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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

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

C.  bond length
1.  different pairs of elements form bonds of different length
2.  move down the periodic table - bond length get longer
3.  multiple bonds are shorter than single bonds

II. Polarity
A.  determining polarity
1.  molecules can be polar or nonpolar
2.  called dipoles
3.  molecule contains only nonpolar bonds - nonpolar molecule
  1. polarity determined by bond polarity and shape

B.  large molecules
1.  molecular polarity often determine its shape
2.  polar and nonpolar side chains cause the molecule bend and fold on itself
3.  polar side chains on the outside and nonpolar inside
4.  shapes
a.  rings
b.  chains
c.  spherical