Polymers & Giant Covalent Structures

• Polymers are long chains of repeating units
• In a polymer, lots of small units are linked together to form a long molecule that has repeating sections
• All the atoms in a polymer are joined by strong covalent bonds
• Instead of drawing out a whole long polymer molecule, you can draw the shortest repeating section, called the repeating unit, like this:

  
• This polymer is called 'poly(ethene)'. The bonds through the brackets join up to the next repeating unit.

  
  

  'n' is a large number. It tells you that the unit's repeated lots of times

  
• To find the molecular formula of a polymer, write down the molecular formula of the repeating unit in brackets, and put an 'n' outside
• The molecular formula for a poly(ethene) polymer is (C2H4)n
• The intermolecular forces between polymer molecules are larger than between simple covalent molecules, so more energy is needed to break them. This means most are solid at room temperature
• The intermolecular forces are still weaker than ionic or covalent bonds, so they generally have lower boiling points than ionic or giant molecular compounds
• Giant covalet structures are macromolecules
• In giant covalent structures, all the atoms are bonded to each other by strong covalent bonds
• Giant covalent structures have very high melting and boiling points as lots of energy is needed to break the covalent bonds between the atoms
• Giant covalent structures don't contain charged particles, so they don't conduct electricity - not even when molten (except for graphite)
• The main examples of giant covalent structures are diamond and graphite, which are both made from carbon atoms only, and silicon dioxide (silica)
• Diamond:

  Each carbon atom forms 4 covalent bonds in a very rigid giant covalent structure

  
• Graphite:

  Each carbon atom forms 3 covalent bonds to create layers of hexagons. Each carbon atom also has 1 delocalised (free) electron

  
• Silicon dioxide:

  Sometimes called silica, this is what sand is made of. Each grain of sand is one giant structure of silicon and oxygen