Small Molecules
Master Small Molecules for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Simple molecular substances have low melting points because only weak intermolecular forces are overcome, not the covalent bonds themselves.
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Topic overview
Small molecules, also called simple molecular substances, consist of separate molecules held together by weak intermolecular forces.
The covalent bonds within each molecule are strong, but the forces between the molecules are weak. When the substance melts or boils, only these weak intermolecular forces are overcome, so relatively little energy is needed and melting and boiling points are low. Many are gases or liquids at room temperature.
They do not conduct electricity, because the molecules have no overall electrical charge and there are no delocalised electrons or free ions to carry it. As the size of the molecules increases, the intermolecular forces become stronger and the melting and boiling points rise.
Revision notes
Structure and bonding
Simple molecular substances consist of separate small molecules.
Strong covalent bonds hold the atoms within each molecule together, but only weak intermolecular forces act between separate molecules.
Low melting and boiling points
When the substance changes state, only the weak intermolecular forces are overcome — the covalent bonds are not broken.
Little energy is therefore needed, so melting and boiling points are low and many such substances are gases or liquids at room temperature.
Conductivity and size
These substances do not conduct electricity because the molecules have no overall charge and there are no free electrons or ions.
As molecules get larger, the intermolecular forces between them become stronger, so more energy is needed and melting and boiling points increase.
Key points
- Simple molecular substances contain separate molecules.
- Covalent bonds within molecules are strong.
- Intermolecular forces between molecules are weak.
- Only the weak forces are overcome on melting.
- They have low melting and boiling points.
- They do not conduct electricity.
Worked examples
Example 1
Explain why simple molecular substances do not conduct electricity. [2 marks]
Model answer
The molecules have no overall electrical charge (1 mark), and there are no delocalised electrons or free ions available to carry charge (1 mark).
Example 2
Explain why the boiling point increases as the molecules get larger. [2 marks]
Model answer
Larger molecules have stronger intermolecular forces between them (1 mark), so more energy is needed to overcome these forces and separate the molecules (1 mark).
Example 3
Explain why chlorine is a gas at room temperature although it contains strong covalent bonds. [3 marks]
Model answer
The strong covalent bonds are within each chlorine molecule (1 mark), but the intermolecular forces between separate molecules are weak (1 mark). Only these weak forces need to be overcome for chlorine to be a gas, so very little energy is required (1 mark).
Common mistakes
Saying covalent bonds break on melting.
Only the weak intermolecular forces are overcome.
Saying the substance has weak bonds.
The bonds are strong; it is the forces between molecules that are weak.
Forgetting there are no charged particles.
That is the reason these substances do not conduct.
Saying larger molecules have stronger covalent bonds.
It is the intermolecular forces that increase with size.
Exam tips
- Always distinguish bonds within molecules from forces between them.
- Explain low boiling points through the weak intermolecular forces.
- Say no overall charge and no free electrons or ions for conductivity.
- Link molecule size to intermolecular force strength.
Key terms
- Simple molecular
- A structure of separate small molecules.
- Intermolecular force
- A weak attraction between separate molecules.
- Covalent bond
- A strong shared pair of electrons within a molecule.
- Volatile
- Easily evaporated, having a low boiling point.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.