C2.1.3 - Ionic Compounds

• The Ionic Lattice

  An ionic compound is a giant structure of ions. The ions have a regular, repeating arrangement called an ionic lattice. The lattice is formed because the ions attract each other and form a regular pattern with oppositely charged ions next to each other.

  
• Ionic Bonding

  An ionic lattice is held together by strong electrostatic forces of attraction between the oppositely charged ions. The forces act in all directions in the lattice. This is called ionic bonding.

  
• Representing Ionic Compounds

  • the two-dimensional space-filling model clearly shows the arrangement of ions in one layer, but it does not show how the next layer of ions is arranged
  • the three-dimensional ball and stick model shows the arrangement of ions in a larger section of the crystal, but using sticks for bonds is misleading because the forces of attraction between ions actually act in all directions
  • the three-dimensional model is also misleading because it shows lots of free space between the ions, which there isn't
• Conducting Electricity in Ionic Compounds


  A substance can conduct electricity if:

  • it contains charged particles, such as ions, and
  • these particles are free to move from place to place

  An ionic compound can conduct electricity when:

  • it has melted to form a liquid, or
  • it has dissolved in water to form an aqueous solution

  Both these processes allow ions to move from place to place. Ionic compounds cannot conduct electricity in the solid state because their ions are held in fixed positions and cannot move.
• Why are there high boiling and Melting points in Ionic Compounds?

  Energy must be transferred to a substance to make it melt or boil. This energy overcomes the strong electrostatic forces of attraction which act in all directions between the oppositely charged ions:

  • some forces are overcome during melting
  • all remaining forces are overcome during boiling

  The more energy needed, the higher the melting point or boiling point. Since the electrostatic forces of attraction between oppositely charged ions are strong, their melting and boiling points are high.