Valence shell electron pair
repulsion (VSEPR) model:
Predict the geometry of the
molecule from the electrostatic
repulsions between the electron
(bonding and nonbonding) pairs.
The idea here is that the bonding
and nonbonding pairs around a
given atom will be positioned as
far apart as possible.
This chart is
NOT provided
on the AP exam!
VSEPR
A = central atom
B = atoms bonded to central atom
E = lone pairs (non-bonding) of electrons on central atom
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
AB2
2
0
Arrangement of
electron pairs
Molecular
Geometry
linear
linear
B
Bond angle
B
Cl
Be
Cl
lone pairs
on to
central
atom
20
atoms
bonded
central
atom
VSEPR
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
AB2
2
0
linear
linear
AB3
3
0
trigonal
planar
trigonal
planar
Arrangement of
electron pairs
Molecular
Geometry
VSEPR – Trigonal Planar arrangement
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
Arrangement of
electron pairs
Molecular
Geometry
trigonal
planar
bent
AB3
3
0
trigonal
planar
AB2E
2
1
trigonal
planar
VSEPR
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
AB2
2
0
linear
linear
AB3
3
0
trigonal
planar
trigonal
planar
AB4
4
0
Arrangement of
electron pairs
tetrahedral
Molecular
Geometry
tetrahedral
VSEPR – Tetrahedral Arrangement
Class
AB4
AB3E
# of atoms
bonded to
central atom
# lone
pairs on
central atom
4
0
3
1
Arrangement of
electron pairs
Molecular
Geometry
tetrahedral
tetrahedral
tetrahedral
trigonal
pyramidal
VSEPR – Tetrahedral Arrangement
# of atoms
bonded to
central atom
# lone
pairs on
central atom
AB4
4
0
tetrahedral
tetrahedral
AB3E
3
1
tetrahedral
trigonal
pyramidal
AB2E2
2
2
tetrahedral
bent
Class
Arrangement of
electron pairs
Molecular
Geometry
O
H
H
CH4
NH3
H2O
Number of
lone pairs
0
1
2
Bond angle
109.5o
107o
104.5o
In these molecules, the arrangement of the electron
pairs predict tetrahedral geometry. Why is the
geometry different from predicted?
Lone pairs require more room than bonding pairs and
tend to compress the angles between the bonding
pairs.
VSEPR
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
AB2
2
0
linear
linear
AB3
3
0
trigonal
planar
trigonal
planar
AB4
4
0
tetrahedral
tetrahedral
0
trigonal
bipyramidal
trigonal
bipyramidal
AB5
5
Arrangement of
electron pairs
Molecular
Geometry
VSEPR – Trigonal Bipyramidal Arrangement
Class
AB5
AB4E
# of atoms
bonded to
central atom
# lone
pairs on
central atom
5
0
trigonal
bipyramidal
trigonal
bipyramidal
1
trigonal
bipyramidal
See-Saw
4
Arrangement of
electron pairs
Molecular
Geometry
(distorted tetrahedron)
VSEPR – Trigonal Bipyramidal Arrangement
# of atoms
bonded to
central atom
# lone
pairs on
central atom
AB5
5
0
trigonal
bipyramidal
trigonal
bipyramidal
AB4E
4
1
trigonal
bipyramidal
See-Saw
AB3E2
3
2
trigonal
bipyramidal
Class
Arrangement of
electron pairs
Molecular
Geometry
T-shaped
F
F
Cl
F
VSEPR – Trigonal Bipyramidal Arrangement
# of atoms
bonded to
central atom
# lone
pairs on
central atom
AB5
5
0
trigonal
bipyramidal
trigonal
bipyramidal
AB4E
4
1
trigonal
bipyramidal
See-Saw
AB3E2
3
2
trigonal
bipyramidal
T-shaped
AB2E3
2
3
trigonal
bipyramidal
linear
Class
Arrangement of
electron pairs
Molecular
Geometry
I
I
I
VSEPR
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
AB2
2
0
linear
linear
AB3
3
0
trigonal
planar
trigonal
planar
AB4
4
0
tetrahedral
tetrahedral
trigonal
bipyramidal
octahedral
Arrangement of
electron pairs
AB5
5
0
trigonal
bipyramidal
AB6
6
0
octahedral
Molecular
Geometry
VSEPR – Octahedral Arrangement
# of atoms
bonded to
central atom
# lone
pairs on
central atom
AB6
6
0
octahedral
octahedral
AB5E
5
1
octahedral
square
pyramidal
Class
Arrangement of
electron pairs
Molecular
Geometry
F
F
F
Br
F
F
VSEPR – Octahedral Arrangement
# of atoms
bonded to
central atom
# lone
pairs on
central atom
AB6
6
0
octahedral
octahedral
AB5E
5
1
octahedral
square
pyramidal
AB4E2
4
2
octahedral
square planar
Class
Arrangement of
electron pairs
Molecular
Geometry
F
F
Xe
F
F
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.
What are the molecular geometries of SO2 and SF4?
(In SF4, S uses an expanded octet of 10.)
F
S
S
F
S
F
AB2E
bent
AB4E
F
See-Saw
(distorted
Tetrahedron)
Dipole Moments and Polar Molecules
electron poor
region
electron rich
region
H
F
d+
d-
A molecule such as HF that has a center of positive charge and
a center of negative charge is said to have a dipole moment.
The dipole moment is often represented by an arrow pointing to
the negative charge center with the tail of the arrow indicating
the positive center of charge.
Which of the following molecules have a dipole moment?
H2O, CO2, SO2, and CH4
O
S
dipole moment
polar molecule
dipole moment
polar molecule
H
O
C
O
no dipole moment
nonpolar molecule
H
C
H
H
no dipole moment
nonpolar molecule