Covalent bonding: Covalently bonded substances may consist of small molecules.

The idea

Many covalently bonded substances are made of small, separate molecules rather than giant structures. A small molecule contains a fixed, small number of atoms joined by strong covalent bonds, for example H₂O, CO₂, O₂, N₂, CH₄ and HCl. The atoms within each molecule are held together strongly, but the molecules themselves are separate and attracted to one another only by weak intermolecular forces. This explains why small-molecule substances often have low melting and boiling points: little energy is needed to overcome the weak forces between molecules, not the strong covalent bonds within them.

For example: Iodine and carbon dioxide are simple molecular substances that are gases or easily vaporised solids at room temperature.

What earns the marks

• A small molecule is a discrete group of a limited number of atoms joined by covalent bonds, such as H₂O or CO₂.
• Small molecules contain strong covalent bonds between the atoms inside the molecule.
• Separate small molecules are held near each other by weak intermolecular forces.
• The weak forces between molecules explain low melting and boiling points and ready vaporisation.
• When a simple molecular substance melts or boils, the covalent bonds within molecules are not broken; the molecules separate from one another.
• Examples include H₂, O₂, N₂, Cl₂, H₂O, CO₂, CH₄ and HCl.

Common mistakes and how to put them right

• Saying that covalent bonds break when a simple molecular substance boils.

  Put it right by stating that the weak forces between molecules are overcome, while covalent bonds remain intact.

• Treating a small molecule as a giant lattice.

  Put it right by describing it as a discrete particle with a fixed small number of atoms.

• Assuming all covalent substances are small molecules.

  Put it right by noting that some covalent substances, such as diamond and silicon dioxide, are giant covalent structures.

Exam tips

• When comparing melting points, identify whether the substance is simple molecular or giant covalent before explaining the energy needed.
• Use the phrase weak intermolecular forces for the attractions between small molecules, and strong covalent bonds for the attractions within them.
• Give a named example, such as H₂O or CO₂, to support an explanation about small molecules.