1. Lecture 10: VSEPR Theory (Ch 8)
Dr. Harris
9/20/12
HW: Ch 8: 19, 23, 29, 33

2. Introduction
To date, we have learned about the Lewis structures of covalent bonds
Lewis structures give insight into how atoms are bonded within a molecule, but does NOT tell us about the shape (molecular geometry), of the molecule
Molecular geometry plays a major role in the properties of a substance. This is particularly true with biochemical reactions.

3. Importance of Molecular Shape and Structure
Thalidomide was a popular drug for the treatment of morning sickness. The chemical formula of Thalidomide is C13H10N2O4
If the synthetic procedure is not properly controlled, either of the two optical isomers (mirror images) of the drug can form
Relieves morning sickness
Severe birth defects

4. Considering Molecular Geometry
If we draw the Lewis structures of water without considering geometry, we would derive the following: H O δ+ δ-
The Lewis structure suggests that water is a linear (straight) molecule.
However, if this were true, then the dipoles moments would be in opposite directions, as described above, and water would be a nonpolar molecule
If this were the case, life as we know it would be very different

5. Considering Molecular Geometry
The actual geometry of water is shown below: H O δ+ δ- + =
104.5o
This is a bent geometry. The angle between the atoms is 104.5o. In this geometry, the molecule has a net dipole moment directed upward, which is why water is polar.
How do we determine the geometry?

6. VSEPR Theory
The images below show balloons tied together at their ends.
There is an optimum geometry for each number of balloons, and the balloons spontaneously attain the lowest-energy arrangement.
In other words, the balloons try to “get out of each other’s way” as best they can. These arrangements maximize the distance between the balloon centers. Electrons behave the same exact way.

7. VSEPR
In the valence-shell electron-pair repulsion theory (VSEPR), the electron groups around a central atom:
are arranged as far apart from each other as possible
have the least amount of repulsion of the negatively
charged electrons
have a geometry around the central atom that determines molecular shape

8. Using VSEPR To Predict Geometry
STEP 1
Figure out the Lewis dot structure of the molecule.

9. Domain Geometry Around
Total
Electron Domains
Domain Geometry Around
Central Atom
Bonding
Domains
Lone Pair
MOLECULAR GEOMETRY B A B 2 2
Linear
Ex. CO2 A B
Trigonal
planar 3 3
Ex. BH3 A B ••
Bent
104.5o 2 1
Ex. NO2-

10. Just a note
Any molecule containing only two atoms must be linear. There is no other possible arrangement. Ex. H2, HCl, CO, etc.

11. Examples Give the chemical structures and geometries of the following:
BeCl2
HCN
SO2 F2

12. 4-Coordinate Molecules Have a Tetrahedral Arrangement
A tetrahedron is a shape consisting of 4 triangular faces. The vertices are separated by an angle of 109.5o, and each position is equivalent.
Another way to view a tetrahedron is to imagine a cube with atoms at opposite corners, with the central atom at the center of the cube.

13. Domain Geometry Around
Total
Electron Domains
Domain Geometry Around
Central Atom
Bonding
Domains
Lone Pair
MOLECULAR GEOMETRY A B
Tetrahedral 4
Ex. CH4 A B ••
Trigonal
Pyramidal 4 3 1
Ex. NH3 A B ••
Bent
104.5o •• 2 2
Ex. H2O

14. Examples Give the chemical structures and geometries of the following:
PF3
OF2

15. Expanded Electron Domains
As stated in the previous lecture, central atoms with a principal quantum number of n>3 can accommodate more than 8 valence electrons.
In many instances, there will be 5 or 6 bonds around these central atoms
The regions occupied by the constituent atoms in a 5-coordinate structure are not equivalent. The constituent atoms may be either equatorial or axial.

16. Five-Coordinate MoleculesZ X
Axial position (z axis)
Equatorial position (x-y plane) Y
Five coordinate molecules assume some variation of the trigonal bipyramidal configuration shown to the left.
If you have a 5 coordinate molecule which contains a lone pair, like SF4, the lone pair will go in an equatorial position.
Lone pair want to be as far away from other electron domains as possible

17. Axial vs. Equatorial Lone PairF ••
In the top arrangement, we have placed the lone pair in an equatorial position. Here, the lone pair has two ‘nearby neighbors’ that are 90o, and two ‘distant neighbors’ 120o away
In the bottom arrangement, the lone pair is in an axial position. The lone pair has three ‘nearby neighbors’ 90o away and one ‘distant neighbor‘ 180o away.
The top arrangement is preferred because the lone pair has less ‘nearby neighbors’ A E E E A S F ••

18. Domain Geometry Around
Total
Electron Domains
Domain Geometry Around
Central Atom
Bonding
Domains
Lone Pair
MOLECULAR GEOMETRY
Trigonal
Bipyramidal A B 5
Ex. PCl5 A B ••
Seesaw 4 1 5
Ex. SF4
T-shaped A B •• 3 2
Ex. ClF3

19. A Five-Coordinate molecule with 3 Lone pairs is LINEAR
Symmetrical about the central atom.
Ex. XeF2
Note: In this chapter, you will find that Xe is actually able to make chemical bonds.

20. Examples Give the chemical structures and geometries of the following:
PBr5
PF4-
TeCl4

21. Six-Coordinate Molecules Take on an Octahedral Geometry
Unlike a trigonal bipyramid, the equatorial and axial positions in an octahedral are equivalent.
When placing lone pairs in the structure, we must still maximize their distance. It is customary to first place lone pair in the axial positions.

22. Lone Pairs Migrate As Far Away From One Another As Possible
If a second lone pair exists, it is placed the maximum distance (180o) from the 1st pair
First electron pair is placed in an axial position

23. Domain Geometry Around
Total
Electron Domains
Domain Geometry Around
Central Atom
Bonding
Domains
Lone Pair
MOLECULAR GEOMETRY
Octahedral B B B 6 A B B B
Ex. SF6 6
Square pyramidal 5 1 •• B B A B B B
Ex. BrF5

24. Domain Geometry Around
Total
Electron Domains
Domain Geometry Around
Central Atom
Bonding
Domains
Lone Pair
MOLECULAR GEOMETRY
Square Planar •• 6 4 2 B B A B B ••
Ex. XeF4

25. Examples Give the chemical structures and geometries of the following:
SeF6
ICl5
NiCl42-

26. Determining Polarity
Now that we know the geometry of molecules, we can determine whether or not the molecule is polar (has an overall dipole moment)
A polar molecule
contains polar bonds, as determined from differences in electronegativity (lecture 14)
has a separation of positive and negative partial charges, called a dipole, indicated with + and –
has dipoles that do not cancel (not symmetrical)

27. - - - O C S N - H H Cl + + + Polar Molecules Overall Dipole moment =δ - +
Overall Dipole moment = - δ - δ O C S δ +
Overall Dipole moment = N - δ + H
Overall Dipole moment =

28. C O - H C - H + H Cl + + Nonpolar Molecules Symmetrical
A nonpolar molecule
contains nonpolar bonds, as determined from differences in electronegativity
Or may be symmetrical
Or dipoles cancel C O δ + -
Symmetrical Cl H H + δ C - δ + H
OVERALL DIPOLE = 0