Structure of matter
How a substance behaves — its melting point, conductivity, hardness — is determined almost entirely by its bonding and structure.
Simple molecular substances
Atoms covalently bonded into small molecules (H₂O, CO₂, Cl₂, CH₄). The covalent bonds are strong, but the intermolecular forces between molecules are weak.
- Low melting and boiling points (often gases or liquids at room temperature).
- Do not conduct electricity — no charged particles free to move.
Giant ionic lattices
Positive metal ions and negative non-metal ions in a regular 3D lattice held by strong electrostatic forces in all directions (e.g. NaCl, MgO).
- Very high melting points.
- Do not conduct as a solid (ions are fixed). Conduct when molten or dissolved (ions are free to move).
Giant covalent — diamond, graphite and graphene
Each carbon atom is covalently bonded into a giant lattice.
| Allotrope | Bonds per C | Properties |
|---|---|---|
| Diamond | 4 | Hard, very high melting point, does not conduct |
| Graphite | 3 (in layers) | Soft (layers slide), high melting point, conducts (delocalised electrons) |
| Graphene | 3 (single layer) | Strong, transparent, excellent conductor |
Silicon dioxide (sand) is also a giant covalent lattice with the same hardness/insulating profile as diamond.
Metallic structures
Lattice of positive metal ions in a "sea" of delocalised electrons.
- High melting point (strong metallic bonds).
- Conduct electricity and heat (delocalised electrons carry charge).
- Malleable and ductile (layers can slide because the bonding is non-directional).
Polymers
Long-chain molecules made of repeating monomer units (e.g. poly(ethene)). Chains are tangled and held by weak forces between them. Stronger than simple molecules but softer than giant covalent — can be melted and reshaped (thermosoftening) or set rigidly (thermosetting).
CCEA tip
A "use the structure to explain why X conducts/melts at Y" question always wants you to (i) name the structure, (ii) name the bonds/forces and (iii) link to mobility of charges or strength. Skipping any of the three forfeits a mark.
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