Intermolecular
Forces
Intermolecular Forces
 Forces
that exist in a collection of
molecules.
 In the gaseous state, these forces
are negligible. The gaseous
molecules do not interact with
each other.
van der Waals Forces
 The
forces of interaction between
molecules.
 In
liquids
and
solids,
the
molecules interact and have a
great influence on one another.
Johannes van der Waals
(1837-1923)
 A Dutch physicist, while working
on the theory of ideal gases,
recognized the existence of
some type of weak force on
particles that are very close to
each other.
Classification of van der
Waals Forces
Dipole-dipole
 In
the solid and liquid state, the
molecules
align
themselves
where the positive end of one
dipole is directed toward the
negative end of the neighboring
dipole.
The presence of dipole-dipole
interaction explains the higher
boiling point of a polar molecule
than a nonpolar molecule of the
same molecular weight.
Instantaneous-induced
dipole or London Forces
This type of interaction exists in a nonpolar
species.
 If at a particular instance an event occurs
that causes a distortion of the electron
cloud, a normally nonpolar species
becomes an instantaneous dipole.
 Following this, a neighboring non-polar
molecule is induced to become a dipole.

Fritz London (1900-1954)
A
German physicist, offered a
theoretical explanation on this
type of intermolecular force.
 He also devised the first quantum
mechanical treatment of the
hydrogen molecule.
A nonpolar
species
A nonpolar species
becomes an
instantaneous dipole
due to the distortion of
the electron cloud
A nonpolar molecule
induced to become a
dipole is called
induced dipole
Hydrogen Bond
A
hydrogen bond occurs only under
special circumstances, that is, if
hydrogen is bonded to highly
electronegative atoms such as
nitrogen, oxygen, and fluorine.
 It is a strong intermolecular force.
Identify what type of van der Waals
Forces exist in the following molecules.
1.
2.
3.
4.
5.
FeO
BF3
N2
NH3
CO2