Molecular Geometry
(Shapes of Molecules)
VSEPR Theory
CP Chemistry
http://www.scl.ameslab.gov/MacMolPlt/Surface.JPG
VSEPR Theory
• Electron groups around the central atom
will be most stable when they are as far
apart as possible – we call this valence
shell electron pair repulsion theory
– because electrons are negatively charged,
they should be most stable when they are
separated as much as possible
• The resulting geometric arrangement will
allow us to predict the shapes and bond
angles in the molecule
Electrons vs. Molecular Geometry
• The geometry of electron pairs around a
central atom is called the electron
geometry.
• The arrangement of bonded nuclei around
a central atom forms the molecular
geometry.
• Lone pair electrons on a central atom will
repel other pairs but will not be visible in
the molecular geometry (no nuclei)
• If there are lone pairs on the central atom
the electron geometry and the molecular
geometry will differ.
Two electron pairs on central atom
Examples:
CS2, HCN, BeF2
3 electron
pairs on central
atom
• All are in bonds
• Trigonal Planar
Shape
Examples:
SO3, BF3, NO3-, CO32-
3 Electron Pairs
• 2 Bonded
• 1 Un-Bonded
Bent Shape
• Un-bonded
electron pair takes
up more space and
“repels” more.
• The bond angle will
change to less than
the original 120o to
about 117o-115o
Other Examples:
About 117o
• SO2, O3, PbCl2, SnBr2
Four electron pairs on
central atom
• All 4 in bonds
Examples:
CH4, SiCl4,
SO42-, ClO4-
4 Electron Pairs
• 3 bonded
• 1 un-bonded
(lone pair)
Trigonal
Pyramidal
Shape
Examples: NH3, PF3, ClO3. H3O+
Bond angles are
reduced from 109.5o to
107o due to extra
repulsion by lone pair
4 Electron Pairs
2 Bonded
2 Un-bonded
(lone pairs)
BENT SHAPE
Examples: H2O, OF2, SCl2
Bond angles are
reduced a little more
due to repulsion
To 104.5o
The steps in determining a molecular shape
Molecular
formula
Step
1
Lewis
structure
Step
2
Electron-group
arrangement
(electron
geometry)
Count all e- pairs around central
atom
Step
3
Bond
angles
Note lone pairs and
double bonds
Step
4
Consider bonding
e- pairs only
Molecular
geometry
Representing 3-Dimensional
Shapes on a 2-Dimensional Surface
• One of the problems with drawing molecules is
trying to show their dimensionality
• By convention, the central atom is put in the
plane of the paper
• Put as many other atoms as possible in the same
plane and indicate with a straight line
• For atoms in front of the plane, use a solid
wedge
• For atoms behind the plane, use a hashed
wedge
Predicting Molecular Shapes with Two, Three, or Four Electron Groups
PROBLEM:
Draw the molecular shape and predict the bond angles
(relative to the ideal bond angles) of (a) PF3
SOLUTION: (a) For PF3 - there are 26 valence electrons, 1 nonbonding
pair
The shape is based upon the tetrahedral arrangement.
F P F
The F-P-F bond angles should be <109.50 due
F
to the repulsion of the nonbonding electron
P
F pair.
F
F
The final shape is trigonal pyramidal.
0
< 109.5
Predicting Molecular Shapes with More Than One Central Atom
PROBLEM:
Determine the shape around each of the central atoms in
acetone, (CH3)2C=O.
Find the shape of one atom at a time after writing the
Lewis structure.
SOLUTION:
tetrahedral
H
H C
H
O
C
H
C H
H
tetrahedral
trigonal planar
O
H
C
H C
C
H
HH
>1200
H
<1200