The Shapes of Molecules
Or in Big Scientific Words
“MOLECULAR GEOMETRY”
Simulate Electron Pair Arrangements
Use Balloons to represent an Electron Pair
 What is a pair of electrons going to do to
another pair of electrons?

THEY REPEL
EACH OTHER

So if we want to mimic what happens in an
atom, we need to get the balloons as far
apart as possible.
First Some Terminology
Nonbonding
Pair of
Electrons
Also called “Lone Pair”,
“Unshared Pair”
Bonding Pair of
Electrons
Also called “Shared Pair”
2 Electron Pairs on Central Atom
Beryllium fluoride


2 Shared Pairs
0 Unshared Pairs
This is a Lewis
Structure and a 3D
drawing
How can we get these
electron pairs
(balloons) as far apart
as possible?
What does the shape
look like?
LINEAR
180º Apart
3 Electron Pairs on Central Atom
Boron trihydride


3 Bonding Pairs
0 Nonbonding Pairs
This is a 3D
drawing of BH3
Get three balloons as far
apart from each other.
How would you describe
the shape?
TRIGONAL
PLANAR
120º Apart
What would it look like if?




We had a Double Bond or
a Triple Bond?
Does a double count as
two pairs regardless?
Since the two pairs are in
the same place do they
count as only one?
Because they are forced
into the same place they
only count once.
Sulfur dioxide
What else new
do you notice?
3 Pairs Around Central Atom

2 Shared Pairs Adjusted
1 Lone Pair

The lone pair is still
there but we can’t see
it.
Lewis Structure
The molecule actually
looks like
BENT
Roughly 120º Apart
3D Shape
4 Pairs Around Central Atom
Silicon tetrafluoride


Think 3D not 2D…
How might you
describe shape?
TETRAHEDRAL
109.5º Apart
Lewis Structure
4 Shared Pairs
0 Lone Pairs
How do you
draw it?
How to Draw a Tetrahedral Shape
Regular Lines
Mean its in the
Plane of the
Paper
Dashed Line Means its
Going Away From You
Lewis Structure
Bold Line Means
its Coming At
You.
3D Shape
4 Electron Pairs Around Center

What does it look like?
Ammonia
TRIGONAL
PYRAMIDAL
Roughly 109.5º Apart
Lewis Structure
How do you draw it?
3
Bonding Pairs
1
Nonbonding Pair
How to Draw Trigonal Pyramidal
4 Electron Pairs around Center

What does this one
look like?
Water
BENT
Roughly 109.5º Apart
How do you draw it?
Lewis Structure
2
Shared Pairs
2
Unshared Pairs
3D Shape
5 Electron Pairs!

Expanded Octets
5 Shared Pairs

Phosphorus
pentafluoride

What does it look
like?
0 Unshared Pairs
Lewis Structure
TRIGONAL BIPYRAMIDAL
Some are 120º Apart. Others are 90º
Try to Draw It
Six Pairs

How do the balloons
arrange themselves?
Sulfur hexafluoride
Lewis Structure
OCTAHEDRAL
Why “octa”?
Octahedron
All 90º apart
How do you
draw it?
6
Shared Pairs
0
Lone Pairs
VSEPR Theory

Valence Shell Electron Pair Repulsion

Electron Pairs try to get as far away from
each other as possible.