1. H C Valence Shell Electron Pair Repulsion Theory 2p 2s 1s Ken Rogers
Miami Killian
Part 1

2. We’ll start with methane, CH4.
VSEPR theory is used to predict the shape of a molecule. In order to do this, however you need to be able to draw an electron dot structure for the molecule.
After that, just count the electron groups around the central atom and count those groups that are bonding groups.
We’ll start with methane, CH4.

3. C H C The electron configuration of carbon is 1s2 2s2 2p2.
The orbital diagram shows the electrons in the 2s sublevel are paired. 1s 2s 2p
In order for carbon to bond with hydrogen to form CH4, a 2s orbital electron would need to move to a 2p sublevel. H C

4. This process of the 2s electron moving to a 2p sublevel is called promotion.
The orbitals and their shapes change.
The new orbitals have a new shape and the blending of the s orbital with the three 2p orbitals is called hybridization.

5. H C 4 valence orbitals repel themselves as far apart as possible.
And 4 orbitals spread out as far as possible in 3 dimensional space is not :
the real shape is:
All bond angles are 109.5o

6. Z X Y s
sp3
px,py,pz
The blending of an s orbital with three p orbitals results in the hybridization type known as sp3 hybridization.

7. Tetrahedral 4, 4 = tetrahedral
Any electron dot structure that has 4 orbitals and all 4 are bonding is tetrahedral shaped.
4, 4 = tetrahedral

8. Now, the electron dot structure for ammonia, NH3 begins with N,
Promotion is not necessary to bond the hydrogens.
4,3 = ?
The number of orbitals is 4, but only 3 of them are bonding.

9. The bonding does not need promotion.
The hybridization is still sp3. All molecules with 4 orbitals have sp3 hybridization.
The non bonding orbital is not stretched out between the atoms so it is larger than the other orbitals.
The larger non-bonding orbital repels the other 3 orbitals closer together.
Bond angles are 107o

10. The 4 atoms form a 3 sided pyramid
4, 3 = trigonal pyramidal
The 4 atoms form a 3 sided pyramid

11. Water is also sp3 hybridization.
4 orbitals, only 2 bonding. H
4,2
4, 2 = bent
Bond angles are 104.5o

12. Cl H 4, 1 = linear 4 orbitals, only 1 bonding. No bond angles
(An angle requires 3 points.)

13. To summarize so far: Copy this chart. #of orbitals, bond type of
Example # bonding shape angle hybrid
CH , tetrahedral o sp3
NH , trigonal pyramidal o sp3
H2O , bent o sp3
HCl , linear none sp3
More to follow

14. F B F F 3,3 = ? The electron dot structure for BF3 is
BF3 is a molecule that doesn’t achieve an octet. F B F F
3,3 = ?
The number of orbitals around the central atom is 3, all 3 of are bonding.

15. This is a new hybridization type.
Boron has two of its electrons paired in the 2s sublevel.
Promotion of a 2s e- to a 2p sublevel occurs. B
2sp2 2s 2p 1s
This is a new hybridization type.
sp2 hybridization

16. F B F
And produces three orbitals arranged as far apart as possible. F
3,3
This is why the theory is called Valence Shell Electron Repulsion.
How do you arrange 3 orbitals as far apart as possible?

17. Bond angles are 120o
120o
120o
3, 3 = trigonal planar

18. S O 3,3 = trigonal planar The electron dot structure for SO3 is
SO3 achieves an octet by forming a double bond. O S
3,3 = trigonal planar
The number of orbitals around the central atom is 3 because the one double bond is considered one bond. All three are bonding.

19. S O 3,3 = trigonal planar This is the ball and stick model of SO3.

20. The electron dot structure for SO2 is
3,2 = ?
The number of orbitals around the central atom is 3 but only two are bonding.

21. The nonbonding pair repels the bonding pairs closer together.
Bond angle about 117o
3,2 = bent

22. Molecular oxygen would have the following shape.
No bond angle
3,1 = linear

23. It’s not necessary to go through all the promotion and hybridization steps to figure out the shape of a molecule.
The shape can be determined from its electron dot structure.
1. Draw the electron dot structure.
2. Count the number of electron groups around the central atom. Then count the groups that are bonding. Double bonds and single bonds are considered one group.

24. C O C O 2,2 triatomic linear Consider carbon dioxide, CO2.
Around the central atom, are 2 electron groups. 2 are bonding.
Two electron groups repelling themselves as far apart as possible would result in a linear arrangement. The shape is triatomic linear.

25. 180o

26. The electron dot structure for N2 is:
2,1 = linear
sp hybridization

27. Finish your chart. To summarize #of orbitals, bond type of
Example # bonding shape angle hybrid
BF3
SO , trigonal planar o sp2
SO , bent o sp2
O , linear none sp2
CO , triatomic linear sp
N , linear none sp