Polymers and Giant Covalent Structures

Polymers are long chains of repeating units

In a polymer, lots of small units are linked together to form a along 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(which can contain thousands or millions of atoms), you can draw the shortest repeating section, called the repeating unit:
- This polymer is called poly(ethene)
- The bonds through the brackets join up to the next repeating unit
- The bit in brackets is the repeating unit
- n is a large number
  - It tells you that the unit’s repeated lots of times
So for poly|(ethene), the molecular formula of the polymer is C2H4n
The intermolecular forces between polymer molecules are larger than between simple covalent molecules, so more energy is needed to break them
- This means most polymers 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

In giant covalent structures, all the atoms are bonded to each other by strong covalent bonds
They have very high melting and boiling points as lots of energy is needed to break the covalent bonds between the atoms
They don’t contain charged particles, so they don’t conduct electricity-not even when molten
- Except for a few weird excepetions such as graphite
The main examples that you need to know about are diamond and graphite, which are both made from carbon atoms only and silicon dioxide
- Diamond:
  - Each carbon atoms forms four covalent bonds in a very rigid giant covalent structure
- Graphite:
  - Each carbon atom forms three covalent bonds to create layers of hexagons
  - Each carbon atom also has one 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