How do shared electrons and a sea of delocalised electrons explain covalent and metallic substances?
Covalent and metallic bonding: shared electron pairs, simple molecular and giant covalent structures (diamond, graphite, fullerenes, graphene), polymers, metallic bonding, and how each structure explains properties.
A focused answer to Edexcel GCSE Chemistry topic 1, covering covalent bonding and dot-and-cross diagrams, simple molecular substances, the giant covalent structures of diamond, graphite, fullerenes and graphene, simple polymers, metallic bonding with delocalised electrons, and how each structure explains melting point and conductivity.
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What this dot point is asking
Edexcel wants you to explain covalent bonding as shared pairs of electrons, draw dot-and-cross diagrams of simple molecules, describe simple molecular and giant covalent structures (including diamond, graphite, fullerenes and graphene), describe simple polymers, explain metallic bonding using a sea of delocalised electrons, and use each structure to explain melting point, conductivity, hardness and malleability. This completes the three bonding types.
Covalent bonding
Molecules can have single, double or triple covalent bonds. A hydrogen molecule has one shared pair; oxygen has a double bond (two shared pairs); nitrogen has a triple bond. Dot-and-cross diagrams show the shared pairs in the overlap between the atoms, with the dots and crosses distinguishing which atom each electron came from. The typical size of an atom or small molecule is of the order m.
Simple molecular substances
Substances such as water, carbon dioxide, methane, chlorine and iodine are made of small molecules.
- The covalent bonds within each molecule are strong, but the intermolecular forces between molecules are weak.
- Melting or boiling only overcomes the weak intermolecular forces, not the covalent bonds, so these substances have low melting and boiling points and many are gases or liquids at room temperature.
- They do not conduct electricity, because the molecules are neutral overall with no free electrons or ions.
As the molecules get larger, the intermolecular forces increase, so the melting and boiling points rise.
Giant covalent structures
In a giant covalent structure, billions of atoms are joined by covalent bonds in a continuous network, giving very high melting points.
- Diamond. Each carbon atom forms four covalent bonds in a rigid tetrahedral lattice. It is very hard, has a very high melting point and does not conduct (no free electrons). Used in cutting tools.
- Graphite. Each carbon atom forms three covalent bonds, arranged in layers of hexagons. The fourth outer electron of each atom is delocalised, so graphite conducts electricity. The layers are held by weak forces and slide over each other, so graphite is soft and used as a lubricant and in electrodes.
- Silicon dioxide (silica). A giant covalent lattice with a high melting point, similar in structure to diamond.
Polymers
A simple polymer such as poly(ethene) consists of very large molecules made of many repeating units (monomers) joined by covalent bonds into long chains. The chains are held together by intermolecular forces, which are stronger than in small molecules because the chains are long, so polymers are solids at room temperature with higher melting points than small molecules but lower than giant covalent or ionic substances.
Metallic bonding
The delocalised electrons come from the outer shells of the metal atoms. This structure explains the typical properties of metals:
- Good electrical and thermal conductors, because the delocalised electrons are free to move and carry charge and energy.
- Malleable and ductile (can be bent, shaped and drawn into wires), because the layers of ions can slide over one another without breaking the metallic bonds.
- High melting and boiling points, because the electrostatic attraction between the ions and the delocalised electrons is strong.
Try this
Q1. State what is meant by a covalent bond. [1 mark]
- Cue. A shared pair of electrons between two non-metal atoms.
Q2. Explain why simple molecular substances have low melting points. [2 marks]
- Cue. Melting overcomes only the weak intermolecular forces between molecules, which needs little energy; the strong covalent bonds are not broken.
Q3. Explain why graphene is useful in electronic devices. [2 marks]
- Cue. It is a single layer of carbon atoms with delocalised electrons, so it conducts electricity well, and it is very strong and light.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20184 marksDiamond and graphite are both forms of carbon, yet graphite conducts electricity and is used as a lubricant while diamond does not conduct and is very hard. Explain these differences in terms of structure and bonding.Show worked answer →
A 4-mark compare-the-allotropes question, very common on Edexcel Paper 1.
In diamond each carbon atom forms four covalent bonds in a rigid giant lattice, so it is very hard and has no free electrons to conduct (1 mark for the four bonds, 1 mark for no free electrons). In graphite each carbon forms only three covalent bonds, leaving one delocalised electron per atom that can move and carry charge, so graphite conducts (1 mark). Graphite is arranged in layers held together by weak forces, so the layers slide over each other, which is why it is a lubricant (1 mark).
Markers reward "four bonds" versus "three bonds plus a delocalised electron" and the sliding layers, not vague answers about hardness.
Edexcel 20213 marksExplain why metals are good conductors of electricity and can be bent and shaped, in terms of metallic bonding.Show worked answer →
A 3-mark metallic-bonding explanation.
A metal is a giant lattice of positive ions surrounded by a sea of delocalised electrons (1 mark). These delocalised electrons are free to move through the structure and carry charge, so the metal conducts electricity (1 mark). The layers of ions can slide over one another without breaking the metallic bonding, so the metal is malleable and can be bent and shaped (1 mark).
Markers want "delocalised electrons free to move" for conduction and "layers slide" for malleability.
Related dot points
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Sources & how we know this
- Pearson Edexcel GCSE (9-1) Chemistry (1CH0) specification — Pearson Edexcel (2016)