Giant Structures: Giant Covalent and Metallic
ChemistryΒ· 2.6, 2.7Β· 12 min read
1. Core: Giant Covalent Lattice Structureβ β ββββ± 3 min
Giant covalent lattice
A 3D network of non-metal atoms held together by strong covalent bonds between adjacent atoms, with no discrete individual molecules present in the structure.
Giant covalent (macromolecular) structures have extremely high melting and boiling points, as very large amounts of heat energy are needed to break the thousands of strong covalent bonds spread throughout the entire lattice. Almost all giant covalent substances do not conduct electricity, as they have no free charged particles to carry charge.
Explain why diamond has a melting point of over 3500Β°C.
- 1
- Identify the structure: Diamond is a giant covalent lattice.
- 2
- Describe the bonding: Every carbon atom in diamond is covalently bonded to 4 other carbon atoms in a rigid 3D tetrahedral network.
- 3
- Link to property: A very large amount of heat energy is required to break the many strong covalent bonds in the lattice, so diamond has an extremely high melting point.
Exam tip:
Always reference the many strong covalent bonds when explaining high melting points of giant covalent structures, not just strong bonds alone, for full marks.
2. Core: Key Giant Covalent Substancesβ β β βββ± 4 min
Three giant covalent substances are specified in the 0620 syllabus: diamond, graphite, and silicon(IV) oxide (silicon dioxide). Their properties are directly linked to their unique structures, as outlined below:
Substance | Structure | Key properties | Common uses |
|---|---|---|---|
Diamond | Each C bonded to 4 other C atoms, tetrahedral 3D lattice | Very hard, high melting point, non-conductive | Cutting tools, jewellery |
Graphite | Each C bonded to 3 other C atoms in hexagonal layers; weak intermolecular forces between layers; 1 delocalised electron per C | Soft/slippery, high melting point, conductive | Lubricants, electrodes for electrolysis |
Silicon(IV) oxide | Each Si bonded to 4 O atoms, each O bonded to 2 Si atoms, 3D tetrahedral lattice matching diamond structure | High melting point, hard, non-conductive | Sand, glass manufacturing |
Explain why graphite is suitable as a lubricant for machine parts that operate at very high temperatures.
- 1
- Link to slipperiness: Graphite has hexagonal layers held together by weak intermolecular forces, so layers can slide over each other easily, making it soft and slippery.
- 2
- Link to high temperature stability: Graphite is a giant covalent lattice with strong covalent bonds within its layers, so it does not melt or break down at high operating temperatures.
- 3
- Conclude: These two properties make it ideal for use as a high-temperature lubricant.
Exam tip:
Graphite is the only common giant covalent substance that conducts electricity β this exception is tested in almost every exam session, so memorise the reason for this property.
3. Extended Only: Metallic Bonding and Structureβ β β ββExtended onlyβ± 2 min
Metallic bonding
The strong electrostatic force of attraction between positively charged metal ions arranged in a regular closely packed lattice, and a 'sea' of delocalised outer shell electrons moving freely throughout the structure.
Each metal atom loses its outer shell electrons to form a positive ion. These electrons are shared across the entire lattice and are not bound to any single ion, so they can move freely through the structure. This bonding model explains all characteristic properties of metals.
Describe the structure and bonding present in a solid sample of magnesium metal.
- 1
- Each magnesium atom loses its 2 outer shell electrons to form a positive MgΒ²βΊ ion.
- 2
- The positive MgΒ²βΊ ions are arranged in a regular, closely packed 3D lattice.
- 3
- The released outer shell electrons form a delocalised 'sea' of negative charge surrounding the positive ions.
- 4
- Metallic bonding is the strong electrostatic attraction between the positive metal ions and the delocalised sea of electrons.
Exam tip:
Never refer to metallic bonding as 'ions attracting electrons' alone β always specify the electrostatic attraction between positive metal ions and delocalised electrons to earn full marks.
4. Extended Only: Properties of Metallic Structuresβ β β ββExtended onlyβ± 3 min
High melting and boiling points: Large amounts of energy are needed to break the strong metallic bonds throughout the lattice.
Good electrical conductivity: Delocalised electrons are free to move and carry electric charge through the structure.
Good thermal conductivity: Delocalised electrons transfer kinetic energy rapidly through the lattice.
Malleable (can be hammered into shape) and ductile (can be drawn into wires): Layers of positive metal ions can slide over each other without breaking the metallic bonding, as the delocalised electrons move with the layers.
Explain why copper metal is used to make electrical wires for household circuits.
- 1
- Electrical conductivity: Copper has a sea of delocalised electrons that are free to move and carry electric charge, so it is an excellent conductor of electricity.
- 2
- Ductility: Copper is ductile, so it can be drawn into thin, flexible wires easily for use in circuits.
5. Common Pitfalls
Wrong move:
Referring to 'weak covalent bonds' when explaining why graphite is soft.
Why:
The covalent bonds within graphite layers are very strong; only the intermolecular forces between layers are weak.
Correct move:
Explicitly distinguish between strong intralayer covalent bonds and weak interlayer intermolecular forces for graphite property questions.
Wrong move:
Stating diamond does not conduct electricity because it has no electrons.
Why:
Diamond has electrons, but all are used in covalent bonds so they are not free to move and carry charge.
Correct move:
Explain that diamond has no free or delocalised charged particles to conduct electricity.
Wrong move:
Describing metallic bonding as attraction between neutral metal atoms and electrons.
Why:
Metal atoms lose their outer electrons to form positive ions; the attraction is between ions and delocalised electrons, not neutral atoms.
Correct move:
Always reference positive metal ions and delocalised electrons when describing metallic bonding.
Wrong move:
Confusing silicon(IV) oxide structure with simple molecular carbon dioxide.
Why:
COβ is a simple molecular substance, while SiOβ is a giant covalent lattice with no discrete molecules.
Correct move:
State that SiOβ has a 3D giant covalent structure similar to diamond when answering questions about its high melting point.
Wrong move:
Stating metals break when hit because ionic bonds repel.
Why:
Metals have metallic bonding, not ionic bonding; layers of ions slide rather than repel when force is applied.
Correct move:
Explain that metallic bonding remains intact as layers of ions slide, making metals malleable.
6. Quick Reference Cheatsheet
Structure type | Key bonding | Core properties | Common examples |
|---|---|---|---|
Giant covalent | Strong covalent bonds across 3D lattice | High mp/bp, mostly non-conductive, hard (except graphite) | Diamond, graphite, SiOβ |
Metallic (Extended) | Electrostatic attraction between + metal ions and delocalised electrons | High mp/bp, conductive, malleable/ductile | All metals (Fe, Cu, Mg) |
Ionic (prerequisite) | Electrostatic attraction between + and - ions | High mp/bp, conductive only when molten/dissolved | NaCl, MgO |
7. Frequently Asked
Why is graphite soft but diamond is very hard?
Graphite has a layered structure held by weak intermolecular forces between layers, so layers can slide over each other easily. Diamond has a rigid 3D tetrahedral lattice of strong covalent bonds between all adjacent carbon atoms, so it cannot be scratched easily.
(Extended only) Why are metals good conductors of electricity?
Metals have a lattice of positive metal ions surrounded by delocalised electrons that are free to move through the entire structure, carrying electric charge.
Going deeper
What's Next
Now that you understand the three types of giant structures tested in CIE IGCSE Chemistry 0620, you can move on to comparing simple molecular and giant structures, a common 3-5 mark structured exam question. You will also apply this knowledge when learning about material properties in later units, and when answering questions that ask you to link structure, bonding and properties for unknown substances. Make sure you can recall the specific structures of diamond, graphite and silicon(IV) oxide, as well as the metallic bonding model for Extended papers, to score full marks on this topic.