MOLECULAR
STRUCTURE
MOLECULAR GEOMETRY
VSEPR THEORY
Valence-Shell Electron-Pair Repulsion Theory:
Because electron pairs repel, molecules adjust their shapes
so that valence electron pairs are as far apart as possible
VSEPR CONTINUED…
Types of electron pairs
Bonding Pairs: Electrons that form bonds
Lone (unshared) Pairs: Electrons that are nonbonding;
no atom trying to bond. These are held closer to the atom they are
associated with than bonding pairs.
Lone Pairs Repel More Strongly Than Bonding Pairs!
LONE PAIR EFFECTS
ON BOND ANGLES
In methane there are no lone
pair electrons so in the
tetrahedral shape the bond
angles are the expected
109.50.
In the ammonia molecule there
is one lone pair. Since it has a
stronger repulsion is closes the
bond angle a little to 1070
In the water molecule there are
2 lone pairs each having
greater repulsion causing the
angle to close even more to
104.50 angle.
DETERMINING
MOLECULAR SHAPE
• Draw the Lewis Diagram
• Count up electron pairs on central atom.
double/triple bonds = ONE pair
• Shape is determined by the # of bonding pairs and lone
pairs.
Be Familiar With the Common Shapes & Their Bond Angles
See p.10 in CRM
COMMON SHAPE-LINEAR
Beryllium Hydride
2 total electron pairs
2 bonding pairs
0 lone pairs
Beryllium does not follow the octet rule.
Beryllium is complete with 4 valence
electrons
Carbon dioxide
2 total electron pairs
2 bonding pairs
0 lone pairs
Carbon does follow the octet rule fulfilling it
through two double bonds in this case
COMMON SHAPE-TRIGONAL
PLANAR
Boron Hydride
3 total electron pairs
3 bonding pairs
0 lone pairs
Boron is also an exception to the octet
rule. Boron is complete with 6 valence
electrons
Formaldehyde
4 total electron pairs
4 bonding pairs
0 lone pairs
Carbon does follow the octet rule,
which is fulfilled by the double bond.
COMMON SHAPE - BENT
Ozone
Generally O would have two
bonding pairs which is seen
by the upper O, the other
two adjust so that all three
can obey the octet rule
Water
4 total pairs
2 bonding pairs
2 lone pair
COMMON SHAPETETRAHEDRAL
Methane
4 total pairs
4 bonding pairs
0 lone pairs
COMMON SHAPE-TRIGONAL
PYRAMIDAL
Ammonia
4 total pairs
3 bonding pairs
1 lone pair
COMMON SHAPE- TRIGONAL
BIPYRAMIDAL
Phosphorous
pentachloride
5 total pairs
5 bonding pairs
0 lone pairs
COMMON SHAPE OCTAHEDRAL
Sulfur Hexafluoride
6 total pairs
6 bonding pairs
0 lone pairs
POLAR MOLECULES
Therefore, polar molecules have…
- asymmetrical shape (lone pairs) or
- asymmetrical atoms