Oxygen exists as simple molecules, O₂. Describe the bonding within an oxygen molecule and explain why oxygen has a low boiling point.

Cambridge International IGCSE Chemistry (0620) — 2.5 Simple molecules and covalent bonds · Describe and Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Oxygen molecules are held together by a covalent bond, where the two oxygen atoms share electrons.
The covalent bond is a double bond – two pairs of electrons are shared between the atoms.
Between oxygen molecules there are only weak intermolecular forces.
Because only a small amount of energy is required to overcome these forces, the boiling point of oxygen is low.

Examiner tips

Use the term ‘covalent double bond’ to show understanding of bonding.
Mention ‘weak intermolecular forces’ to link to low boiling point.
Explain that low energy is needed to break these forces for a low boiling point.
Keep the answer concise and use the exact wording from the mark scheme.

Common mistakes

Confusing the double bond with a single bond.
Failing to mention the weak intermolecular forces.
Using vague terms like ‘bonding’ without specifying covalent or double bond.

Mark scheme (4 marks)

Oxygen molecules are held together by a covalent bond / sharing of electrons between the two oxygen atoms
The covalent bond is a double bond / two pairs of electrons are shared
There are weak intermolecular forces between oxygen molecules
Only a small amount of energy is needed to overcome these intermolecular forces, so the boiling point is low

Key terms in this question

boiling point · simple molecule