1. Molecular Geometry (Shapes of Molecules)
VSEPR Theory
Honors Chemistry

2. VSEPR Theory
Electron groups around the central atom will be most stable when they are as far apart as possible – we call this valence shell electron pair repulsion theory
because electrons are negatively charged, they should be most stable when they are separated as much as possible
The resulting geometric arrangement will allow us to predict the shapes and bond angles in the molecule

3. Electron-group repulsions and the five basic molecular shapes.
linear
trigonal planar
tetrahedral
trigonal bipyramidal
octahedral

4. Electrons vs. Molecular Geometry
The geometry of electron pairs around a central atom is called the electron geometry.
The arrangement of bonded nuclei around a central atom forms the molecular geometry.
Lone pair electrons on a central atom will repel other pairs but will not be visible in the molecular geometry (no nuclei)
If there are lone pairs on the central atom the electron geometry and the molecular geometry will differ.

5. Two electron pairs on central atom
Examples:
CS2, HCN, BeF2

6. 3 electron pairs on central atom All are in bonds
Trigonal Planar Shape
Examples:
SO3, BF3, NO3-, CO32-

7. Bent Shape 3 Electron Pairs 2 Bonded 1 Un-Bonded
Un-bonded electron pair takes up more space and “repels” more.
The bond angle will change to less than the original 120o to about 117o-115o
Other Examples:
SO2, O3, PbCl2, SnBr2
About 117o

8. Four electron pairs on central atom All 4 in bonds
Examples:
CH4, SiCl4, SO42-, ClO4-

9. Trigonal Pyramidal Shape
4 Electron Pairs
3 bonded
1 un-bonded (lone pair)
Trigonal Pyramidal Shape
Examples: NH3, PF3, ClO3. H3O+
Bond angles are reduced from 109.5o to 107o due to extra repulsion by lone pair

10. 2 Un-bonded (lone pairs) BENT SHAPE
4 Electron Pairs
2 Bonded
2 Un-bonded (lone pairs)
BENT SHAPE
Examples: H2O, OF2, SCl2
Bond angles are reduced a little more due to repulsion
To 104.5o

11. Five electron pairs on central atom
All Bonded = Trigonal Bipyramidal

12. 4 Bonded and 1 Un-bonded (lone pair)
5 Electron Pairs
4 Bonded and 1 Un-bonded (lone pair)
Also called an Irregular Tetrahedron

13. 5 Electron Pairs
3 Bonded
2 Un-Bonded
T-Shaped
Molecule
5 Electron Pairs
2 Bonded
3 Un-Bonded
= LINEAR Molecule!!!
Bond Angles = 180o

14. Six electron pairs on central atom
All Bonded = Octahedral Shape

15. 6 Electron pairs
5 Bonded
1 Un-Bonded Lone Pair
Square Pyramidal shape

16. 6 Electron pairs
4 Bonded
2 Un-Bonded Lone Pair
Square Planar shape

17. The steps in determining a molecular shape
Molecular formula
Lewis structure
Electron-group arrangement (electron geometry)
Bond angles
Molecular geometry
Count all e- pairs around central atom
Note lone pairs and double bonds
Consider bonding e- pairs only
Step 1
Step 2
Step 3
Step 4

18. Representing 3-Dimensional Shapes on a 2-Dimensional Surface
One of the problems with drawing molecules is trying to show their dimensionality
By convention, the central atom is put in the plane of the paper
Put as many other atoms as possible in the same plane and indicate with a straight line
For atoms in front of the plane, use a solid wedge
For atoms behind the plane, use a hashed wedge

19. Predicting Molecular Shapes with Two, Three, or Four Electron Groups
PROBLEM:
Draw the molecular shape and predict the bond angles (relative to the ideal bond angles) of (a) PF3
SOLUTION:
(a) For PF3 - there are 26 valence electrons, 1 nonbonding pair
The shape is based upon the tetrahedral arrangement.
The F-P-F bond angles should be < due to the repulsion of the nonbonding electron pair.
The final shape is trigonal pyramidal.
<

20. Predicting Molecular Shapes with Five or Six Electron Groups
PROBLEM:
Determine the molecular shape and predict the bond angles (relative to the ideal bond angles) of (a) SbF5 and (b) BrF5.
SOLUTION:
(a) SbF valence e-; all electrons around central atom will be in bonding pairs; shape is trigonal bipyramidal.
(b) BrF valence e-; 5 bonding pairs and 1 nonbonding pair on central atom. Shape is square pyramidal.

21. Predicting Molecular Shapes with More Than One Central Atom
PROBLEM:
Determine the shape around each of the central atoms in acetone, (CH3)2C=O.
Find the shape of one atom at a time after writing the Lewis structure.
SOLUTION:
tetrahedral
tetrahedral
trigonal planar
>1200
<1200