Four types of crystal structures

• The four types of crystal structures are ionic metallic molecular and giant covalent (macromolecular)
• You should be able to draw the following diagrams or describe the structure in words to show the four different types of crystal. You should also be able to explain the properties of these solids. The tables earlier in the revision guide explain these properties.
• Ionic sodium chloride- giant ionic lattice showing alternate Na+ and Cl- ions

  
• Metallic magnesium or sodium

  • use this diagram for any metal
  • Giant metallic lattice showing close packing magnesium ions

  
• Molecular iodine - regular arrangement held together by weak van der waals forces

  
• Molecular ice -

  • This diagram is difficult to draw
  • The main point to show is a central water molecule with two ordinary covalent bonds in a tetrahedral arrangement
  • The molecules are held further apart than in liquid water and this explains the lower density of ice

  
• Macromolecular diamond

  • Tetrahedral arrangement of carbon atoms. 4 covalent bonds per atom

  Macromolecular graphite

  • Planar arrangement of carbon atoms in layers. 3 covalent bonds per atom in each layer. 4th outer electron per atom is delocalised. Delocalised electrons between atoms

  Both these macromolecular structures have very high melting points because of strong covalent forces in the giant structure. It takes a lot of energy to break the many strong covalent bonds