2.3.1Elements, compounds and mixtures
Carbon — OCR Gateway GCSE Combined Science A J250 - Chemistry Revision Notes & Topical Questions
5 min read 7 parts Lesson 16 of 18
Section 1 of 7Why carbon is special
Four outer electrons
- Four outer electrons Carbon is in Group 4 of the periodic table, so its atoms have four electrons in the outer shell.
- Each atom can therefore form four covalent bonds, sharing one electron in every bond.
Chains and rings
- Chains and rings Carbon atoms can bond to one another to build long chains and closed rings.
- These skeletons are the basis of huge families of similar compounds, such as the hydrocarbons.
Natural and synthetic compounds
- Natural and synthetic compounds Because the possible arrangements are almost endless, there are vast numbers of natural compounds (in living things) and synthetic ones (such as plastics).
Section 2 of 7Allotropes of carbon
Same element, different structures
What is an allotrope?
- What is an allotrope? Allotropes are different structural forms of the same element in the same physical state.
- Their atoms are identical, but the pattern of bonding differs, so their physical properties differ.
Diamond
- Diamond Each carbon atom is covalently bonded to four others in a rigid, three-dimensional giant lattice.
Graphite
- Graphite Each carbon atom is bonded to three others, forming flat layers of hexagons.
Graphene
- Graphene A single layer of graphite: a sheet of carbon atoms one atom thick, joined in a continuous hexagonal pattern.
Section 3 of 7Diamond
1Four strong bonds per atom
- Four strong bonds per atom All the outer electrons are used in covalent bonds, and the bonds extend throughout the giant lattice.
2Very hard, very high melting point
- Very hard, very high melting point A huge amount of energy is needed to break so many strong covalent bonds,
- so diamond is extremely hard and melts only at a very high temperature.
3Does not conduct electricity
- Does not conduct electricity There are no free electrons or ions to carry charge.
4Uses
- Uses Cutting tools, drill bits and saw blades tipped with diamond, and jewellery.
Section 4 of 7Graphite and graphene
Graphite: layers that slide
- Graphite: layers that slide Strong covalent bonds hold atoms together within each layer, but only weak forces act between layers.
- The layers slide over each other easily, so graphite is soft and slippery and is used in pencils and as a lubricant.
Graphite: conducts electricity
- Graphite: conducts electricity Each carbon uses only three of its four outer electrons in bonding.
- The fourth becomes a delocalised electron that is free to move along the layers,
- so graphite conducts electricity and heat and is used for electrodes in electrolysis.
Graphene
- Graphene Because it is a single layer of strongly bonded atoms, graphene is extremely strong yet very light, flexible and almost transparent.
- Its delocalised electrons make it an excellent conductor, so it is used in electronics and in composite materials.
Section 5 of 7Comparing the allotropes
| Property | Diamond | Graphite | Graphene |
|---|---|---|---|
| Bonds per carbon atom | 4 | 3 | 3 |
| Structure | 3D giant lattice | Layers of hexagons | One layer of hexagons |
| Hardness | Extremely hard | Soft and slippery | Very strong, flexible |
| Electrical conductivity | None | Good | Very good |
| Melting point | Very high | Very high | Very high |
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Allotrope | A different structural form of the same element in the same state, such as diamond and graphite. |
| Giant covalent structure | A huge network of atoms joined by covalent bonds throughout the structure. |
| Delocalised electron | An electron that is not fixed to one atom and is free to move through the structure. |
| Graphene | A single layer of graphite, one atom thick. |
| Lattice | A regular, repeating three-dimensional arrangement of particles. |
Section 7 of 7Exam technique
1
Saying graphite conducts because of ions. It conducts because of delocalised electrons; there are no ions.
2
Writing that graphite layers are joined by weak covalent bonds. The weak forces are between the layers, and the covalent bonds inside layers are strong.
3
Explaining the softness of graphite by saying the covalent bonds break easily. No covalent bonds are broken when layers slide.
4
Claiming diamond and graphite are different elements. They are both pure carbon and differ only in structure.
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