
Hybridize s bonds only
hybridized
sp orbitals
unhybridized
p orbitals
69
Hybridizing Shortcuts
1.Single bonds: sigma2.Double bonds: one sigma, one pi3.Triple bonds: one sigma, two pi4.Hybridize: add the number of
sigma bonds plus lone pairs
s p d
70
You Try It
Describe the bonding and
hybridization for each atom in:
•formaldehyde, CH2O
•hydrogen cyanide, HCN
(assign formal charges)
71
Delocalized Molecular Orbitals
An explanation of resonance.
O OOO O O
ozone
Ozone is a blend of the two
resonance structures.
Delocalized Molecular Orbitals
72
The p component of the double bond
is delocalized over the molecule.
sp2
sp2
sp2
73
Delocalized “MOs”
e- in s bonds– localized
e- in p bonds-- delocalized
H
H
H
H
H
H
H
H
H
H
H
H
benzene
74
Benzene
sp2
sp2
C
C
C C
C
C
Sigma bonds
(localized)
75
Benzene
p bonds
(delocalized)
sp2 pz
+
76
Benzene
Bond order 1.5
H
H
H
H
H H
“Conjugated” double bonds are
more stable (less reactive)
77
Carbonate Ion
+ resonance
structuresO—CO
O
2-
C: sp2 fors bonds (localized)
plus a 2pz for p bond (delocalized)
O: each has a 2pz orbital (delocalized)
78
Carbonate Ion
o
o
oC
Additional stability due to
delocalization of the pi bond.o
o
oC
79
You Try It
Would you predict the NO3- anion
to have additional stability due to
delocalization of the p electrons?
80
Polyatomic Ions
Many common polyatomic ions
have delocalized pi bonds,
partially accounting for their
additional stability in chemical
reactions.
CO3-2, NO3
- , ClO3- etc.
81
Ionic & Covalent
CaCO3
C
O
OOCa
2-
2+
Trigonal planar, sp2, 120o
Ionic compounds containing polyatomic
ions have both ionic and covalent bonds!