Covalent bonding: Some covalently bonded substances have very large molecules, such as polymers.

The idea

Not all covalent substances are small molecules. In a polymer, many identical small units called monomers are joined by strong covalent bonds into a very large molecule. The formula is often written with a repeating unit and the subscript n, for example poly(ethene) as –(C₂H₄)ₙ–. While the atoms within each chain are held by strong covalent bonds, the intermolecular forces between separate polymer molecules are relatively strong compared to those between small molecules, due to the large size of the polymer chains. This cumulative strength of intermolecular forces explains why polymers are usually solids at room temperature with higher melting points than small-molecule substances like CH₄. To recognise a large covalent molecule, look for a repeating unit and the subscript n.

What earns the marks

• State that polymers are covalently bonded substances made from many small monomer units joined together.
• Recognise a polymer from a formula showing a repeating unit and the subscript n, such as –(C₂H₄)ₙ–.
• Explain that the atoms within a polymer molecule are joined by strong covalent bonds.
• State that the intermolecular forces between large polymer molecules are relatively strong compared to those between small molecules.
• Compare polymers with small molecules such as CH₄, noting that polymers have much larger molecules and higher melting points due to stronger intermolecular forces.
• Use the idea of large molecules and relatively strong intermolecular forces to explain why polymers are solids at room temperature.

Common mistakes and how to put them right

• Thinking that all covalent substances are small molecules.

  Put it right: Polymers are covalent but contain very large molecules made of many repeating units.

• Stating that covalent bonds are broken when a polymer melts.

  Put it right: It is the intermolecular forces between the polymer chains that are overcome, not the strong covalent bonds within the chains.

• Assuming intermolecular forces in polymers are weak like in small molecules.

  Put it right: Because polymer molecules are so large, the intermolecular forces between them are relatively strong, making them solid at room temperature.

Exam tips

• Look for a repeating unit or the subscript n in a displayed formula to identify a polymer.
• When comparing polymers with small molecules, refer to the size of the molecule and the relative strength of the intermolecular forces.
• Use the terms monomer and polymer correctly, and link polymerisation to the joining of many small units.