Chapter 10 Bonding Theory and Molecular Structure
1
I. Molecular Shapes
A. The VSEPR model
1. electron-pair geometries
2. molecular geometries
B. Molecular polarity
II. Valence Bond Theory
A. Covalent bonding and orbital overlap
B. Hybrid orbitals
1. sp hybrid orbitals
2. sp2 hybrid orbitals
3. sp3 hybrid orbitals
4. hybridization involving d orbitals
C. Multiple bonds
1. double bonds
2. triple bonds
III. Molecular Orbital Theory
A. First-row diatomics
B. Second-row diatomics
IV. Benzene and Aromatic Compounds
2
I. Molecular Shapes
A. The VSEPR model
1. electron-pair geometries
Valence Shell Electron Pair Repulsion Theory: regions of electron density
(single, double, or triple bonds or lone pairs) arrange themselves around an
atom to be as far apart as possible (electron pair repulsion).
Electron pair geometries:
180°
BeF2
90°
F
F
Be
linear
F
BF3
F
B
PF5 F
120°
F
F
P F
F
F
P
F
F
F
trigonal bipyramidal
F
trigonal planar
Cl
CCl4
F
120°
90°
109.5°
C Cl
Cl
Cl
tetrahedral
SF6
F
F
F
S
F
F
F
F
F
F
octahedral
S
F
F
F
3
4
I. Molecular Shapes
A. The VSEPR model
2. molecular geometries
5
I. Molecular Shapes
A. The VSEPR model
2. molecular geometries
Electron pair geometry: tetrahedral
Molecular geometry:
tetrahedral
trigonal pyramidal
bent
6
7
8
9
10
I. Molecular Shapes
A. The VSEPR model
2. molecular geometries
NI3
SO2
PCl4–
NO3–
OF2
SO32–
BrCl3
PO43–
11
I. Molecular Shapes
B. Molecular polarity
Molecular polarity  physical and chemical properties
bonds: if DX > 0  polar bond
d+ d–
A—B
dipole
molecules and ions: if dipoles do not exactly cancel, molecule will be polar
BeCl2
BF3
CH2O
CCl4
CHCl3
NH3
12
I. Molecular Shapes
B. Molecular polarity
PCl3F2
CO32–
CHO2–
13
II. Valence Bond Theory
A. Covalent bonding and orbital overlap
Bonds are formed using valence electrons and orbitals:
atomic orbitals
e.g.,
overlap
H +H
1s
1s
molecular orbitals (covalent bonds)
H H
molecular orbital
1s (sigma bond)
atomic orbitals
14
II. Valence Bond Theory
A. Covalent bonding and orbital overlap
H + Cl
H
Cl + Cl
H Cl
Cl
Cl
1s
3s
3p
3p
1s
Cl
3p
But what about CH4?
3p
 m.o.
 m.o.
H
C H
H
H
Tetrahedral, all bonds
equivalent. How do we
get this from s and p a.o.s?
15
II. Valence Bond Theory
B. Hybrid orbitals
1. sp hybrid orbitals
BeH2 facts: H
Be
H 2 equivalent bonds
linear, 180°
Be
2s
2p
promote
electron
2s
atomic configuration
can't form two bonds
sp hybrid a.o.s:
hybridize
2p
sp
hybrid
a.o.s
two bonds, but not
equal, not 180°
and
180° !
= H Be
H(1s) Be(sp) H(1s)
(linear)
atomic orbitals
2p
H
(spBe + 1sH)
molecular orbitals
16
II. Valence Bond Theory
B. Hybrid orbitals
2. sp2 hybrid orbitals
H
BH3 facts:
H
B
2s
B
trigonal planar,
3 equivalent bonds
H
promote
electron
2p
hybridize
2s
2p
2p
sp2
hybrid
a.o.s
sp2 hybrid a.o.s:
H
3H
B(sp2)
trig. plan.
=
H
B
H
(sp2B + 1sH)
17
II. Valence Bond Theory
B. Hybrid orbitals
3. sp3 hybrid orbitals
H
CH4 facts:
C H
H
H
C
2s
2p
tetrahedral,
4 equivalent bonds
promote
electron
hybridize
2s
2p
H
sp3
4H
hybrid a.o.s:
C(sp3)
tetrahedral
C H
H
H
sp3
hybrid
a.o.s
(sp3C + 1sH)
18
II. Valence Bond Theory
B. Hybrid orbitals
3. sp3 hybrid orbitals
NH3
N
2p
2s
sp3
lone pair in sp3 a.o.
sp3 hybridized
N H
H
H
H2O
O
2s
(sp3N + 1sH)
2p
sp3
lone pairs in sp3 a.o.s
sp3 hybridized
O H
H
(sp3O + 1sH)
19
II. Valence Bond Theory
B. Hybrid orbitals
4. hybridization involving d orbitals
PCl5
3p
3s
3s
3d
3p
sp3d
Cl
Cl
Cl P Cl
Cl
sp3d hybrid a.o.s:
trigonal
bipyramidal
SF6
3s
3p
3s
3d
3p
sp3d2
F
F
F
sp3d2 hybrid a.o.s:
octahedral
S
F
F
F
20
II. Valence Bond Theory
B. Hybrid orbitals
Summary:
e– pair geometry
hybridization
linear
sp
trigonal planar
sp2
tetrahedral
sp3
trigonal bipyramidal
sp3d
octahedral
sp3d2
21
II. Valence Bond Theory
B. Hybrid orbitals
What is the hybridization of the central atom in each of the following?
CCl4
BrCl3
BF3
SF6
NH3
BeCl2
PCl4–
XeF4
22
II. Valence Bond Theory
C. Multiple bonds
1. double bonds
trigonal planar = sp2
C2H4 facts: H
all six atoms lie
in same plane
H
C C
H
1s
H
C
1s
2p
sp2
C
H
H
H
H
H
1s
2p
sp2
 bond
2p
H
1s
C C
H
overlap
p orbitals
H
(sp2
(sp2C
+
C + 1sH)
sp2C)
H
H
C C
H
H
H
all atoms coplanar
for p orbital overlap
H
C
=
H
C
H
double bond =
1  bond +
1  bond
23
II. Valence Bond Theory
C. Multiple bonds
2. triple bonds
C2H2 facts: H
H
sp
2p
C
sp
linear = sp
1s
2p
2  bonds
2p
H
C H
H
1s
C
C
C C
H
(spC + 1sH)
(spC + spC)
H
C C
H = H
C
C
H
triple bond =
2  bonds +
1  bond
24
II. Valence Bond Theory
What is the hybridization of each indicated atom in the following
molecule? How many sigma and pi bonds are in the molecule?
H
O
H
C O C H
C
H
C
H
C
N
25
III. Molecular Orbital Theory
Fact: O2 is paramagnetic!
O O
Lewis structure
VSEPR
Valence bond theory
•sp2 hybridized
•lone pairs in sp2 hybrid orbitals
•bonding pairs in  and  bonds
All show
all electrons
paired.
26
III. Molecular Orbital Theory
Overlap of wave functions:
+
+
–
–
+
+
–
–
constructive
overlap
–
destructive
overlap
27
III. Molecular Orbital Theory
A. First-row diatomics
Overlap of 1s orbitals:
*1s
–
+
E
+
antibonding m.o.
(higher energy than
separate atoms)
1s
bonding m.o.
(lower energy than
separate atoms)
28
III. Molecular Orbital Theory
A. First-row diatomics
(no. of e– in bonding m.o.s) - (no. of e– in antibonding m.o.s)
bond order =
2
*1s
H2
E
1s
1s
1s
b.o. = 1 (i.e., lower energy than separate atoms)
29
III. Molecular Orbital Theory
A. First-row diatomics
E
He2
He2+
*1s
*1s
1s
1s
1s
1s
1s
1s
b.o. = 0
b.o. = 0.5
30
III. Molecular Orbital Theory
B. Second-row diatomics
Overlap of 2s and 2p orbitals
2s  2s and *2s
(same as 1s),
then 2p orbitals give:
(i.e., 8 a.o.s  8 m.o.s)
z
z
x
x
y
y
31
III. Molecular Orbital Theory
B. Second-row diatomics
*2p
*2p *2p
2px 2py 2pz
2px 2py 2pz
2p
E
2p 2p
*2s
2s
2s
2s
32
III. Molecular Orbital Theory
B. Second-row diatomics
N2
O2
F2
*2pz
*2px, *2py
2pz
2px, 2py
*2s
2s
bond order:
magnetic:
33
III. Molecular Orbital Theory
B. Second-row diatomics
CO
NO
ClO
*2pz
*2px, *2py
2pz
2px, 2py
*2s
2s
bond order:
magnetic:
34
IV. Benzene and Aromatic Compounds
H
H
benzene
C6H6
H
C
C
sp2
all
120º
C
C
H
C
C
H
=
H
planar
hexagon
6 e– in a cyclic,
planar  system
 aromatic stabilization
35
IV. Benzene and Aromatic Compounds
CH3
CH3
CH3
CH3
CH3
CH3
methylbenzene
toluene
1,2-dimethylbenzene (meta-xylene)
CH3
ortho-dimethylbenzene
(para-xylene)
(o-xylene)
Cl
Cl
p-dichlorobenzene
naphthalene
benzo[a]pyrene
(carcinogen)
36