Covalent bonding: where n is a large number.

The idea

In the shorthand formula of a polymer, n is written outside the brackets and stands for a large number of repeating units joined together. It is not a small fixed value such as 2 or 3; a polymer sample contains chains of many different lengths, so n is variable and large. For example, in poly(ethene), n might be several thousand, meaning thousands of —CH₂—CH₂— units are covalently bonded in one chain. The value of n affects the size and properties of the polymer, such as its strength and melting point, because longer chains have stronger intermolecular forces between them. Understanding n helps explain why polymers are macromolecules rather than simple molecules.

What earns the marks

• n represents the number of repeating units in the polymer chain.
• n is a large number, not a small fixed number such as 2 or 3.
• Different polymer molecules in a sample can have different values of n, so n is variable.
• The size of n affects the length of the polymer chain and therefore properties such as strength and melting point.
• The repeating unit inside the brackets is joined many times, which is why the polymer is a macromolecule.

Common mistakes and how to put them right

• Stating that n is a fixed number such as 2.

  Put it right by saying n is a large, variable number of repeating units.

• Confusing n with the number of atoms in one repeating unit.

  Put it right by explaining that n counts repeating units, not atoms.

• Thinking n affects only the drawing and not the real molecule.

  Put it right by linking larger n to longer chains and different physical properties.

Exam tips

• When defining n, use the phrase 'a large number of repeating units' rather than just 'many'.
• Link n to polymer properties such as strength or melting point to gain explanation marks.
• Avoid giving a single numerical value for n unless the question provides one; n is generally variable.