Properties of Simple MoleculesOxford AQA IGCSE Chemistry: Revision notes
Section 1
What are simple molecular substances?
Substances made of simple molecules consist of a small, fixed number of atoms joined together by strong covalent bonds, for example H₂, Cl₂, H₂O and CH₄. Unlike metals or ionic compounds, simple molecular substances exist as separate individual molecules, not as one giant structure.
Section 2
What are the physical states of simple molecular substances?
Substances consisting of simple molecules can be gases, liquids or solids at room temperature, but they generally have relatively low melting and boiling points compared with giant structures such as ionic or metallic compounds or giant covalent structures.
Section 3
Why do simple molecular substances have low melting and boiling points?
Simple molecular substances have only weak forces of attraction between their molecules, called intermolecular forces. When a simple molecular substance melts or boils, it is these weak intermolecular forces that are overcome — not the strong covalent bonds within each molecule. Because intermolecular forces are weak, only a small amount of energy is needed to separate the molecules, giving low melting and boiling points.
A very common error is saying that the covalent bonds break when a simple molecular substance melts or boils — they do not. Only the weak intermolecular forces between molecules are overcome.
Examiners award marks for explicitly stating that it is the intermolecular forces, not the covalent bonds, that are broken on melting or boiling.
Section 4
Why don't simple molecular substances conduct electricity?
Simple molecular substances do not conduct electricity, in any physical state. This is because the molecules have no overall electric charge — there are no free ions or delocalised electrons available to carry electrical current.
Section 5
How does molecule size affect properties?
As the size of simple molecules increases, the strength of the intermolecular forces between them tends to increase, because larger molecules have a greater surface area over which these forces act. This means larger molecules generally have higher melting and boiling points than smaller ones, even though both are still "simple molecular" substances.
Methane (CH₄) is a small molecule and is a gas at room temperature, whereas larger hydrocarbon molecules with many more atoms are liquids or solids, because their intermolecular forces are stronger.
Must Know
- Simple molecular substances are gases, liquids or solids with relatively low melting/boiling points
- Atoms within a molecule are held by strong covalent bonds
- Melting/boiling only overcomes weak intermolecular forces between molecules, not the covalent bonds
- Simple molecular substances do not conduct electricity because the molecules carry no overall charge
- Larger molecules generally have stronger intermolecular forces and so higher melting/boiling points
That's the notes covered.
Carry on to the next subtopic.