Carbon Allotropes (Edexcel IAL Chemistry)

Spec point — 3.12

Carbon Allotropes

Covalent bonding & giant covalent lattice structures

• Allotropes of carbon are substances that only contain carbon atoms but due to the differences in bonding arrangements they are physically completely different

• Giant covalent lattices have very high melting and boiling points

  • These compounds have a large number of covalent bonds linking the whole structure
  • A lot of energy is required to break the lattice

• The compounds can be hard or soft

  • Graphite is soft as the forces between the carbon layers are weak
  • Diamond and silicon(IV) oxide are hard as it is difficult to break their 3D network of strong covalent bonds

• Most compounds are insoluble with water

• Most compounds do not conduct electricity however some do

  • Graphite has delocalised electrons between the carbon layers which can move along the layers when a voltage is applied
  • Diamond and silicon(IV) oxide do not conduct electricity as all four outer electrons on every carbon atom are involved in a covalent bond so there are no freely moving electrons available

Graphite

• Each carbon atom in graphite is bonded to three others forming layers of hexagons, leaving one free electron per carbon atom

• These free electrons migrate along the layers and are free to move and carry charge, hence graphite can conduct electricity

• The covalent bonds within the layers are very strong

  • But, the layers are held together by weak van der Waals' forces of attraction, which are easily overcome
  • This allows the layers to slide over each other, making graphite soft and slippery

Diagram showing the structure and bonding arrangement in graphite

Diamond

• In diamond, each carbon atom bonds with four other carbons, forming a tetrahedron
• All the covalent bonds are identical, very strong and there are no intermolecular forces

Diagram showing the structure and bonding arrangement in diamond

Graphene

• Graphene consists of a single layer of graphite which is a sheet of carbon atoms covalently bonded forming a continuous hexagonal layer
• It is essentially a 2D molecule since it is only one atom thick
• It has very unusual properties make it useful in fabricating composite materials and in electronics

Graphene is a truly remarkable material that has some unexpected properties