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GCSE Chemistry Revision
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GCSE Chemistry revision
Giant covalent structures
How bonding and structure are related to the properties of substances
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Key knowledge
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Introduction
- Allotropes are different structural forms of the same element existing in the same physical state, for example, diamond, graphite, and fullerenes are all allotropes of carbon in the solid state.
Giant Covalent Structures
- Both diamond and graphite are giant covalent structures, meaning they consist of a regular lattice of covalently bonded atoms.
- Because the covalent bonds in giant covalent structures are very strong, these materials tend to have high melting points and are generally very strong materials.
Structure of Diamond
- In diamond, each carbon atom is covalently bonded to four other carbon atoms, forming a rigid, regular three-dimensional lattice.
- The large number of strong covalent bonds means diamond is extremely hard and has a very high melting point.
- Diamond does not conduct electricity because it has no free electrons or ions available to carry charge.
Structure of Graphite
- In graphite, each carbon atom is covalently bonded to only three other carbon atoms, forming hexagonal rings that make up large, flat sheets called layers.
- The layers in graphite are held together only by weak intermolecular forces, allowing them to slide over one another, making graphite relatively soft.
- Despite being soft, graphite still has a very high melting point because the covalent bonds within each layer are very strong.
Delocalised Electrons in Graphite
- Because each carbon atom in graphite forms only three covalent bonds, it has one spare electron that becomes delocalised, meaning it is free to move throughout the structure.
- These delocalised electrons allow graphite to conduct both electricity and heat, unlike diamond.
Comparing Diamond and Graphite
- Diamond is harder than graphite because all four of its carbon bonds are used in a rigid 3D structure, whereas graphite's layers can slide due to weak forces between them.
- Both allotropes share a very high melting point due to the strong covalent bonds present within their structures, but only graphite conducts electricity.
Introduction to Graphene and Fullerenes
- A single isolated layer of graphite is known as graphene, which is an incredibly thin and strong material.
- Scientists can use graphene layers to construct other structures such as spheres and tubes, which are collectively known as fullerenes.