BACK                                                          Molecular Shape

I.  The Shape of Small Molecules
A.  molecular geometry
1.  missing from structural formulas
2.  chemists use ball-stick models to show geometry
3.  geometry is usually symmetrical
4.  due to shared and unshared  electron repulsion forces
5.  valence-shell electron pair repulsion theory (VSEPR theory)
a.  states that in a small molecule, the pairs of valence electrons are arranged as far apart
from each other as possible
b.  unshared -unshared repulsion > unshared-shared repulsion > shared-shared repulsion
c. does not apply to many compounds that contain transition elements
6. 5 basic shapes
a.  linear
1. connected in straight line
2. all 2 atom molecules and many 3 atom molecules
3.  can use bond angles - angle between two adjacent bonds
4.  straight line - bond angle is 180o
b.  trigonal planar
1.  triangular and flat
2.  bond angle - 120o
3.  central atom with no unshared pairs of electrons bonded to three atoms
c.  tetrahedral
1.  tetra - 4 ;  hedral - surfaces
2.  molecules with four surfaces
3.  bond angle - 109.5o
d.  pyramidal
1.  central atom with an unshared pair of electrons bonded to three atoms
2.  bond angle - 107o
3.  angle smaller than a tetrahedral because of the greater repulsion of the unshared pair   
of  electrons
e.  bent
1.  bond angle - 105o
2.  bond angle less than pyramidal because of the greater unshared-unshared repulsion forces
B.  hybrid orbitals
1.  changes in atomic orbitals due to bonding
2.  properties of the new orbitals are a combination
3.  s and p --> two sp -- linear
4.  s and two p --> three sp2 -- trigonal planar

5. s and three p (no multiple bonds) --> sp3 -- tetrahedral