Students should be able to explain the properties of graphite in terms of…

The idea

Graphite is a giant covalent structure of carbon atoms arranged in hexagonal layers. Within each layer, every carbon atom forms three strong covalent bonds, so three outer electrons are localised in bonds and the fourth is delocalised. The many strong covalent bonds make graphite very high melting, while the delocalised electrons allow it to conduct electricity along the layers. Between layers there are only weak forces, so layers can slide over one another, making graphite soft and slippery and useful as a lubricant and in pencils. The weak forces also mean the layers are far apart, giving graphite a lower density than diamond. To explain a property, always name the structural feature responsible and then link it to the property.

What earns the marks

• Graphite has a giant covalent structure in which each carbon atom is covalently bonded to three others in a hexagonal layer.
• Strong covalent bonds within layers require a large amount of energy to break, giving graphite a very high melting point.
• One delocalised electron per carbon atom is free to move within a layer, so graphite conducts electricity.
• Weak forces between layers allow layers to slide over one another, making graphite soft and slippery.
• The weak forces between layers also mean the layers are separated by relatively large distances, so graphite has a lower density than diamond.
• Graphite is insoluble in water because the covalent bonds within layers are too strong to be broken by water molecules.

Common mistakes and how to put them right

• Saying graphite melts easily because layers slide.

  Put it right by distinguishing weak forces between layers from strong covalent bonds within layers, which give a very high melting point.

• Stating that graphite conducts because it contains ions.

  Put it right by explaining that delocalised electrons, not ions, carry charge in graphite.

• Claiming graphite is soft because its covalent bonds are weak.

  Put it right by stating the covalent bonds are strong but the forces between layers are weak.

Exam tips

• For each property, name the structural feature first and then state the consequence, for example 'weak forces between layers, so layers slide'.
• Use comparative language when contrasting graphite with diamond, such as 'graphite has three bonds per carbon atom whereas diamond has four'.
• Check that every property you mention is linked to a specific structural feature rather than given as a standalone fact.