1. “The shape of things to come”
Molecular Geometry
“The shape of things to come”
Watson and Crick with
their famous 3-D
model of DNA
How did they know?
Shape of molecules important as influences their physical and chemical behavior

2. What Determines the 3-D Shape of a Molecule?
Need to understand electrostatic repulsion
Need to draw Lewis structures
Need to differentiate between lone pair electrons and bonding pair electrons around the central atom in a molecule
Need to apply a theory based on all of the above

3. Lewis Structure
Lone pair e-
Bonding pair e-
Electron pairs are also called domains
Electron domains try to stay out of each
others way (to minimize repulsion)

4. Valence Shell Electron Pair Repulsion Theory
VSEPR, for short
Proposed by English chemist Ron Gillespie in the 1950s
Based on Lewis structures of molecules and electron repulsion

5. VSEPR Theory Rules
In a molecule, electron domains (pairs) will orient themselves around the central atom in an arrangement that minimizes the repulsions among them.
The shapes of different molecules or ions depend on the number of electron domains surrounding the central atom.
Lone pair electrons count as 1 electron domain; single, double or triple bonding pair electrons count as 1 electron domain.

6. Applying VSEPR Rules LiH Draw Lewis Structure Li : H
2. Count # of lone pair and bonding pair
e- around central atom.
3. 0 l.p. e-, 1 b.p. e- = linear geometry
Li-H
Generalization – molecules having 0 lone pair e- and 1 bonding pair e- around the central atom will have a linear shape

7. Other Linear Molecules
CO2
Lewis Structure
2 bonding pair e-, 0 lone pair e-
2 b.p. e- get as far apart as possible (180°)
Shape is Linear
Bonding pair e-
Central atom

8. Other Linear Molecules
BeH2
Lewis Structure
2 bonding pair e-, 0 lone pair e-
Shape = Linear HF
Lewis structure Linear
1 b.p. e-, 3 l.p. e-

9. BH3 Lewis Structure 3 bonding pair e-, 0 lone pair e-
Minimization of repulsion dictates the 3 bonding pair e- orient themselves toward the corners of a triangle
Triangular or Trigonal Planar

11. CH4 Lewis Structure 4 bonding pair e-, 0 lone pair
Minimization of repulsion = orientation of b.p. e- towards the corner of a tetrahedron
Tetrahedral

12. NH3 Lewis Structure 3 bonding pair e- 1 lone pair e-
Minimization of repulsion = pyramidal shape

14. H2O Lewis Structure 2 lone pair e- 2 bonding pair e-
Minimization of repulsion = Bent or V shape

15. Polarity of Molecules Polar molecules must meet 2 criteria
Bonds in molecule must be polar covalent
Shape of molecule must be asymmetrical
Examples NH3, H2O
Formula
3-D shape
3-D shape with bond dipoles ( )
NH3
H2O

16. Polarity of Molecules