This document introduces valence shell electron pair repulsion (VSEPR) theory, which is used to predict the molecular geometry of molecules based on the number of electron domains around the central atom. It explains that electron pairs repel each other and will spread out as far as possible in three dimensions. The number of electron domains determines the domain geometry, while the molecular geometry is based only on the arrangement of bonding domains. Examples are used to demonstrate how to draw dot diagrams and determine the domain and molecular geometries. Non-bonding pairs can cause a difference between domain and molecular geometry. Common molecular geometries that result from different numbers of bonding domains are also outlined.

1. VSEPR Models

2. Purpose
• Now that you know how to draw dot diagrams, we will translate them into
shapes for the molecules called molecular geometry. This will ultimately
allow us to understand properties of the substances. For example, if the
water molecule was linear rather than bent, much of the water on earth
would vaporize since the boiling point of water would drop significantly.
Similarly, the function of proteins in our body is dependent on the shape
of the protein, which is determined from the arrangements of the bonds
in the protein molecule.

3. Example – H2O
• We will start with water as an example here. The water molecule has a
bent geometry with the hydrogens with an angle of 104.5o between them.
We need to understand why water takes this geometry.
H H
O

4. The Big Idea – Valence Shell Electron Pair
Repulsion (VSEPR)
• Pairs of electrons whether bonding or non-bonding will try to
get as far away from each other in three dimensions as
possible since they repel one another.
• We call a pair of electrons whether bonding or non-bonding
an electron domain.
• Note: A single domain is either a non-bonding pair, a single
bond, a double bond, or a triple bond.

5. Number of electron domains determines the
domain geometry.
• 2 domains – Linear Geometry, 180o angle
between domains
• 3 domains – Trigonal Planar Geometry,
120o angle between domains
• 4 domains – Tetrahedral geometry, 109.5o
angle between domains

6. Some examples:
• CO2
– Dot Diagram:
– Number of domains – two – each double bond counts as a single domain
– Geometry – linear
• AlCl3
– Dot Diagram:
– Number of domains – three – each single bond counts as a domain.
– Geometry – trigonal planar
• CH4
– Dot diagram
– Number of domains – four – each single bond counts as a domain.
– Geometry - tetrahedral

7. What about when we have non-bonding pairs?
• The examples we have looked at so far had all bonding pairs.
When we have non-bonding pairs, the domain geometry does
not change, but the molecular geometry does.
• Example – H2O
– Dot diagram:
– Number of domains – four – each bond and each non-bonding pair
counts as a domain
– Domain Geometry – tetrahedral because four domains
– Molecular geometry - bent

8. The Big Idea
• Non-bonding pairs influence the geometry of the molecule, however
when naming the molecular geometry, we look at the arrangement of the
bonding pairs.
• H2O – the four domains take the tetrahedral geometry. The two domains
in blue represent the non-bonding pairs and the ones in gray represent
the bonds to hydrogen. Looking at just the bonds to the hydrogens, we
can see that the shape is bent
Domain Geometry – tetrahedral –
shows all domains
Molecular Geometry – bent –
only looks at the bonds

9. Misconception Alert
• Many students see the dot diagram of water and want to call it linear.
Remember that dot diagrams are two dimensional structures on paper.
They do not show geometry; rather, they are used to help predict
geometry.
The dot diagram is correct, but water is not
a linear molecule!

10. Molecular Geometries
• Now that we understand the difference between domain and molecular
geometries, we will look at the different molecular geometries for up for
four domains.

11. Molecular Geometries for three domains.
• Trigonal Planar Molecular
Geometry: All three domains are
bonding pairs
• Bent Molecular Geometry: Two
of three domains are bonding
pairs

12. Molecular Geometries based on Tetrahedral
• Tetrahedral Molecular Geometry:
All four domains are bonding
pairs
• Trigonal Pyramidal Molecular
Geometry: Three of four domains
are bonding pairs
• Bent Molecular Geometry: Two
of four domains are bonding pairs

13. How to approach these problems…
1. Draw the dot diagram of the molecule.
2. Count the domains to determine the domain geometry.
3. Count the bonding domains to determine the molecular geometry.

14. Summary of Domain and Molecular Geometries
Total Number
of domains
Number of
Bonding
Domains
Molecular
Geometry
Example Dot
Diagram
Example
Structural
Formula
2 2 Linear
3 2 Bent
3 Trigonal Planar
4 2 Bent
3 Trigonal
Pyramidal
4 Tetrahedral

15. Misconception Alert!
• Many students do not like to draw the dot diagram. They look at a
molecule like XY3 and just guess either trigonal planar or trigonal
pyramidal. Similarly, they call anything with the formula XY2 linear, when
it is often bent. You must draw the dot diagram to determine if there are
non-bonding pairs influencing the geometry. Take your time.

16. Pause and Practice
• Draw the dot diagram and determine the domain and
molecular geometries of the following:
– NF3
– SiCl4
– BH3
– SF2

17. Pause and Practice - Answers
• Draw the dot diagram and determine the domain and molecular
geometries of the following:
– NF3
• Dot diagram→
• Domain geometry: 4 domains – tetrahedral
• Molecular Geometry: 3 of 4 domains are bonding – trigonal pyramidal
– SiCl4
• Dot diagram→
• Domain geometry: 4 domains – tetrahedral
• Molecular Geometry: 4 of 4 domains are bonding – tetrahedral
– BH3
• Dot diagram→
• Domain geometry: 3 domains – trigonal planar
• Molecular Geometry: 3 of 3 domains are bonding – trigonal planar
– SF2
• Dot diagram→
• Domain geometry: 4 domains – tetrahedral
• Molecular Geometry: 2 of 4 domains are bonding – bent

18. Try the exercises