Intermolecular Forces
Intermolecular Forces:
- Forces between molecules
- Responsible for the state of matter: solid, liquid, or gas
- affect the melting and boiling points of compounds as well as the
solubility of one substance in another
- weaker than covalent bonds within molecules, since molecular
compounds melt easily (melting doesn’t break the bond between
atoms, but between the molecules themselves!)
o example: melting ice to water to steam will break apart the
intermolecular forces and change state, but not decompose
into H2 and O2
Intramolecular Force (Force-Force attraction):
- Forces within molecules
- the attractive force between the
valence electrons of one atom
and the nucleus of another atom
would be considered
intermolecular forces
- Covalent bonds are examples of
intramolecular forces
The covalent
intramolecular
bond in I2 is
very strong.
I
I
I
I
I
I
I
I
I
I
I
I
There are weaker
intermolecular forces which
hold covalent molecules
together in a molecular
solid. Since they are
relatively weak, Iodine has
a low melting point.
TYPES OF FORCES
Type of
Force
London
Forces (Vanderwal
forces)
Exists between
Explanation
- all molecules, but
especially important in
non polar covalent
molecules
Dipoledipole
- polar covalent
molecules
- results from the random movement of the
electrons in the molecule which generates
temporary positive and negative regions in the
molecule.
- very weak
- strength increases with atomic number
- the only type of intermolecular force operating
between non-polar molecules (e.g. H2, CO2)
- attraction between opposite dipoles on two
molecules in a compound
- stronger than London forces but weaker than
hydrogen bonding
- causes an increase in mp and bp relative to non
polar molecules (e.g. PCl3,CH3Cl)
- interaction of a non-bonding pair of electrons
with a hydrogen which is attached to an
electronegative element such as oxygen
- found in water and results in water having a
higher mp and bp than other molecules of the
same type
- Hydrogen bonds are about five times stronger
than regular dipole-dipole bonds
- It occurs when N, O, or F are bonded to H.
-large EN value
- about one-tenth the strength of a covalent bond
(e.g. H2O, NH3)
- the dipole is attracted to the oppositely charged
ion
+ –
+

–
+ –
 –
+
Hydrogen
bond
- polar covalent
molecules
Dipole-Ion
- between an ion and
one end of a polar
molecule (eg in
dissolving of ionic
compounds in water)
What kind(s) of intermolecular forces are present in the following substances:
a) NH3, b) SF6, c) PCl3, d) LiCl, e) HBr, f) CO2
(hint: consider EN and molecular shape/polarity)
a) NH3: Hydrogen bonding (H + N), London.
b) SF6: London only (it is symmetrical).
c) PCl3: EN=2.9-2.1. Dipole-dipole, London.
e) HBr: EN=2.8-2.1. Dipole-dipole, London.
d) LiCl: EN=2.9-1.0. Ionic, (London).
f) CO2: London only (it is symmetrical)
London forces
Instantaneous dipole:
Induced dipole:
Eventually electrons A dipole forms in one atom
are situated so that
or molecule, inducing a
tiny dipoles form
dipole in the other
Why oil and water don’t mix
+
+
–

+
+
+
–

+
–

+
+
–

+
+
+
–

–
+
+
 
The non-polar substance is
pushed away. If it were
moving faster it might break
+ – +

through the attractive forces.
Solubility is a balance be+ – +

tween speed and attraction.
Also, the more similar the strength of their dipoles
the more likely two compounds are to mix.
+

–