Intermolecular Forces
What causes different substances to exist in different states of matter under the
same conditions?
Forces that hold the particles together. Interparticle or Intermolecular Forces
(IMF)
What are the forces between particles that cause these properties?
Case 1: Substance is Ionic:
 Forces:
Ionic Bond
 Properties:
Crystalline Solid at Room Temp, Hard, Brittle, High Melting, Soluble in water,
forms electrolyte when dissolved in water.
Case 2: Substance is a pure metal:
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 Forces:
Metallic Bond
 Properties: Strong, malleable, ductile, conducts heat and electricity, most are
solids, HIGH MELTING POINT.
Case 3: Substances is molecular:
 Forces:
Intermolecular Forces
 Properties:
Vary, depending on the type of Intermolecular force.
A. General Classes of Intermolecular Forces
1. Van der Waals forces
Weak Intermolecular Forces
a. Dispersion forces (also called London Dispersion or induced
dipoles)
The Weakest IMF
i. Dispersion forces are exhibited by:
Nonpolar Molecules, and atoms that don’t bond.
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ii. Strength of dispersion force depends on:
The size of the electron cloud that distorts. The bigger the cloud, the
n
more it distorts, the stronger the dispersion force.
iii. Examples:
O2, Ne, Xe, CH4, C3H8
b. Dipole Forces:
Weak Imf (Van Der Waals) but stronger than Dispersion Forces.
i. Dipole forces are exhibited by:
Molecules with PERMANENT DIPOLES (Polar Molecules)
ii. Strength of dipoles force depends on:
The polarity of the molecules. (This depends on the
electronegativities of the atoms, and the shape of the molecule.)
iii. Examples:
HCl, SO2,CH3Cl
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2. Hydrogen “Bonds”
i. Hydrogen Bonds are exhibited by:
Molecules with extreme polarity
Polar Molecules that contain Hydrogen bonded to 1 of the following: F,O,N.
In a molecule of HF:
H-F------ H-F-----H-F
In a molecule of water:
H-O----H-O---H-O:
In NH3:
N-H----N-H---N-H
ii. Strength of H Bonding depends on:
numbers! There are hundreds of thousands of Hydrogen
bonds in an ice cube!
iii. Examples:
H2O, CH3OH,CH3COOH
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Hydrogen Bonding and Biological Molecules
1. The amazing water molecule – the amazing properties of water are all due to:
THE AMAZING HYDROGEN BOND!

Polar solvent – Dissolves polar and ionic compounds.

High Boiling Point/ Heat of Vaporization

Less dense as a solid than as a liquid

High Surface Tension

High Specific Heat
How do the amazing properties of water affect life on earth?
2. What gives DNA its curl?
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Intermolecular Forces and Properties:
In a liquid, the strength of the intermolecular forces causes the following properties:
1.
Boiling point/ Heat of vaporization/ Specific Heat
2.
Viscosity/ Surface Tension/ Capillary Action
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Did you ever wonder????
What causes different substances to exist in different states of matter under the
same conditions?
Why is the air you breathe a gas, and the water you drink a liquid, when the temperature
(kinetic energy of the molecules) is the same?
As a liquid is heated, the kinetic energy of its particles increases. As the boiling point,
the kinetic energy is enough to overcome the forces of attraction between the liquid’s
particles. The particles pull away from each other and enter the gas phase. Boiling
point is therefore a good measure of the force of attraction between particles of a
liquid. The higher the boiling point, the stronger the forces between particles.
Use the table of boiling points to assess the relative strength of the forces of
attraction at work in the substances listed.
Low= -100 or lower
Low/Med = 0 to -100
Med = 0 to 100
High = above 100
Substance
H2
Boiling Point  C
-253
Bonding Type
NPC
Strength of Forces
Low
7
Cu
2567
Metallic
High
H2S
-61
PC
Low - medium
PH3
-88
PC
Low - medium
C6H6
80
NPC
Med
O2
-183
NPC
Low
HF
20
PC
Med
NaCl
1413
Ionic
High
Cl2
-34
NPC
Low - medium
CCl4
77
PC – nonpolar
Med
molecule
ICl
97
PC
Med
Fe
2750
metallic
High
HF
20
PC
Med
8
Br2
59
NPC
Med
CH4
-164
NPC
Low
H2O
100
PC
Med
W
5660
Metallic
High
NH3
-33
PC
Low - medium
HCl
-85
PC
Low - medium
MgF2
2239
Ionic
High
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