1. The VSEPR Theory
Section 4.3

2. VSEPR VSEPR (Valence Shell Electron Pair Repulsion)
valence electrons stay as far apart as possible to minimize repulsion
Tells us the geometry of the molecule
When looking at a molecule we look specifically at the central atom (the one that has the most bonding electrons) to determine the geometry

3. Rules of VSEPR
Only the valence shell electrons of the central atom(s) are important for molecular shape
Valence shell electrons are paired or will be paired in a molecule or polyatomic ion
Bonded pairs or electrons and lone pairs of electrons are treated approximately equal
Valence shell electron pairs repel each other electrostatically (“like repels like”)
The molecular shape is determined by the positions of the electron pairs when they are a maximum distance apart.

4. Representing 3-D Structures
The solid wedge represents atoms/pairs of electrons coming TOWARDS the viewer
The dashed wedge represents atoms/pairs of electrons going AWAY from the viewer

5. Using VSEPR
First draw Lewis structures of the molecule, including the electron pairs around the central atom
Count the total number of bonding pairs and lone pairs around the central atom
Refer to table 1 on pg. 245 or Appendix C3 to predict the shape

6. These are the major types. Others shown in Appendix C3 include:
Linear
AX2
AX3
Trigonal planar
Tetrahedral
AX4
Trigonal pyramidal
AX3E
Bent
AX2E2
These are the major types. Others shown in Appendix C3 include:
AX5 – Trigonal bypyramidal
AX6 – Octahedral
AX5E – Square pyramidal
AX4E2 – Square planar
A represents the central atom
X represents the bonded pairs
E represents lone pairs of electrons

8. Examples What are the shapes of the following molecules:
nitrogen trihydride dihydrogen sulfide
methane boron trichloride

9. Some Things you should know…
Secrets we’ve been keeping:
Sulfur can have several different valence configurations:
It is actually found in nature as S8
Can have 2, 4, 6 unpaired electrons
Phosphorous
Can have 3, 4, 5 unpaired electrons
Basically the larger the atom the more electrons it can have.
And some noble gases form compounds!
When in doubt use the method of adding up valence electrons from yesterday to determine the # of l.p. of e- on the central atom.

10. phosphorous pentafluoride bromine pentachloride
xenon tetrafluoride sulfur hexafluoride

11. Assign Geometries for:
Iodine dichloride ion (ICl2-)
Chlorine trifluoride (ClF3)
Sulfur tetrafluoride (SF4)
When you are done complete the worksheet.
(we are working on this tomorrow too!)