Unit 1.5

• Coordination number


  The number of ions surrounding a central ion in an ionic solid
• Ionic solids


  • Giant lattices of positive and negative ions
  • The structure of the crystal depends on the relative number of ions and their sizes
  • Structures have the same base unit repeated over and over again
  • The ions are arranged to maximise the electrostatic attraction between oppositely charged ions and minimise the electrostatic repulsion between ions with the same charge
• Ionic solids with 1:1 ratio


  • Sodium chloride
  • Caesium chloride
• Sodium chloride


  Each Na+ ion is surrounded by 6 Cl- ions and vice versa. The coordination number of each ion is 6.
• Caesium chloride


  Each Cs+ ion is surrounded by 8 Cl- ions and vice versa. The coordination number of each ion is 8.
• The physical properties of an ionic solid are decided by the structure and bonding of the compound
• Typical characteristics of ionic solids

  • High melting temperatures
  • Often soluble in water
  • Hard but brittle
  • Poor electrical conductivity when solid, but good when molten or dissolved
• Giant covalent solids


  Networks of covalently bonded atoms that stretch throughout the whole structure, also called macromolecules
• Allotropes of carbon


  • Diamond
  • Graphite
• Diamond


  Each carbon atom is covalently bonded to four others in a tetrahedral arrangement
• Physical properties of diamond


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


  Consists of layers of hexagonal rings, with each carbon atom joined to three others by strong covalent bonds. The fourth electron from each carbon atom is delocalised within the layer.
• Physical properties of graphite


  • Very high melting temperature
  • Soft, slippery feel
  • Insoluble in water
  • Good conductor of electricity parallel to the layers, poor conductor perpendicular to the layers
  • Low density
• Delocalised


  An electron is not attached to a particular atom, but can move around between atoms
• 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
  • Soft
  • Normally insoluble in water, except those that can form hydrogen bonds
  • Poor conductors of electricity
• Simple molecular solids


  • Iodine
  • Ice
• Iodine


  Atoms are covalently bonded in pairs to form diatomic molecules, held together by weak van der Waals forces
• Ice


  Molecules of water are arranged in rings of six, held together by hydrogen bonds. The structure creates large areas of open space, making ice less dense than liquid water.
• Physical properties of metals

  • High melting temperatures
  • Hard
  • Insoluble in water
  • Good conductors of electricity and heat
  • Malleable and ductile
• Metallic bonding


  Metal atoms lose control over their outer electrons, which become delocalised and free to move throughout the metal. This leaves a lattice of positive metal cations surrounded by a 'sea' of delocalised electrons.