Molecular Geometry
Predict with VSEPR Theory
Valence Shell Electron Pair Repulsion Theory
- Electron clouds repulse each other
- Distribute around a central atom
 as far apart as possible
- 1 electron cloud can be: 1 lone pair
1 single bond
1 double bond
1 triple bond
Arrangement of Electron Clouds - 5 basic arrangements
around a central atom.
2 clouds
linear
3 clouds
trigonal planar
4 clouds
tetrahedral
5 clouds
trigonal bipyramidal
6 clouds
octahedral
• MOLECULAR GEOMETRIES are named for the
positioning of the ATOMS (not including lone pairs)
• If all clouds lead to an atom, the molecular and
electron cloud geometries are the same.
Electron
cloud
geometry
Number Number Molecular
of
of lone geometry
bonding pairs
e- pairs
Linear
2
0
Linear
Trigonal
3
0
Trigonal
Planar
planar
2
1
Bent or
"
angular
Tetrahedral
4
0
Tetrahedral
3
1
Pyramidal
"
2
2
Bent
"
Bond
Angle
Example
180o
120o
CO2
BF3
180o
SO2
109.5o
<109.5o
CH4
NH3
<109.5o
H2O
PF5
ClF3
Trigonal
bipyramidal
5
0
"
4
1
Trigonal
pyramidal
See-saw
"
3
2
T-shaped
90o
120o
90o
<120o
90o
"
Octahedral
2
3
Linear
180o
XeF2
"
6
5
0
1
90o
90o
SF6
BrF5
"
4
2
Octahedral
Squarepyramidal
Squareplanar
90o
XeF4
SF4
Factors Affecting Bond Angles
- The bond angle decreases as the number of lone pairs
on the central atom increases.
- Lone pairs take-up more space, so they push-back the
chemical bonds.
Note: For the trigonal bipyramidal - lone pairs are always
placed in equatorial positions.
Net Polarity of Molecules
NONPOLAR
1. All nonpolar bonds
2. Two or more polar bonds, but they cancel by geometry.
Look for:
- Outside atoms alike AND no lone pairs on central
atom
OR
- Even number of lone pairs on central atom uniformly
distributed in space
POLAR (molecule is called a dipole)
1. Only 1 polar bond in the molecule
3. More than 1 polar bond
- Outside atoms not all alike AND/OR
- Only one lone pair on the central atom
OR
- 2 or more lone pairs on not evenly distributed in
space
Net Dipole Moments of Polyatomic Molecules
Example:
CO2, each C-O dipole is canceled because the molecule is
linear.
Example:
H2O, the H-O dipoles do not cancel because the molecule
is bent.