Structure and properties

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.

Worked examples

Example 1

Substance X melts at 1610°C, does not conduct electricity as a solid or when molten, and does not dissolve in water. What is its structure?

  1. A very high melting point means a giant structure with many strong bonds: not simple molecular.
  2. It does not conduct when molten, so it has no ions that can move: not ionic.
  3. It does not conduct as a solid, so it has no delocalised electrons: not metallic.
  4. So X is giant covalent (it is silicon dioxide).

Answer: giant covalent

Example 2

Explain why graphite conducts electricity but diamond does not.

  1. Carbon has 4 outer electrons.
  2. In diamond each carbon atom uses all 4 in covalent bonds, so no electrons are free.
  3. In graphite each carbon atom bonds to only 3 others. The fourth electron is delocalised and can move along the layers, carrying charge.

Answer: graphite has one delocalised electron per carbon atom; in diamond every outer electron is in a bond

Example 3

A cube-shaped nanoparticle has sides of 10 nm. Find its surface area to volume ratio. What happens to the ratio if the side is 1 nm?

  1. A cube has 6 square faces. 6 × 10² = 600 nm²
  2. Find the volume. 10³ = 1000 nm³
  3. Divide. 600 ÷ 1000 = 0.6 per nm
  4. For a 1 nm cube: 6 ÷ 1 = 6 per nm. The side is 10 times smaller, so the ratio is 10 times bigger.

Answer: 0.6 per nm; 6 per nm, 10 times bigger