1.5 Solid structures

• 6:6 Coordination 


  The sizes of the atoms are relatively similar.

  An example is Sodium chloride.

  
• 8:8 Coordination 


  When the sizes of the ions are different.

  An example is Caesium chloride.

  
• Physical properties of ionic solids

  • High melting points. The ionic lattices are held together by strong electrostatic forces
  • Often soluble in water. Due to water molecules being polar, it is attracted to the ions of the lattice.
  • Hard but brittle. When force is applied layers of ions slide over each other leading to same charged ions repelling one another.
  • Poor electrical conductivity when solid, but good when molten or dissolved. Due to ions being free to move.
• Physical properties of diamond

  • Very high melting temperature
  • Extremely hard
  • Insoluble in water
  • Poor conductor of electricity
• Graphite

  It consists of layers of hexagonal rings. In a layer, each carbon is joined to 3 others by strong covalent bonds. The fourth electron from each carbon is delocalised. The layers are held by Van der Waals forces.
• Diamond

  Each carbon is bonded to four others. In a tetrahedral shape. The bonding forces are uniform throughout the structure.
• Physical properties of graphite

  • Very high melting temperature, strong covalent bonds in the layers.
  • It has a soft, slippery feel. Due to weak VDW forces between layers.
  • Insoluble in water.
  • Good electrical conductor, the delocalised electron is free to move along the layer.
  • Low density, due to the space between layers.
• Simple molecular solids


  Simple molecular solids have covalent bonds within molecules held together by weak intermolecular forces.
• Physical properties of Simple molecular solids

  • Low melting and boiling temperatures. The intermolecular forces are very weak; easy to break.
  • Soft. The weak intermolecular forces are weak and are easily broken.
  • Normally insoluble in water, no ions to attract polar water molecules. Apart from molecules that can form hydrogen bonds.
  • Poor conductors of electricity.
• Metals 


  When metal atoms are close to each other, each atom loses control over its outer electrons. They become delocalised, so they are free to move around the metal; it will lead to a positive cation.
• Metallic bonding

  Metals will consist of a regular arrangement of metal cations surrounded by a sea of delocalised electrons. There are electrostatic forces of attraction between the nucleus of the cation and delocalised electrons.
• Physical properties of metals.

  • High melting temperatures. Large energy is required to overcome the force of attraction between cations and delocalised electrons.
  • Hard
  • Insoluble in water
  • Good conductors of electricity due to delocalised electrons.
  • Malleable and ductile. Metals can slide over one another.
• Ionic solids

  Giant lattice of positive and negative ions. The structure of the crystal depends on the size of ions. They are arranged in a way that the attractive force is greater than the repulsive force.
• Giant covalent solids 


  Networks of covalently bonded atoms that stretch throughout the whole structure. Different forms of the same element are known as allotropes.