Giant covalent structures

O Level / IGCSE Chemistry · Atoms, elements and compounds. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

Test this topic freeAll O Level / IGCSE Chemistry topics

The idea

In a giant covalent structure a very large number of atoms are joined by strong covalent bonds in a continuous network. There are no small molecules. Melting means breaking many covalent bonds, so these substances have very high melting points.

In diamond each carbon atom is bonded to four others, making a rigid three-dimensional network. Diamond is very hard and does not conduct electricity. Silicon(IV) oxide has a similar structure, so it has similar properties.

In graphite each carbon atom is bonded to only three others, forming flat layers of hexagons. The layers are held together by weak forces and can slide. The fourth outer electron of each atom is delocalised, so graphite conducts electricity.

Rules to remember

Common mistake

Graphite is soft, but that does not give it a low melting point. Sliding the layers only overcomes weak forces; melting graphite means breaking the strong covalent bonds inside the layers.

Worked example

Diamond and graphite are both made only of carbon atoms. Why can graphite be used as an electrode while diamond cannot?

  1. A carbon atom has 4 outer electrons. To conduct electricity a solid needs charged particles that are free to move.
  2. In diamond each carbon atom uses all 4 outer electrons in covalent bonds to 4 other atoms, so no electrons are free.
  3. In graphite each carbon atom uses only 3 outer electrons in covalent bonds. The fourth electron is delocalised and can move along the layers.

Answer: Graphite has delocalised electrons that are free to move and carry the current; diamond has none.

Practice questions

Try each one, then open the answer.

1. Why is diamond used in the tips of cutting tools?

  1. A
    its layers can slide over each other, which lets it cut through rock
  2. B
    it conducts electricity, so it heats up and melts what it touches
  3. C
    it has a very low density, which makes the tool light and easy to hold
  4. D
    it is extremely hard, with every carbon atom held by four covalent bonds
Show answer

Answer: D. Diamond is a rigid giant covalent network; there are no weak points, so it is the hardest natural substance.

2. Graphite is soft, yet its melting point is higher than 3600 °C. Why is its melting point so high?

  1. A
    it is a metal, so its positive ions are held by strong metallic bonds
  2. B
    it contains ions, and the attractions between them are strong
  3. C
    the forces between the layers are very strong
  4. D
    the strong covalent bonds within each layer must be broken to melt it
Show answer

Answer: D. Softness comes from the weak forces between layers, but to melt graphite the strong covalent bonds inside each layer must break, which needs a lot of energy.

3. Why is graphite a good lubricant?

  1. A
    it is a liquid at room temperature and flows between surfaces
  2. B
    its atoms are not bonded to each other at all, so it crumbles
  3. C
    it contains ions that reduce friction between moving parts
  4. D
    the weak forces between its layers let them slide over each other
Show answer

Answer: D. Graphite is layers of carbon atoms, each bonded to three others. Weak forces between layers let them slide.

More questions on this topic

Read the full notes

Keep going

← Simple molecules and covalent bondsMetallic bonding →All rules on one pageStuck? Ask a question