VESPR Theory

1
VESPR Theory

• Chemistry 332

2
How realistic is the Lewis dot pictures to actual
molecule shape?

• Just as a picture may fail to do your appearance
  justice.
• Electron dot structures fail to reflect the
  three-dimensional shapes of molecules
• Get out a sheet of paper and grab a partner from
  around you and go get a model kit
• Let the building begin!!!

3
Perform the following tasks

• Draw the Lewis dot structure for CH4
• Using your model kits build this molecule.
• Draw a picture under the Lewis dot as it look in
  the model you just built

4
METHANE MOLECULE

• Methane molecules really exist in three
  dimensions
• The geometric shape is called a regular
  tetrahedron
• In this arrangement, all the H-C-H angles are
  109.5

5
Tetrahedral Shape

• The methane molecule has four bonding electron
  pairs and no unshared pairs
• The bonds are farthest apart when the angle
  between the central carbon and its attached
  hydrogens is 109.5

6
VSEPR theory

• The Valence-Shell Electron-Pair Repulsion theory
• States that because electron pairs repel,
  molecules adjust their shapes so that the
  valence-electron pairs are as far apart as
  possible.

7
More Building

• Draw the electron dot structure for ammonia, NH3
• Build the model and draw an accurate picture of
  what you see from your model.

8
Pyramidal Structure

• Unshared pairs of electrons are important when
  you are trying to predict the shapes of
  molecules.
• The unshared pared pair of electron in the NH3
  molecule strongly repels the bonding pairs,
  pushing them closer together
• Bond angle is only 107

9
Back to building

• Draw the Lewis dot structure for water, H2O
• Build the model using the model kits.
• Draw the picture of the model you have created

10
Bent shape

• Two bonding pairs and two unshared pairs of
  electrons from a tetrahedral arrangement around
  the central oxygen.
• The water molecule is planar (flat) but bent
• Measure bond length is about 105

11
Back to building

• Draw the Lewis dot structure for CO2
• Build the CO2 model using the model kits
• Draw a picture of the model you just made

12
Linear molecule

• The carbon in CO2 has no unshared pairs
• The double bonds joining the oxygens to the
  carbon are farthest apart when the bond angle is
  180.

13
Trigonal planar

• Draw the Lewis dot structure for NO3-1
• You will notice that there is a double bond with
  no unshared pair of electrons
• Now build this model
• This new shape is called trigonal planar

14
Trigonal Planar

• Has bond angles 120
• Consist of one double bond no lone pair of
  electrons

15
Build the following molecules and predict their
shape

• N2
• NH3
• HF
• CH3OH
• H2S
• CHCl3
• I2
• O2

16
Here are the answers

• N2 linear
• NH3 pyramidal
• HF linear
• CH3OH tetrahedral
• H2S bent
• CHCl3 tetrahedral
• I2 linear
• O2 linear

17
(No Transcript)