Polymers and giant covalent structures
Comparing structures
A substance's properties depend on its structure and bonding:
| Structure | Example | Melting point | Conducts? |
|---|---|---|---|
| Small molecules | Water, CO₂ | Low | No |
| Polymers | Poly(ethene) | Medium (solid at room temperature) | No |
| Giant covalent | Diamond, SiO₂ | Very high | No (except graphite and graphene) |
| Giant ionic | NaCl | High | Only when melted or dissolved |
| Giant metallic | Copper | High | Yes |
To melt a substance made of molecules, you only overcome . To melt a giant structure, you must break strong bonds throughout it.
Polymers
are very large molecules made of thousands of atoms joined in long chains by . They are made by joining many small molecules called .
Because polymer molecules are so large, the between them are relatively strong, much stronger than between small molecules. So most polymers are solids at room temperature.
Longer chains have stronger forces of attraction between them, making stronger materials.
Key terms
Giant covalent structures
In a , all the atoms are joined to each other by in a huge network. There are no separate molecules.
To melt or boil one, many strong covalent bonds must be broken, which needs a lot of energy. So giant covalent substances have .
Examples:
- and — forms of carbon (see Carbon structures)
- (SiO₂, silica) — the main compound in sand. Like diamond, it is hard and has a very high melting point.