AQA · GCSE · Combined Science

    Course version: 8464 · Version from Sept 2018

    Graphite: In graphite, one electron from each carbon atom is delocalised.

    Carbon has four outer-shell electrons.

    Checked against the AQA 8464 specification (opens in a new tab) on .

    • Point 2 of 4
    • 3 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Carbon has four outer-shell electrons. In graphite, each carbon atom forms three covalent bonds to three other carbon atoms in a hexagonal layer, using three of those electrons. The fourth electron from each carbon atom is not used in bonding and becomes delocalised: it is free to move throughout the layer rather than staying with one atom. Because every carbon contributes one delocalised electron, the layer contains a sea of moving electrons. This explains why graphite conducts electricity along its layers, unlike diamond where all four outer electrons are localised in covalent bonds. A useful check is to count bonds: three sigma bonds per carbon means one electron remains, so the ratio of delocalised electrons to carbon atoms is 1:1.

    What earns the marks

    • Each carbon atom in graphite forms three covalent bonds to neighbouring carbon atoms in the same layer.
    • Three of the four outer-shell electrons per carbon atom are used in these covalent bonds.
    • The remaining fourth outer-shell electron per carbon atom becomes delocalised.
    • Delocalised electrons are free to move within the layer, so graphite conducts electricity.
    • The ratio of delocalised electrons to carbon atoms is 1:1, because every carbon atom contributes one delocalised electron.
    • Delocalisation occurs within layers; movement between layers is limited, which helps explain why conduction is mainly along the layers.

    Common mistakes and how to put them right

    • Saying all four electrons are delocalised.

      Put it right by counting three covalent bonds per carbon atom, leaving only one electron delocalised.

    • Confusing graphite with diamond and stating that all outer electrons are localised in bonds.

      Put it right by noting that diamond has four bonds per carbon and no delocalised electrons.

    • Describing delocalised electrons as 'free atoms' or 'ions'.

      Put it right by stating they are electrons that are not associated with any single atom and can move through the layer.

    Exam tips

    • Link the number of bonds per carbon atom directly to the number of delocalised electrons in your answer.
    • Use the phrase 'delocalised electrons' rather than 'free electrons' when explaining conduction in graphite.
    • If asked to compare graphite and diamond, state the bonding count for each before drawing the conductivity conclusion.

    Try the lesson

    The start of the Graphite: In graphite, one electron from each carbon atom is delocalised. lesson, made from checked teaching notes for this specification point.

    Have a go first

    A sample of graphite contains 6.0 × 10²³ carbon atoms. How many delocalised electrons are present in the sample?

    Listen

    Carbon has four outer-shell electrons. In graphite, each carbon atom forms three covalent bonds to three other carbon atoms in a hexagonal layer, using three of those electrons. The fourth electron from each carbon atom is not used in bonding and becomes delocalised: it is free to move throughout the layer rather than staying with one atom. Because every carbon contributes one delocalised electron, the layer contains a sea of moving electrons. This explains why graphite conducts electricity along its layers, unlike diamond where all four outer electrons are localised in covalent bonds. A useful check is to count bonds: three sigma bonds per carbon means one electron remains, so the ratio of delocalised electrons to carbon atoms is 1:1. A strong answer earns marks for these points. Each carbon atom in graphite forms three covalent bonds to neighbouring carbon atoms in the same layer. Three of the four outer-shell electrons per carbon atom are used in these covalent bonds. The remaining fourth outer-shell electron per carbon atom becomes delocalised. Delocalised electrons are free to move within the layer, so graphite conducts electricity. A common slip: Saying all four electrons are delocalised. Instead, counting three covalent bonds per carbon atom, leaving only one electron delocalised. Exam tip: Link the number of bonds per carbon atom directly to the number of delocalised electrons in your answer.

    See it

    A flowchart running from top to bottom. The first box states that a carbon atom has four outer-shell electrons. From it, two arrows lead onwards. The first arrow goes to a box saying three covalent bonds are formed to three other carbon atoms in a hexagonal layer, which leads to a box saying three outer-shell electrons are used in bonding. The second arrow goes to a box saying the fourth outer-shell electron is not used in bonding, which leads to a box saying the fourth electron becomes delocalised. That leads to a box saying delocalised electrons are free to move within the layer, which finally leads to a box saying graphite conducts electricity along its layers.
    It shows how each carbon atom in graphite uses three outer-shell electrons in covalent bonds while the fourth becomes delocalised, allowing conduction along the layers.

    Key words

    Say what each word means before you turn the card.

    Delocalised electrons

    Try answering aloud or in your head before turning the card.

    Electrical conductivity

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    Structure and bonding

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    See every definition
    Delocalised electrons
    Electrons that are not used in bonding and are free to move throughout the layer rather than staying with one atom.
    Electrical conductivity
    Graphite conducts electricity along its layers because the delocalised electrons are free to move within the layer.
    Structure and bonding
    Each carbon atom in graphite forms three covalent bonds to neighbouring carbon atoms in the same layer, using three of its four outer-shell electrons.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A flowchart running from top to bottom. The first box states that a carbon atom has four outer-shell electrons. From it, two arrows lead onwards. The first arrow goes to a box saying three covalent bonds are formed to three other carbon atoms in a hexagonal layer, which leads to a box saying three outer-shell electrons are used in bonding. The second arrow goes to a box saying the fourth outer-shell electron is not used in bonding, which leads to a box saying the fourth electron becomes delocalised. That leads to a box saying delocalised electrons are free to move within the layer, which finally leads to a box saying graphite conducts electricity along its layers.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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