Chemical Bonding II:
Molecular Geometry and
Hybridization of Atomic Orbitals
Chapter 10
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Molecular Geometry
•  Problem: Lewis structures show connectivity
between atoms, but not overall geometry of molecule
•  The geometry that the molecule ultimately assumes
is determined by electron repulsion
•  Bonding and lone pair electrons are positioned as far
apart from each other as possible
•  We need a model to describe the molecular geometry
of molecules
Molecular Geometry
Valence shell electron pair repulsion (VSEPR) model:
Predicts the geometry of the molecule from the electrostatic
repulsions between the electron (bonding and nonbonding) pairs.
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
Arrangement of
electron pairs
Molecular
Geometry
AB2
2
0
linear
linear
B
B
10.1
1
0 lone pairs on central atom
Be
Cl
Cl
2 atoms bonded to central atom
10.1
Molecular Geometry
VSEPR
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
AB2
2
0
linear
linear
0
trigonal
planar
trigonal
planar
AB3
3
Arrangement of
electron pairs
Molecular
Geometry
10.1
Molecular Geometry
BF3
10.1
2
Molecular Geometry
VSEPR
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
AB2
2
0
linear
linear
trigonal
planar
tetrahedral
Arrangement of
electron pairs
AB3
3
0
trigonal
planar
AB4
4
0
tetrahedral
Molecular
Geometry
10.1
10.1
VSEPR
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
AB2
2
0
linear
linear
trigonal
planar
Arrangement of
electron pairs
Molecular
Geometry
AB3
3
0
trigonal
planar
AB4
4
0
tetrahedral
tetrahedral
0
trigonal
bipyramidal
trigonal
bipyramidal
AB5
5
10.1
3
10.1
VSEPR
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
AB2
2
0
linear
linear
trigonal
planar
Arrangement of
electron pairs
Molecular
Geometry
AB3
3
0
trigonal
planar
AB4
4
0
tetrahedral
tetrahedral
trigonal
bipyramidal
octahedral
AB5
5
0
trigonal
bipyramidal
AB6
6
0
octahedral
10.1
10.1
4
10.1
Molecular Geometry
•  2 types of molecules
•  A molecule without lone pair of electrons on central atom
•  A molecule with lone pair of electrons on central atom
•  Lone pair of electrons are more repulsion, occupy more space,
and contain more electron density than bonding electrons
Lone pair e-
CH4
bonding-pair vs.
bonding-pair repulsion
NH3
<
lone-pair vs. bonding
pair repulsion
H 2O
<
lone-pair vs. lone-pair
repulsion
5
VSEPR
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
AB3
3
0
AB2E
2
1
Arrangement of
electron pairs
Molecular
Geometry
trigonal
planar
trigonal
planar
trigonal
planar
bent
10.1
VSEPR
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
Arrangement of
electron pairs
Molecular
Geometry
AB4
4
0
tetrahedral
tetrahedral
AB3E
3
1
tetrahedral
trigonal
pyramidal
10.1
VSEPR
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
Arrangement of
electron pairs
Molecular
Geometry
AB4
4
0
tetrahedral
tetrahedral
AB3E
3
1
tetrahedral
trigonal
pyramidal
AB2E2
2
2
tetrahedral
bent
O
H
H
10.1
6
VSEPR
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
AB5
5
AB4E
4
Arrangement of
electron pairs
Molecular
Geometry
0
trigonal
bipyramidal
trigonal
bipyramidal
1
trigonal
bipyramidal
distorted
tetrahedron
Seesaw
10.1
VSEPR
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
AB5
5
0
AB4E
4
1
AB3E2
3
2
Arrangement of
electron pairs
Molecular
Geometry
trigonal
bipyramidal
trigonal
bipyramidal
trigonal
bipyramidal
trigonal
bipyramidal
distorted
tetrahedron
T-shaped
F
F
Cl
F
10.1
VSEPR
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
AB5
5
0
AB4E
4
1
AB3E2
3
2
AB2E3
2
3
Arrangement of
electron pairs
Molecular
Geometry
trigonal
bipyramidal
trigonal
bipyramidal
trigonal
bipyramidal
trigonal
bipyramidal
distorted
tetrahedron
trigonal
bipyramidal
T-shaped
linear
I
I
I
10.1
7
VSEPR
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
Arrangement of
electron pairs
Molecular
Geometry
AB6
6
0
octahedral
octahedral
AB5E
5
1
octahedral
square
pyramidal
F
F
F
Br
F
F
10.1
VSEPR
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
Arrangement of
electron pairs
Molecular
Geometry
AB6
6
0
octahedral
octahedral
AB5E
5
1
octahedral
AB4E2
4
2
octahedral
square
pyramidal
square
planar
F
F
Xe
F
F
10.1
10.1
8
Predict the geometries of the following molecules and ion using
the VSEPR method:
(a) CBr4
(b) BCl3
(c) NF3
(d) H2Se
(e) NO2(f) TeCl4
Geometry of Molecules with More Than
One Central Atom
•  Draw Lewis structure of methanol
•  It’s difficult to determine overall shape of molecule, but we can
determine shape around central atoms
•  Central atoms are C and O
•  What’s the geometric shape around C and O?
Predicting Molecular Geometry
1.  Draw Lewis structure for molecule.
2.  Count number of lone pairs on the central atom and
number of atoms bonded to the central atom.
3.  Use VSEPR to predict the geometry of the molecule.
10.1
9
Dipole Moments
•  Dipole moments are used to determine whether or not a
molecule is polar or non-polar overall
•  Why do you care?
•  Rule: Like dissolves like
•  Polar molecules are miscible in polar molecules (vice versa)
Dipole Moments and Polar Molecules
Shift in electron density
(due electronegativity) can
be quantitatively measured
as a dipole moment (µ)
m=Qxr
electron poor
region
electron rich
region
H
F
δ+
δ-
Q is the charge (C)
r is the distance between charges (m)
Polar molecule
1 D = 3.36 x 10-30 C m
10.2
Proof of polarity (partially + and – charges) is their alignment in electric field
10.2
10
Sum of all dipoles
results in overall dipole
of molecule, and
determines polarity
(polar or non-polar)
10.2
Which of the following molecules have a dipole moment?
H2O, CO2, SO2, and CH4
Typically, symmetrical molecules are non-polar
10.2
Does BF3 have a
dipole moment?
No (non-polar molecule)
10.2
11
Does CH2Cl2 have
a dipole moment?
10.2
There are three different structures of C2H2F2 are they all polar?
Isomers
Dipole Moments
Other polar molecules: HCl, HF, H2O, SO2, and NO
Non-polar molecules: H2, O2, F2, CO2, CH4, and benzene
10.2
12
Chemistry In Action: Microwave Ovens
Dipole Moments at Work
Microwaves are generated in
magnetron
Negative pole follows + region of wave Guided and reflected to food by
waveguide and rotating blades,
respectively
Polar molecules mimic
oscillating wave, therefore
they rotate; collisions with
food causes it to heat up
13