Students should be able to recognise polymers from diagrams showing their…

The idea

Polymers are large molecules built from many small monomer units joined by covalent bonds. In diagrams, look for a long chain of atoms connected by single lines representing covalent bonds, often with a repeating unit shown in brackets and a subscript n. The chain backbone is usually carbon atoms, and each carbon may also bond to hydrogen or other atoms. Recognising a polymer means identifying this repeating pattern. For example, poly(ethene) is drawn as a chain of –CH₂–CH₂– units; the brackets and n show that the unit repeats many times. The intermolecular forces between polymer molecules are relatively strong, so these substances are solid at room temperature. However, they still have lower melting points than giant covalent structures, while the covalent bonds within each chain remain strong.

What earns the marks

• Identify a polymer diagram as a long chain of many atoms joined by covalent bonds, often with a repeating unit.
• Recognise the repeating unit shown in brackets with a subscript n, indicating many identical units joined together.
• Distinguish polymers from small molecules by the large number of atoms and the chain structure.
• State that atoms within a polymer chain are held by strong covalent bonds, while separate chains are held by relatively strong intermolecular forces.
• Use the displayed formula to identify the monomer repeat unit, for example –CH₂–CH₂– in poly(ethene).

Common mistakes and how to put them right

• Mistaking a polymer diagram for a small molecule.

  Put it right by checking for a repeating unit and a subscript n, which show many units joined.

• Thinking intermolecular forces between polymer chains are weak.

  Put it right by stating that because polymer molecules are large, the intermolecular forces between them are relatively strong, making them solid at room temperature.

• Confusing a polymer with a giant covalent structure.

  Put it right by noting that a polymer is made of many separate chains, not one continuous giant lattice.

Exam tips

• When shown a diagram, first look for brackets and a subscript n; these are strong clues that the structure is a polymer.
• If asked to name the polymer, identify the repeating unit and relate it to the monomer, for example ethene forms poly(ethene).
• Use the terms 'monomer', 'repeating unit' and 'polymer chain' accurately in your answers, and remember that intermolecular forces between chains are relatively strong.