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VSEPR Theory
4.2.7
14.1.1
Molecular Geometry - VSEPR
One way to predict the shape of a
molecule is the Valence Shell Electron
Pair Repulsion (VSEPR) Theory.
 Q – Do electrons like to hang out
together?

 A – Heck no!

The basic idea is that electron pairs in a
molecule, whether being shared or not,
will move as far apart as possible to
minimize the repulsion between them.
Molecular Geometry - VSEPR

VSEPR Steps
1. Write the Lewis electron-dot structure for the
compound.
2. Determine the number of electron pair
groups (Steric Number) surrounding the
central atom(s).
○
Double and triple bonds count as one pair
3. Determine the geometric shape that
maximizes the distance (bond angle)
between the electron groups.
Example – BeCl2

How many electron pair groups?
2

When you have 2 pair groups the shape is
linear.
Example – BCl3

How many electron pair groups?
3

When you have 3 pair groups the shape is
trigonal planar.
Example – CH4

How many electron pair groups?
4

When you have 4 pair groups the shape is
tetrahedral.
Example – PCl5

How many electron pair groups?
5

When you have 5 pair groups the shape is
trigonal bipyramidal.
Example – SF6

How many electron pair groups?
6

When you have 6 pair groups the shape is
octahedral.
Example – H2O

How many electron pair groups?
4
When you have 4 pair groups the shape is
tetrahedral.
 Is that what water looks like?

 Heck no!
Example – H2O

When you have tetrahedral geometry but
two of the sides are unshared pairs of
electrons, you end up with a bent shape.
 In the bent shape, the electron pairs are there
in a tetrahedral form but we don’t see them.
Question

What would happen to the bond angle if
there was 1 lone pairs instead of two?
 The bond angle would continue to increase.
Example – NH3

How many electron pair groups?
4

When you have 4 pair groups the shape is
tetrahedral, but if one is a lone electron
pair, we get a trigonal pyramidal.
More on bond angles

What is a little odd
about the geometries
of CH4 and NH3?
More on bond angles

Even though both
CH4 and NH3 follow a
tetrahedral pattern,
they don’t have the
same bond angles
because a lone pair
repels a little harder
than a bonding pair.
Homework

VSEPR Theory Wkst
 Try a couple tonight, entire worksheet (both
sides) is due Monday.