Structures

• Solids can be divided into four groups based on structure:

  • ionic crystals
  • atomic/ macromolecular/ giant molecular crystals
  • simple molecular crystals
  • metallic crystals
• Ionic crystals are composed of an ionic lattice, where ions are held together by strong electrostatic forces
• Giant molecular crystals are composed of an atomic lattice All atoms are held together by strong covalent bonds. They are also known as macromolecular or atomic crystals
• Example of giant molecular crystals:

  crystal, graphite
• Diamond and graphite are allotropes of carbon
• Allotrope - different forms of the same element in the same physical state, which have the same chemical properties and different physical properties
• Difference in Melting Points:

  • Sodium Chloride: fairly high, strong ionic bonds require a lot of energy to break
  • Diamond: very high, strong covalent bonds need large amounts of energy to break
  • Graphite: very high
• Difference in Electrical Conductivity:

  • Sodium Chloride: conducts only when molten or dissolved in water
  • Diamond: does not conduct electricity
  • Graphite: conducts electricity when solid (one free electron from each carbon atom)
• Simple Molecular Crystals are composed of a molecular lattice, where small molecules are held together by weak intermolecular forces e.g iodine, sulphure, ice
• Metallic Crystals are composed of a metallic lattice in which metal cations are held together by mobile, valency electrons