Intermolecular
Forces
Pages 112-113;
167-169
Intermolecular Forces
 Forces
of attraction
between molecules
 Link molecules together
instead of atoms
 Not very strong
Three Types
 Dispersion
Forces (AKA
London Dispersion Forces)
 Dipole Interactions
 Hydrogen Bonding
Dispersion Forces
 Weakest
of the three
intermolecular forces
 Occur between nonpolar
molecules
 Caused by the movement of
electrons around the molecule
Dispersion Forces
H-H share electrons equally,
but as electrons are zipping
around the orbitals, they
might both be found in the
same area at the same time.
This would give the molecule a
temporary or instantaneous
dipole.
Dispersion Forces

The larger the molecule (the more
electrons it contains), the stronger the
dispersion force becomes.
Nonpolar
Molecule
Boiling Point
(°C, 1 atm)
# electrons
Comparison
of strength
H2
-253
2
weakest
O2
-183
16
Cl2
-34
34
Br2
59
70
strongest
Dipole Interactions
 Also
known as dipole-dipole
 Occur when polar molecules are
attracted to one another
 Positive end of a polar molecule
attracts to the negative end of
another polar molecule
Dipole Interactions
Dipole Interactions
Dipoles can be
induced on
nonpolar
molecules as
well.
 Example:
Water and
Oxygen

O
H
H
O
O
Dipole Interactions
O
H
H
O
Induced dipoles
O
Hydrogen Bonding
 The
intermolecular attraction
between a hydrogen atom which
is covalently bonded to N, O or
F AND an unshared pair of
electrons on another N, O, or F
atom
 H-F
H-O
H-N
Hydrogen Bonding
 Because
H is so small, it can
approach the unshared pair of
electrons very closely.
 Dotted lines are used to show
hydrogen bonding
 Hydrogen bonding is a very
strong IMF.
Hydrogen Bonding in Water
O
O
H
H
H
H
O
H
H
O
O
H
H
H
H
Practice Problem 1

What type of intermolecular force
will happen between the following
molecules?
 F2
 ICl
 CCl4
 H2S
 NH3
 HF
Practice Problem 3
Water (H2O) and ammonia (NH3)
are both relatively small polar
molecules. Using IMF, explain
why water’s boiling point is 100°C
while ammonia’s is -33°C.
Practice Problem 2
 Rank
the molecules below in
order of lowest boiling point to
highest boiling point.
 H2O
 CH4
 HBr