Giant Covalent Structures
A concise revision guide to giant covalent structures: how diamond, graphite and graphene are bonded, why their properties differ so much, and how the structure explains melting point, hardness and electrical conductivity.
What Is a Giant Covalent Structure?
In a giant covalent structure (also called a macromolecular or giant molecular structure) every atom is joined to its neighbours by strong covalent bonds that extend throughout the whole crystal. There are no separate molecules: a single crystal of diamond is, in effect, one enormous molecule. Compare this with a simple molecular substance such as iodine, where strong covalent bonds hold each I2 molecule together but only weak intermolecular forces act between the molecules.
| Feature | Giant covalent (diamond, graphite, graphene) | Simple molecular (iodine, ice, carbon dioxide) |
|---|---|---|
| Particles | Atoms in a continuous network | Discrete molecules |
| Forces broken on melting | Strong covalent bonds | Weak intermolecular forces |
| Melting point | Very high (diamond sublimes above 3600 °C) | Low |
| Solubility in water | Insoluble | Usually low; some polar molecules dissolve |
| Electrical conductivity | Usually none; graphite and graphene conduct | None |
Key idea: Melting a giant covalent structure means breaking covalent bonds, which needs a great deal of energy, so these substances have very high melting points and are hard or, in the case of graphite, strongly bonded within each layer.
Diamond
In diamond each carbon atom forms four single covalent bonds to four other carbon atoms arranged tetrahedrally around it, with bond angles of 109.5°. The result is a rigid three-dimensional network in which every atom is locked in place by strong C-C bonds.
| Property | Explanation from the structure |
|---|---|
| Very high melting and sublimation point | Many strong covalent bonds must be broken |
| Extremely hard | The rigid tetrahedral network resists any distortion; diamond is used in cutting tools and drill tips |
| Does not conduct electricity | All four outer electrons of each carbon are used in bonding, so there are no delocalised electrons |
| Insoluble in all solvents | No solvent interaction can compete with the covalent bonds |
Diamond: every carbon atom is bonded tetrahedrally to four others in a rigid three-dimensional network.
Silicon has the same structure as diamond, which is why it is also a hard, high-melting solid, although its Si-Si bonds are weaker than C-C bonds.
Exam sentence: Diamond has a giant covalent structure in which each carbon atom is bonded to four others by strong covalent bonds, so a large amount of energy is needed to break the bonds and it has a very high melting point.
Graphite
In graphite each carbon atom forms three covalent bonds to three other carbon atoms, giving flat hexagonal layers with bond angles of 120°. The fourth outer electron of each carbon is not used in a localised bond; it sits in a p orbital above and below the layer and becomes delocalised across the whole sheet. The layers are held to each other only by weak intermolecular forces (induced dipole-dipole or van der Waals forces), and are relatively far apart.
| Property | Explanation from the structure |
|---|---|
| Very high melting point | The strong covalent bonds within the layers must be broken |
| Soft and slippery | The weak forces between layers let them slide over each other; graphite is used as a lubricant and in pencils |
| Conducts electricity | The delocalised electrons can move along the layers when a voltage is applied |
| Low density compared with diamond | The layers are widely spaced |
| Insoluble | Covalent bonding throughout the layers |
Graphite: strong covalent bonds within hexagonal layers, weak forces between them, and delocalised electrons that conduct electricity.
Exam focus: Graphite conducts because it has delocalised electrons; it is soft because the layers are held together only by weak intermolecular forces. Do not say the layers are held by covalent bonds, and do not say graphite conducts because it is a metal.
Graphene
Graphene is a single layer of graphite: a sheet of carbon atoms one atom thick, each bonded covalently to three others in a hexagonal pattern, with one delocalised electron per carbon spread over the sheet. It is described as a two-dimensional giant covalent structure.
| Property | Explanation | Application |
|---|---|---|
| Extremely strong for its thickness | Strong covalent bonds throughout the sheet | Reinforcing composites and lightweight materials |
| Excellent electrical conductor | Delocalised electrons move freely across the sheet | Flexible electronics, touchscreens, sensors |
| Very good thermal conductor | Vibrations pass easily through the rigid covalent network | Heat management in electronic devices |
| Transparent and flexible | A single atomic layer absorbs little light and can bend without breaking bonds | Transparent conducting films |
Graphene: one layer of graphite, a two-dimensional giant covalent structure that is strong, flexible and conducting.
Exam sentence: Graphene is a single layer of graphite in which each carbon atom is covalently bonded to three others; the delocalised electrons make it an excellent conductor and the covalent network makes it very strong.
Comparing the Three Carbon Structures
| Diamond | Graphite | Graphene | |
|---|---|---|---|
| Bonds per carbon atom | 4 | 3 | 3 |
| Arrangement | Tetrahedral, 109.5°, three-dimensional | Hexagonal layers, 120° | One hexagonal layer, 120° |
| Delocalised electrons | None | One per carbon, within each layer | One per carbon, across the sheet |
| Electrical conductivity | None | Along the layers | Excellent |
| Hardness | Extremely hard | Soft, layers slide | Very strong sheet |
| Melting point | Very high | Very high | Very high |
Key idea: The same element gives three very different materials because the bonding arrangement differs: four localised bonds give hardness and no conduction; three bonds plus a delocalised electron give layers that conduct.
Where This Sits in the Edexcel International Specification
Topic 3 point 3.12 asks you to describe the structures of graphite, diamond and graphene and to discuss their applications, alongside the ionic, covalent and metallic bonding types in Topic 3.
Common Exam Mistakes
- Saying graphite is soft because its covalent bonds are weak. The covalent bonds are strong; it is the forces between layers that are weak.
- Saying diamond conducts electricity, or that graphite conducts because it contains ions or is metallic. Only delocalised electrons explain the conduction.
- Describing melting as breaking intermolecular forces. In a giant covalent structure the covalent bonds themselves must be broken.
- Forgetting the bond angle: 109.5° in diamond, 120° in graphite and graphene.
- Drawing a diamond fragment with carbon atoms that have only three bonds, or a graphite layer with four bonds per carbon.
Exam sentence: Giant covalent structures have very high melting points because many strong covalent bonds must be broken; only those with delocalised electrons, graphite and graphene, conduct electricity.
Check Your Understanding
Use these short activities to apply the ideas to structures not used on this page.
Master Covalent Bonding and Shapes of Molecules for Edexcel International A Level Chemistry
Continue from these free revision notes into the full Topic 3B/C Covalent Bonding and Shapes of Molecules course, with guided video teaching, diagnostic MCQ practice, teacher-marked short-answer questions and a specification assignment with a personalised progress report.
Guided video teaching
Learn covalent bonding, dot-and-cross diagrams, giant covalent structures, electronegativity, polarity and molecular shapes through structured video lessons with worked examples and walkthroughs.
Instant MCQ feedback
Auto-marked MCQ quizzes provide immediate diagnostic feedback for every answer choice.
Teacher-marked SAQs
Submit written exam responses and receive chemistry specialist feedback with improvement guidance.
Progress tracking
Identify strengths and weaknesses across covalent bonding, giant covalent structures, polarity, VSEPR theory and molecular shapes with targeted reporting.
See how the course works
Click play to start the course preview animation.
The shapes of some species are being compared.
Which species is not tetrahedral?
Explain this difference in terms of structure and bonding.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.