Attractions Between Molecules or
Intermolecular Forces (IM Forces)
Chapter 8.4
Learning Objectives
• Understand the difference between intramolecular and
intermolecular forces
• Identify the 4 common types of intermolecular forces
We have already talked about
intramolecular forces.
Intramolecular forces hold
atoms together in a molecule.
Covalent bonds would be an example.
Intermolecular forces are
attractive forces between
molecules.
Intermolecular Forces
Intramolecular = strong
Intermolecular = weak
They do control physical properties such as
boiling and melting points, vapor pressure,
and viscosity
Types of Intermolecular Forces
Van der Waals Forces
• Dipole-dipole interactions
• Ion-Dipole interactions
• London dispersion forces
Hydrogen bonding
Dipole-Dipole Interactions
Molecules that have
permanent dipoles are
attracted to each other.
Ion-Dipole Interactions
Attractive forces between an ion and a polar molecule
Ion-Dipole Interaction
The larger the charge the stronger the force
London Dispersion Forces
Occasionally electrons wind up on the same
side of the atom.
London Dispersion Forces
At that instant, the helium atom is polar.
London Dispersion Forces
This polar helium atom then induces a dipole
on a neighboring helium atom.
London Dispersion Forces
London dispersion forces, or dispersion
forces, are attractions between an
instantaneous dipole and an induced dipole.
London Dispersion Forces
• These forces are present in all molecules,
whether they are polar or nonpolar.
• The tendency of an electron cloud to distort in
this way is called polarizability.
Types of Intermolecular Forces
Hydrogen Bonding
• The dipole-dipole interactions
experienced when H is bonded to
N, O, or F are unusually strong.
• We call these interactions
hydrogen bonds.
Hydrogen Bonding: Water
Hydrogen Bonding: Water