“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

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

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)

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

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.

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

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

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-

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

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

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

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

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

Polarity of Molecules