Graphite

Section 1 of 6Structure of graphite

Layers of carbon atoms

• Layers of carbon atoms Graphite is another allotrope of carbon.
  • Its atoms are arranged in flat sheets in which they form interlocking hexagonal rings.
• Each sheet is a giant covalent layer that is only one or a few atoms thick.

Three bonds per atom

• Three bonds per atom Each carbon atom forms three strong covalent bonds with neighbouring atoms in the same layer.
• This uses only three of its four outer electrons, leaving one spare.

Weak forces between layers

• Weak forces between layers There are no covalent bonds joining one layer to the next.
  • Only weak forces hold the layers together, so they are far apart compared with the spacing within a layer.

Section 2 of 6Properties and explanations

Property	Explanation
Soft and slippery	The layers are held by weak forces only, so they can slide over each other easily and rub off onto paper.
High melting point	It is a giant covalent structure with many strong covalent bonds within the layers, and a great deal of energy is needed to break them.
Conducts electricity (HT only)	The fourth outer electron of each atom is delocalised and free to move between the layers, carrying charge through the graphite.
Conducts heat (HT only)	The delocalised electrons also carry thermal energy quickly through the structure.
Low density for a giant structure	The widely spaced layers make graphite less dense than diamond.

Section 3 of 6Why graphite conducts but diamond does not

1Three bonds only

• Three bonds only Each carbon atom uses just three outer electrons for bonding.

2One electron left over

• One electron left over The spare electron is not fixed in any bond (HT only).

3Electrons delocalise

• Electrons delocalise The spare electrons spread out and can move freely along and between the layers.

4Charge flows

• Charge flows Moving electrons carry an electric current, so graphite behaves a little like a metal.

Section 4 of 6Everyday uses

Lubricant

• Lubricant The slippery layers make graphite a useful dry lubricant for machine parts, especially where oil would burn or contaminate.

Pencil leads

• Pencil leads Layers peel away onto paper as a pencil is drawn across it.

Electrodes

• Electrodes Graphite is a conductor with a high melting point and is unreactive in many electrolytes, so it is used for electrodes in electrolysis.

Section 5 of 6Key terms

Term	Meaning
Allotrope	A different structural form of the same element in the same state.
Hexagonal ring	A six-sided ring of carbon atoms; the building block of each layer in graphite.
Delocalised electron	An electron that is not attached to one atom and is free to move through the structure.
Lubricant	A substance that reduces friction between moving surfaces.
Giant covalent structure	A huge network of atoms held together by strong covalent bonds.

Section 6 of 6Exam technique

1

Explaining that graphite is soft because its covalent bonds are weak. The bonds within layers are strong; it is the forces between layers that are weak.

2

Saying graphite conducts because it has free ions. It conducts because of delocalised electrons.

3

Forgetting that graphite still has a high melting point. Melting needs the strong covalent bonds to be broken, not just the weak forces between layers.

4

Giving graphite four bonds per carbon atom in a diagram. Each carbon has three bonds in its layer.