Structure, Properties and Carbon Allotropes
Properties of Ionic Compounds
Ionic compounds form giant ionic lattices — a regular arrangement of alternating positive and negative ions. The strong electrostatic forces between the ions act in all directions, giving ionic compounds these properties:
- High melting and boiling points — a lot of energy is needed to overcome the strong ionic bonds.
- Conduct electricity when molten or dissolved — the ions are free to move and carry charge. They do not conduct when solid because the ions are fixed in position.
Ionic compounds: high melting points (strong bonds in all directions), conduct when molten or dissolved (ions free to move), do not conduct when solid (ions fixed in lattice).
Properties of Simple Molecular Substances
Simple molecules (e.g. \(H_2O\), \(CO_2\), \(O_2\)) have strong covalent bonds within the molecule but weak intermolecular forces between molecules. This gives them:
- Low melting and boiling points — little energy is needed to overcome the weak intermolecular forces (not the covalent bonds).
- Do not conduct electricity — no free electrons or ions to carry charge.
As molecules get larger, the intermolecular forces become stronger, so larger molecules tend to have higher boiling points.
Giant Covalent Structures
Giant covalent structures have millions of atoms connected by covalent bonds in a continuous network. They have very high melting points because many strong covalent bonds must be broken. Examples include diamond, graphite and silicon dioxide.
Diamond: each carbon atom bonds to four others in a rigid tetrahedral structure. Very hard, high melting point, does not conduct electricity (no free electrons or ions).
Graphite: each carbon bonds to three others, forming flat layers with weak forces between them. The fourth electron is delocalised, allowing graphite to conduct electricity. Layers can slide over each other, making it soft and slippery (useful as a lubricant).
Graphene: a single layer of graphite — one atom thick. It is very strong, light, and an excellent conductor of electricity and heat. Fullerenes (e.g. \(C_{60}\) buckminsterfullerene) are carbon molecules shaped like hollow spheres or tubes. Carbon nanotubes are cylindrical fullerenes with remarkable strength and conductivity.
Diamond: 4 bonds per carbon, very hard, doesn’t conduct. Graphite: 3 bonds per carbon + delocalised electrons, conducts electricity, layers slide. Graphene: single layer of graphite, strongest known material, conducts.
Polymers are very large molecules made up of many repeating units called monomers. They have relatively strong intermolecular forces between the long chains, so they tend to be solid at room temperature. Examples include poly(ethene) and poly(propene).