Some covalently bonded substances have very large molecules, such as…

The idea

Covalent bonds form when non-metal atoms share pairs of electrons. In some substances, thousands of atoms join by covalent bonds into very large molecules called polymers. A polymer forms when many small molecules, called monomers, join together; for example, ethene molecules join to form poly(ethene). Each monomer contributes two carbon atoms to the chain, and the double bond between them opens so each carbon forms single bonds to neighbouring units. The result is a long chain with a repeating unit. Because polymer molecules are very large, the intermolecular forces between the chains are relatively strong. This means polymers have high melting points and are usually solid at room temperature, even though the chains themselves are not joined to each other by covalent bonds.

What earns the marks

• Covalent bonds involve shared pairs of electrons between non-metal atoms.
• A polymer is a very large molecule built from many small monomer molecules joined by covalent bonds.
• In addition polymerisation, the double bond in each monomer opens and the monomers link into a long chain.
• The repeating unit of a polymer shows the atoms that recur along the chain, with bonds continuing on both sides.
• Polymers have high melting points because the intermolecular forces between the large polymer chains are relatively strong and require significant energy to overcome.

Common mistakes and how to put them right

• Thinking a polymer is a mixture of separate small molecules.

  Put it right by describing one very large molecule made of many repeating units joined by covalent bonds.

• Drawing the polymer repeating unit with a double bond still present.

  Put it right by showing single bonds only, because the double bond opens during polymerisation.

• Attributing the high melting point of polymers to breaking covalent bonds.

  Put it right by stating that melting overcomes the relatively strong intermolecular forces between the chains, not the covalent bonds.

Exam tips

• When asked to identify a polymer, look for a long chain or a repeating unit rather than a small fixed group of atoms.
• If a diagram shows a monomer, count the carbon atoms and check whether a double bond is present before predicting the repeating unit.
• Link the large size of the molecule to relatively strong intermolecular forces between the chains when explaining a high melting point, not to the covalent bonds.