1.7.4Principles of Chemistry

Giant Covalent Structures — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions

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Part 1 of 10What a giant covalent structure is

Giant covalent structure
- billions of non-metal atoms each joined to neighbours by covalent bonds in a lattice.
Allotropes
- diamond and graphite are different forms of carbon with different bonding arrangements.
- Same atoms, different properties
  - both contain only carbon, but they are physically very different.

Bonding
- each carbon atom is bonded to four others in a tetrahedral arrangement.
Bonds
- all identical, very strong, with no intermolecular forces.
- Electrons
  - all four outer electrons are held in bonds.

Formula
- SiO₂, also called silica, occurring naturally as sand and quartz.
Bonding
- each silicon bonds to four oxygens and each oxygen bonds to two silicons.
- Shape
  - a tetrahedron of one silicon and four oxygens, as in diamond.

Layers
- each carbon bonds to three others, forming layers of hexagons.
Delocalised electrons
- one free electron per carbon is delocalised between the layers.
- Forces between layers
  - only weak intermolecular forces hold the layers together.
- Bonds within layers
  - strong covalent bonds.

Does not conduct
- all outer electrons are in bonds, so no charge can move.
Very high melting point
- a large amount of energy is needed to break the strong lattice of bonds.
Extremely hard and dense
- each carbon is bonded to four others.
Uses
- jewellery, and tips of cutting discs and heavy-duty drill bits for brick and concrete.

Conducts electricity
- delocalised electrons move and carry charge.
Very high melting point
- strong covalent bonds within the layers.
Soft and slippery
- layers slide over each other because of weak forces between them.
Density
- lower than diamond.
Uses
- pencils, industrial lubricant in engines and locks, and unreactive electrodes for electrolysis.
Pencil lead
- it is graphite, not the metal lead; the name graphite comes from Latin for writing.

Substance	Bonding	Electrical conductivity	Hardness	Melting point
Diamond	Each C bonded to 4 others	None	Extremely hard	Very high
Graphite	Each C bonded to 3 others in layers	Conducts	Soft, slippery	Very high
Silicon(IV) oxide	Si to 4 O, O to 2 Si	None	Very hard	Very high

Silica uses
- cheap and natural, so used for sandpaper and to line furnaces.
Other properties
- insoluble in water.
- Cause
  - many strong covalent bonds and no intermolecular forces.

Diamond
- strong bonds in every direction make it hard with a very high melting point, and no free electrons make it an insulator.
Graphite
- strong bonds within layers give a high melting point, while weak forces between layers make it soft and slippery.
Conductivity
- delocalised electrons between the layers carry charge.
- Lubricant
  - layers slide easily over each other.
Silicon(IV) oxide
- the giant structure of strong bonds makes it hard, high melting and non-conducting.
Link
- its structure is similar to diamond, so its properties are similar.

Name the type of structuregiant covalent.
State the particles involved and the bonds between them.
Explain the property by referring to the strong bonds, free electrons or weak forces.
Link the property to a use.

Common trap
- do not say that bonds break when graphite slides;
  - the layers move apart because the weak forces between them are overcome.
Hard and brittle
- hardness refers to resisting scratches, not to being unbreakable.

Explain why diamond is very hard but graphite is soft. [3]3 marks
1. In diamond each carbon forms four strong covalent bonds in a 3D lattice.
2. In graphite the layers are held together by weak forces.
3. The layers can slide over each other, so graphite is soft.
Explain why graphite conducts electricity but diamond does not. [2]2 marks
1. Each carbon in graphite uses three electrons in bonds, leaving one delocalised electron that can move.
2. In diamond all four outer electrons are held in bonds.
Explain why silicon(IV) oxide has a very high melting point. [2]2 marks
1. It is a giant covalent structure with many strong covalent bonds.
2. A very large amount of energy is needed to break them.
State a use of diamond and a use of graphite, each linked to a property. [2]2 marks
1. Diamond cuts concrete because it is extremely hard.
2. Graphite is a lubricant because its layers slide easily.

Term	Meaning
Allotropes	Different forms of the same element in the same state.
Giant covalent structure	A huge lattice of atoms joined by covalent bonds.
Delocalised electron	An electron free to move through a structure.
Tetrahedron	A shape with four bonds pointing to the corners, as in diamond.
Silicon(IV) oxide	SiO₂, a giant covalent compound found as sand and quartz.

Confusing hard with strong — diamond is hard but brittle and can be smashed by a hammer.
Saying graphite has weak bonds — the bonds in layers are strong; only the forces between layers are weak.
Mixing pencil lead with the metal lead — pencil lead is graphite.
Saying giant covalent structures contain molecules — they have a continuous lattice of atoms.

Last updated: 20 October 2026