Structure and properties · Year 12
The properties of a substance depend on its structure and bonding. Giant structures (ionic lattices, giant covalent networks such as diamond, and metals) have high melting points, because many strong bonds must be overcome. Simple molecular substances have low melting points, because only weak forces hold their molecules together. Carbon alone makes diamond, graphite, graphene and fullerenes, and nanoparticles behave differently because of their huge surface area.
Key facts
- Giant ionic: high melting point; conducts only when melted or dissolved.
- Simple molecular: low melting and boiling points; does not conduct.
- Giant covalent (diamond, silicon dioxide): very high melting point, hard, does not conduct (graphite is the exception).
- Metallic: usually a high melting point; conducts when solid and liquid; can be bent into shape.
- Diamond: each carbon atom is bonded to 4 others. Graphite: layers of carbon atoms each bonded to 3 others, with delocalised electrons, so it is soft and conducts. Graphene is a single layer of graphite; fullerenes such as C60 are hollow molecules.
- Nanoparticles are 1–100 nm across. Their surface area to volume ratio is very large, so they make good catalysts.