Higher Chemistry
Unit 1(d)
Intermolecular forces of
attraction
After today’s lesson you should be able
to:
 Explain the difference between intramolecular
and intermolecular forces of attraction.
 Explain what is meant by a permanent dipole,
temporary dipole and induced dipole.
 Give examples of where these dipoles are found.
 Explain where hydrogen bonding occurs.
 State the relative strengths of each
intermolecular force.
What are intermolecular forces?
Intermolecular forces are the weak
forces of attraction that occur between
covalent molecules and the
monoatomic gases when thy are in
their liquid and solid states.
 They influence the physical properties
of molecules e.g. melting and boiling
points, solubility in water and other
solvents, viscosity (the flow rate) and
the density.

There are three types of intermolecular
attractions:
1.
Temporary dipole – induced dipole
(Van der Waal attractions)
2.
Permanent dipole – permanent
dipole
3.
Hydrogen bonds
Temporary dipoles


Occur in all non- metal atoms and molecules.
Occur due to constantly moving electrons
around the nucleus; this results in the electrons
being unevenly distributed. Occasionally there
are more electrons on one side of the atom or
molecule than the other.
no
dipole


temporary
dipole
This results in the atom or molecule having a
slightly positive side and a slightly negative
side.
Only occur for a split second.
Induced dipoles

Occur when a permanent or
temporary dipole induce (force) the
electrons in another unpolarised atom
or molecule to move to one side
resulting in a dipole.

Electrons can be attracted or repelled
within the atom being induced.
Permanent dipoles

A polar covalent bond is a permanent
dipole because one atom has a
permanent δ- charge whereas the other
a permanent δ+ charge.
e.g. HCl, H2O, NH3.
polar covalent bond OR
permanent dipole
Van der Waal’s attractions
Weakest intermolecular force
 Occur between ALL non-metal atoms
and molecules.
 Strongest for large atoms and large
molecules because both have lots of
electrons which can form temporary
dipoles.

Permanent dipole – permanent dipole
Additional to Van der Waals’ forces
 Stronger than Van der Waals’ forces
 Occur between polar molecules.
 The slightly positive charge of one
molecule is attracted to the slightly
negative charge of another and vice
versa.

permanent
dipole
permanent
dipole
permanent dipole to
permanent dipole
attraction
Hydrogen bonding
Occurs between molecules which have
a hydrogen atom directly bonded to a
nitrogen, oxygen or fluorine atom.
 Additional to Van der Waals’ forces and
permanent dipole-permanent dipole
interactions
 Strongest intermolecular force

Exercise

What type of dipole would be present in a molecule of
each of the following?
 Hydrogen fluoride
 Iodine
 Methane
 Carbon dioxide

What type of intermolecular force(s) would occur
between molecules of the following?
 Hydrogen fluoride
 Chlorine
 Poly(ethene)
 Water
Molecules containing polar covalent bonds
and symmetry
 If a molecule contains polar covalent bonds
(permanent dipoles) and is symmetrical the
charges are cancelled out and the overall
molecule is non-polar.
 The intermolecular forces will not be
permanent dipole-permanent dipole
attractions but Van der Waals’ forces.
e.g.


If a molecule containing polar bonds is
non-symmetrical then the charges do not
cancel out and the overall molecule is
polar.
The intermolecular forces will be Van der
Waals’ forces and permanent dipolepermanent dipole attractions.