SCH 3U
Intermolecular Forces
There are two ways of examining a molecule; the internal structure of the molecule itself and how the
molecule interacts with its environment.
Intramolecular Forces – describe the forces of attraction within a molecule. This would describe the
internal structure of a molecule (ie. Covalent bonding, etc.)
Intermolecular Forces – describe the forces of attraction between two molecules. This would describe
how two neighbouring molecules would interact with each other.
Properties of matter can be predicted based on the type of IMF present.
There are 3 types of Intermolecular Forces (IMF):
1. London Dispersion Forces (LDF)
- Intrinsic form of attraction present between molecules and atoms
- Present in ALL matter.
- WEAKEST form of IMF
Therefore, a very low mp/bp and usually found as a gas.
2. Dipole-Dipole Forces (D-D):
- the force of attraction between a slightly positive side of a molecule to the slightly negative
side of a neighbouring molecule.
- Present in all POLAR molecules.
- STRONGER form of IMF
Therefore, a medium mp/bp and usually found as a liquid.
3. Electrostatic Attractions
- the attraction between a positive particle and a negative particles
- present only in ions.
- STRONGEST form of IMF
Therefore a high mp/bp and usually found as a solid.
The properties of a substance depends on the IMFs present. The IMF will be determined by the
structure of the molecule.
Molecular Geometry ( A little reminder)
A molecule that has symmetry is non-polar.
A molecule that is asymmetrical can be polar.
Steps for determining a molecule’s polarity:
1. Draw the Lewis structure.
2. Draw the partial charges as appropriate
3. If the compound has polar bonds, identify the geometry
4. If all the geometric areas of electron density are the same, then the molecule has symmetry and
is a non-polar molecule.
Putting it all together!
Fill in the following chart. Be sure to identify the state based on the IMFs present.
Compound
Cl2
N2
HCN
CBr4
SO2
BH3
CF4
MgF2
H2S
∆EN and
Bond Type
Lewis
Structure
Geometry
Symmetry
Molecular
Polarity
IMF
State