SCH3U
Properties of Molecular and Ionic Compounds
Molecular
Covalent
Ionic
Polar Covalent
State at room
temperature
Electronegativity
Difference Range (EN)
Structure
Melting Point
Examples
(with melting point)
Solubility in Water
Electrical Conductivity in
the Solid State
Electrical Conductivity in
the Liquid State
Electrical Conductivity in
the Aqueous Solution
Molecular Compounds
Molecular compounds have very weak intermolecular bonds (i.e. bonds between molecules)
despite their strong intramolecular bonds (i.e. bonds within the molecules). This creates
very low melting and boiling points since these weak bonds are easily broken.
N2 (s)
N2 (l)
N2 (g)
As the bond polarity of the covalent bonds in a molecule increases, the intermolecular bonds
become stronger. This increases the melting and boiling points:
H2O (s)
H2O (l)
H2O (g)
Only the weak intermolecular
bonds (called hydrogen bonds) are
broken.
The very strong intramolecular
(covalent) bonds remain.
Ionic Compounds
In ionic compounds, there are no intermolecular forces since these
substances only contain ionic bonds. Although ionic bonds are
weaker than covalent bonds, the lattice of ionic bonding requires
more energy to break apart than intermolecular bonds.
This results in the very high melting and boiling points typical of
ionic compounds.
Shape and Molecular Polarity
Although the bond between carbon and oxygen is polar covalent ((EN= 0.9), this substance is
actually a non-polar molecule. Compare the shape of water and carbon dioxide:

In water the bent shape creates a positive and negative ends (poles) on
the molecules. This results in stronger intermolecular forces.

In carbon dioxide, the linear shape cancels out the effect of each polar
bond. Since it has no positive or negative pole, it is a non-polar
molecule.
Questions:
Refer to p. 78 – 93 of Chemistry 11 (McGraw-Hill)
1. In general, what happens to the melting point of a substance as the bond polarity
increases?
2. In general, what happens to the solubility in water of a substance as the bond polarity
increases?
3. Explain why ionic compounds can conduct electricity in the aqueous or liquid states,
while molecular compounds cannot. (Refer to p. 78 – 79 of Chemistry 11)
4. Explain why molecular compounds are usually soft and ionic compounds are hard.
5. Both bond polarity and the shape of a molecule affects polarity. Determine the type of
bond(s) present and identify the shape of each molecule. For each, explain why each
molecule is either polar or non-polar.
a) carbon tetrafluoride (CF4)
c) ammonia (NH3)
b) hydrogen chloride (HCl)
d) methanol (CH3OH)
6. A group of chemists were investigating a white solid that was known to be either
calcium bromide (CaBr2) or menthol (C1oH20O). They tested the solid and noticed it had
a distinctive odour, melted easily and was not soluble in water. What was the identity
of the solid? Explain your answer by referring to the physical properties of the unknown
substance.