Chemical Bonding
Molecular Geometry
What is Molecular Geometry?
• The study of the physical structure, bonding,
and shapes of molecules.
• Involves the connection between formula,
valence, hybridization, and bonding.
• Is governed by the Valence Shell Electron Pair
Repulsion theory…
So what is this Valence Shell
Electron Pair Repulsion Theory??
• A statement that essentially says that electron
pairs do not like each other.
– Why would that be true ????
The Result…
• Electron Pairs around a central atom will
be oriented in space to be as far apart
(from each other) as possible.
Example #1 – A molecule with the
formula AX2
This molecule has two
bonds.
• What hybrids will enable
this? (sp hybrids)
• What will the valence be
around the central atom
after the bonding? (4)
• How will the bond pairs
be arranged in space to
follow the VSEPR?
X
A
X
What is the bond angle X-A-X in
this molecule? (180 degrees)
What is the name given to
describe the shape of this
molecule? (linear)
Example #2 – A molecule with the
formula AX3
This molecule has 3 bonds:
• What hybrids will be
needed to form the three
bonds? (sp2)
X
X
Side View
•What will the valence be
around the central atom
after the bonding? (6)
• How will the bond pairs be
arranged in space to follow
the VSEPR?
X
X
A
X
X
Top View
Example #2 – A molecule with the
formula AX3
X
X
What will the bond angle, X-A-X
be in this molecule? (120°)
A
Is the molecule symmetrical in
shape? (yes)
X
What name will be given to the
shape of the molecule? (trigonal
planar)
Example #3 – A change to 3dimensional thinking…
Given: A molecule with the
formula AX4
• How many bonds are there
in the molecule? (4)
• What was the valence of the
central atom before the
hybridization? (4)
• What will the valence be
around the central atom after
the bonding? (8)
What hybrids are used
to form these bonds?
(sp3)
What element could be
the central atom in this
molecule? (C or Si)
The Lewis Dot Diagram for the AX4
molecule would look like this:
X
X
A
X
X
But, that is two dimensional
thinking….
Here is what really
happens…
The molecule is not “flat” like a
Lewis Dot Diagram would
suggest.
Instead, the “X” atoms are at the
corners of a regular tetrahedron.
X
A
X
X
X
The shape of the molecule is
described as tetrahedral and the
bond angle is 109.5 degrees
Continuing: The AX5 Molecule
First things first:
How many bonds are formed?
(5)
What hybrids are used? (sp3d)
What is the valence around the
central atom after all of the
bonding? (10)
What elements could be the
central atom in the molecule?
Why just these? (P, As - b/c they
have d orbitals available)
X
X
X
A
X
X
Now for the 3-dimensional version:
There is obviously a lot going on
here…
1. We define axial and equatorial
atoms.
X
X
X
X
X
2. We define an axial bond angle as
180 degrees.
3. We define an equatorial bond
angle as 120 degrees.
4. Note that the axis and the
equatorial plane are
perpendicular.
5. The complete molecule is described
as having a trigonal bipyramidal
shape.
How to draw the AX5 molecule:
You should visualize the
drawing as having the central
atom where the “hole” is and
the “X” atoms at the ends of the
axis and also at the corners of
the triangle.
We commonly call this representation the “Pizza on a Stick” model.
Finally, the AX6 molecule.
There are obviously 6 bonds here:
1. What hybrids does the central
atom use? (sp3d2)
2. What elements are capable of
hybridizing this way? (S, Se)
3. What will the valence be
around the central atom once
the bonding has occurred? (12)
4. What would a Lewis Dot
Diagram of this molecule look
like?
X
X
X
A
X
X
X
Now in 3 Dimensions.
If you look carefully (imagine…)
there are 8 faces in this structure.
We name this shape “octahedral”.
All of the bond angles are 90
degrees. The molecule is fully
symmetrical.
How we draw the molecule.
for now….