Topic 3 · page 9 of 15
Giant covalent structures and carbon
Giant atomic structures have very high melting temperatures because every atom is held by strong covalent bonds. Diamond, graphite and graphene are built from the same element with different bonding patterns, which gives different properties and uses.
Read first: Covalent bonds and dot-and-cross diagrams
Giant atomic structures as evidence 3.103.10(i)
The first piece of evidence in 3.10 is the physical properties of giant atomic structures. In a giant atomic (giant covalent) structure, a very large number of atoms are held together in one lattice by covalent bonds. There are no separate small molecules.
Diamond and silicon dioxide are examples. Both have:
- a very high melting temperature (diamond is above 3500 °C),
- great hardness.
To melt the solid, a huge number of covalent bonds must be broken. A very large amount of energy is needed, so the covalent bonds must be strong. This is the evidence that covalent bonding is the strong electrostatic attraction between nuclei and shared pairs.
Diamond 3.12
- Each carbon atom forms 4 covalent bonds, arranged tetrahedrally. (Methane has the same arrangement of four bonding pairs, see shapes with no lone pairs.)
- The structure is a giant three-dimensional lattice of strong covalent bonds.
- Properties: very hard, and a very high melting temperature, because many strong covalent bonds must be broken.
- It does not conduct electricity: all four outer electrons of each carbon are used in bonds, so there are no delocalised electrons and no free charge carriers.
- Uses: cutting tools and drill tips (hardness) and jewellery (hard, and it sparkles).
Graphite 3.12
- Each carbon atom forms 3 covalent bonds in flat hexagonal layers.
- The fourth outer electron of each carbon is delocalised along the layer.
- The layers are held together by weak forces, which are shown dashed in the figure.
- It conducts electricity along the layers, because the delocalised electrons can move.
- It is soft and slippery, because the layers slide over each other when the weak forces between them are overcome.
- It still has a high melting temperature, because within each layer the covalent bonds are strong and a lot of energy is needed to break them.
- Uses: electrodes (conducts and has a high melting temperature), and a lubricant and pencil "lead" (the layers slide).
Graphene 3.12
Graphene is a single layer of graphite, one atom thick.
- Each carbon forms 3 bonds, and one electron per carbon is delocalised.
- It is very strong for its mass, because of the many strong covalent bonds in the layer.
- It conducts electricity well, because of the delocalised electrons.
- It is almost transparent, because it is only one atom thick.
- Uses: electronics and touchscreens (conducts and is transparent), and strong, light composite materials.
Comparing the three 3.12
| Diamond | Graphite | Graphene | |
|---|---|---|---|
| Bonds per carbon | 4 (tetrahedral) | 3 (in layers) | 3 (single layer) |
| Delocalised electrons | No | Yes, along the layers | Yes |
| Conducts electricity | No | Yes | Yes |
| Key property | Very hard | Soft and slippery | Very strong, light, transparent |
| Use | Cutting tools | Electrodes, lubricant | Electronics, composites |
MethodLinking a use to a property
Pick the property, then the structure that causes it, then the use.
- Name the property (hard, conducts, slippery, light and transparent).
- Give the structure reason (tetrahedral lattice, delocalised electron, weak forces between layers, single layer).
- State the use that needs it.
Check yourself
- Explain why diamond has a very high melting temperature.
- Explain why diamond does not conduct electricity but graphite does.
- Explain why graphite is soft even though its melting temperature is very high.
- State the number of covalent bonds formed by each carbon atom in diamond, in graphite and in graphene.
- Give one use of each of diamond, graphite and graphene, and link each use to a property.
- A student says: "silicon dioxide has a high melting temperature because the strong forces between its molecules must be overcome". Correct the statement.
Answers cover these until you have tried the questions
- Diamond is a giant lattice in which every carbon atom is joined to four others by strong covalent bonds. A very large number of strong covalent bonds must be broken to melt it, so a lot of energy is needed.
- In diamond all four outer electrons of each carbon are used in covalent bonds, so there are no delocalised electrons to carry charge. In graphite each carbon forms three bonds and the fourth electron is delocalised and can move along the layers, so graphite conducts.
- The layers are held by weak forces, so they slide over each other easily and graphite is soft. The melting temperature is high because within each layer the covalent bonds are strong, and a lot of energy is needed to break them.
- Diamond: 4. Graphite: 3. Graphene: 3.
- Diamond: cutting tools, because it is very hard (a giant lattice of strong bonds). Graphite: electrodes, because it conducts (delocalised electrons) and has a high melting temperature; or lubricant, because the layers slide. Graphene: touchscreens and electronics, because it conducts and is transparent; or light composites, because it is strong for its mass.
- Silicon dioxide is a giant atomic structure, so it has no molecules. It has a high melting temperature because a large number of strong covalent bonds must be broken, which needs a lot of energy.