Ch. 9 – Molecular Structure
II. Molecular
Geometry
I
II
III
A. VSEPR Theory

Valence Shell Electron Pair
Repulsion Theory

Electron pairs orient themselves in
order to minimize repulsive forces.
A. VSEPR Theory

Types of e- Pairs
 Bonding pairs - form bonds
 Lone pairs - nonbonding e-
Lone pairs repel
more strongly than
bonding pairs!!!
A. VSEPR Theory

Lone pairs reduce the bond angle
between atoms.
Bond Angle
B. Determining Molecular Shape

Draw the Lewis Diagram.

Tally up e- pairs on central atom.
 double/triple bonds = ONE pair

Shape is determined by the # of
bonding pairs and lone pairs.
Know the common shapes
& their bond angles!
C. Common Molecular Shapes
2 total
2 bond
0 lone
BeH2
LINEAR
180°
C. Common Molecular Shapes
3 total
3 bond
0 lone
BF3
TRIGONAL PLANAR
120°
C. Common Molecular Shapes
3 total
2 bond
1 lone
SO2
BENT
104.5°
C. Common Molecular Shapes
4 total
4 bond
0 lone
CH4
TETRAHEDRAL
109.5°
C. Common Molecular Shapes
4 total
3 bond
1 lone
NH3
TRIGONAL PYRAMIDAL
107°
C. Common Molecular Shapes
4 total
2 bond
2 lone
H2O
BENT
104.5°
C. Common Molecular Shapes
5 total
5 bond
0 lone
PCl5
TRIGONAL
BIPYRAMIDAL
120°/90°
C. Common Molecular Shapes
6 total
6 bond
0 lone
SF6
OCTAHEDRAL
90°
D. Examples

PF3
4 total
3 bond
1 lone
F P F
F
TRIGONAL
PYRAMIDAL
107°
D. Examples

CO2
2 total
2 bond
0 lone
O C O
LINEAR
180°
C. Bond Polarity

Electronegativity Trend
 Increases up and to the right.
A. Dipole Moment

Direction of the polar bond in a molecule.

Arrow points toward the more e-neg atom.
+

H
Cl

B. Determining Molecular Polarity

Depends on:
 dipole moments
 molecular shape
B. Determining Molecular Polarity

Nonpolar Molecules
 Dipole moments are symmetrical
and cancel out.
F
BF3
B
F
F
B. Determining Molecular Polarity

Polar Molecules
 Dipole moments are asymmetrical
and don’t cancel .
O
H2O
H
H
net
dipole
moment
B. Determining Molecular Polarity

Therefore, polar molecules have...
 asymmetrical shape (lone pairs) or
 asymmetrical atoms
H
CHCl3
Cl
Cl
Cl
net
dipole
moment
C. Bond Polarity

Most bonds are
a blend of ionic
and covalent
characteristics.

Difference in
electronegativity
determines bond
type.
EN Difference
0.0 - 0.4
 0.4 - 1.0
 1.0 - 2.0
 > 2.0

Nonpolar
Moderately Polar Covalent
Very Polar Covalent
Ionic
C. Bond Polarity
Examples:
3.0-3.0=0.0
Nonpolar

Cl2

HCl 3.0-2.1=0.9
Moderately Polar
Covalent

NaCl 3.0-0.9=2.1
Ionic
C. Bond Polarity

Electronegativity
 Attraction an atom has for a shared
pair of electrons.
 higher e-neg atom   lower e-neg atom +
____
C. Bond Polarity

Nonpolar Covalent Bond
 e- are shared equally
 symmetrical e- density
 usually identical atoms
C. Bond Polarity

Polar Covalent Bond
 e- are shared unequally
 asymmetrical e- density
 results in partial charges (dipole)
+

