Graphite: bonded carbon layers explain every one of its properties
Definition
Graphite
A form of carbon with a giant covalent structure made of layers of carbon atoms arranged in hexagonal rings.
- Graphite is a form of the element carbon (C\text{C}C) that has a giant covalent structure.
- Each carbon atom forms three strong covalent bonds with three other carbon atoms.
- This uses three of each atom's four outer electrons, leaving one electron spare.
- The bonded atoms join into flat layers built from hexagonal rings of carbon.
- There are no covalent bonds between the layers.
- Only weak intermolecular forces hold neighbouring layers together.
Key Idea
Within each layer the carbon atoms are held by strong covalent bonds, but only weak forces act between the layers.
Strong bonds raise the melting point, while weak forces let the layers slide
Definition
Covalent bond
A covalent bond is a strong electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms.
- Graphite has a very high melting point.
- Melting it means breaking many strong covalent bonds across the giant structure.
- Breaking these bonds needs a large amount of energy.
- The weak forces between the layers are much easier to overcome than the covalent bonds.
- Because nothing bonds the layers together, they can slide over one another.
- This sliding makes graphite soft and slippery.
Example
- When you write with a pencil, layers of graphite slide off and stay on the paper.
- Graphite is used as a lubricant because its layers slide over one another easily.
Delocalised electrons let graphite conduct like a metal
Definition
Delocalised electron
A delocalised electron is an electron that is not tied to one atom or bond and can move through part or all of a structure.
- Each carbon atom uses three of its four outer electrons to form the three covalent bonds.
- The remaining electron from every carbon atom becomes delocalised.
- So one delocalised electron is released per carbon atom.
- These delocalised electrons are free to move along the layers.
- The moving electrons carry charge, so graphite conducts electricity and thermal energy.
- Graphite is similar to a metal because both contain delocalised electrons that carry charge.
- Graphite is still a non-metal, even though it conducts like one.
Common Mistake
- Do not say graphite conducts because its carbon atoms move.
- The current is carried by delocalised electrons, not by atoms or ions.
Property answers must link structure, bonding and behaviour
- For the high melting point, link the many strong covalent bonds to the large energy needed to break them.
- For soft and slippery, link the absence of bonds between layers to the layers sliding.
- For electrical conductivity, link the delocalised electrons to their movement through the layers carrying charge.
- A full-mark answer always names the structural feature and connects it directly to the property.
Self review
- How many covalent bonds does each carbon atom form in graphite?
- What shape are the carbon rings within each layer?
- Why can the layers of graphite slide over one another?
- Why does graphite have a high melting point?
- Which particles carry electrical charge through graphite?
