Molecular Geometry

1. Molecular Geometry • 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 “The shape of things to come”

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. Lone pair e- Lewis Structure 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 andelectron 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 Bonding pair e- 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 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

16. Polarity of Molecules