Solid Structures

• Substances with high melting points are usually ionic compounds or giant covalent structures.
• Ionic solid


  Lattices of positive and negative ions
• Ionic solids


  • NaCl (6:6)
  • CaO (8:8)
• Cation is larger than anion


  More cations can fit in the crystal
• Ionic solids


  • High melting point due to strong ionic bonds
  • Require large amount of energy to overcome strong electrostatic forces between oppositely charged ions
• Electrical conductivity of ionic solids

  None when solid, conductive in aqueous solution or molten state as ions become mobile and can carry charge
• Ionic solids


  • Hard but brittle
  • Dissolve in water as ions become hydrated
• Ionic and covalent solids

  Both have a crystalline structure
• Covalent solids


  • Layers of atoms held together by strong covalent bonds
  • Weak van der Waals forces between layers allow layers to slide over each other
  • High melting point due to strong covalent bonds
  • Poor conductors as they lack delocalised electrons or ions
• Covalent solids


  • Diamond, graphite (carbon)
• Covalent solids


  • Very hard and rigid due to numerous strong covalent bonds
• Molecular solids


  • Molecules held together by weak intermolecular forces
  • Low melting and boiling points despite strong covalent bonds within molecules
• Molecular solids do not contain delocalised electrons or ions
• Metallic solids


  Lattice of positive metal ions with delocalised electrons
• Metallic solids


  • High melting point due to strong metallic bonds
  • Good conductors of electricity and heat due to delocalised electrons
  • Malleable and ductile as layers of ions can slide over each other with delocalised electrons preventing repulsion
• Giant covalent structures consist of many atoms held together by strong covalent bonds, leading to high melting points.
• Diamond is the most common giant covalent structure, consisting of carbon atoms arranged in a tetrahedral lattice.