1. MOLECULAR SHAPES
Valence Shell Electron Pair Repulsion Theory VSEPR – a model for predicting 3-D Molecular Shapes

2. Importance of Molecular Shapes
Our universe is three-dimensional – true properties of molecules can only be understood by looking at actual arrangements of molecules in space.
Example application: Chiral molecules – most drugs contain a carbon bonded to 4 different other atoms or groups of atoms.

3. In 2-D molecules look identical, in 3-D they are not.
Link to chiral molecules
In 2-D, It looks as though the left structure can be converted into the right by rotating 180o. In 3-D, we can see that these 2 molecules are actually mirror images of each other and are not identical.

4. Chiral Molecules – binding surfaces in your body recognize one arrangement but not the other

5. Chiral blockbuster drugs
C&E News, June 14, 2004, p. 47

6. Effect of thalidomide mutations
Thalidomide – 1 form of drug treats morning sickness, the other causes mutations
Effect of thalidomide mutations

7. VSEPR Theory
VSEPR (Valence Shell Electron Pair Repulsion) Theory allows us to predict the shape of molecules.
Each molecule has pairs of electrons-
The bonding pairs form covalent bonds.
The nonbonding pairs occupy the same orbital.
Each of these pairs can be thought of as an electron cloud- a cloud of negative charge.
Link to electrical charges.
Link to Phet Simulation

8. Like Charges Repel
These clouds of electrons, areas of negative charge repel each other in such a way as to get as far apart from each other as possible.
When there are only two electron clouds, the farthest they can get away from each other is 180o, a straight line.
Link to tutorial

9. Key ideas
Molecular geometry is determined by the arrangement of bonding and non-bonding electron pairs around the central atom.
The bonding and nonbonding valence electron pairs arrange themselves as far APART in space as possible in order to minimize ELECTROSTATIC REPULSION between negative charges.

10. Effect of the number of electron pairs around the central atom
4 charge clouds, tetrahedral
3 charge clouds, trigonal planar
2 charge clouds, linear
6 charge clouds, Octahedral
5 charge clouds, trigonal bipyramidal

11. Steps for Applying VSEPR:
Draw Lewis Dot structure (be sure to include all nonbonding (lone) pairs on central atom).
Count the number of bonding and nonbonding electron pairs on the central atom.
Double and triple bonds count as only ONE bonding pair (one bonding “region”).
Consult chart from your notes below that describes arrangement of electron pairs in space that minimizes electrostatic repulsions.

12. Effect of Nonbonding Electrons Pairs on Bond Angles
Nonbonding electrons pairs are more spreadout and therefore exert a greater repulsion than bonding pairs.
Example: Bond angles for CH4, NH3, H2O
VSEPR animations

13. Conventions for Drawing in 3-D
Use to represent bonds in plane of paper
Use to represent bonds receding into plane of paper
Use to represent bonds coming out of plane of paper
Use to represent nonbonding (lone) electron pairs on central atom

14. Linear 2 bonding electrons pairs (2 atoms attached to center atom)
0 Non Bonding e- pairs
(lone pairs)
Bond angle = 180o
Type: AB2
Ex. : BeF2, CO2
Link to Linear Shape Animation

15. Three Clouds
With three electron clouds, the farthest the electron clouds can get away from each other is 120o, the corners of an equilateral triangle. This shape is known as trigonal planer.
Link to Triangular Planar Animation

16. Trigonal Planar
3 bonding electron pairs (3 atoms attached to center atom)
0 nonbonding pairs
Bond angle = 120o
Type: AB3
Ex. : BCl3

17. Bent (3 total e- pairs) 2 Bonding e- pairs 1 nonbonding e- pairs
Bond angle < 120 o
Type: AB2E2
Ex. : O3
Link to 2,1 Bent

18. Four clouds
When four clouds are attached to a central atom, the farthest they can get away from each other is the four corners of a tetrahedron. The tetrahedral angle is 109.5o.
Link to animation

19. Tetrahedral 4 Bonding electron pairs 0 Non-Bonding e- pairs
Bond angle = 109.5o
Type: AB4
Ex. : CH4

20. Trigonal Pyramidal 3 Bonding e- pairs 1 Non-bonding e- pair
Bond angle = 107o
Type: AB3E
Ex. : PH3
Link to video

21. Bent (4 total e- pairs) 2 Bonding e- pairs 2 Non Bonding e- pairs
Bond angle 104.5o
Type: AB2E2
Ex. : H2S
Link to 2,2, bent

22. Trigonal Bipyramidal 5 atoms attached to center atom 0 lone pairs
Bond angle =
equatorial -> 120o
axial -> 90o
Type: AB5
Ex. : PBr5

23. Octahedral 6 atoms attached to center atom 0 lone pairs
Bond angle = 90oType: AB6
Ex. : SF6

25. Molecules with multiple centers
A central atom is any atom with more than one atom bonded to it
Perform exercise individually for each atom
Molecular shape will refer only to the atoms/lone pairs immediately attached to that atom