• Created by: gcolem
  • Created on: 01-12-16 14:03

The structure of a typical ionic solid - sodium ch

  • Consists of a giant lattice of ions - described as having a giant ionic structure
  • Sodium chloride is described as being 6:6-co-ordinated - centre sodium ion is being touched by 6 chloride ions


  • The more attraction between the +ve and -ve ions the more energy is released - the more energy that is released the more energetically stable the structure becomes
  • To gain maximum stability you need the maximum number of attractions
  • In this case each ions surrounds itslef with 6 ions of the opposite charge as this is the maximum number that you can fit around before the chloride ions start touching each other
  • If they start touching you introduce repulsions into the crystal which makes it less stable
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Why are the caesium chloride and sodium chloride s

  • When attractions are set up between two ions of opposite charges energy is released
  • The more energy that can be released the more stable the system becomes - The more contact there is between negative and positive ions the more stable the crystal should become
  • Sodium is smaller han caesium - if chloride ions are in contact with sodium ions the whole arrangement would shrink bringing the chloride ions into contact with each other which introduces repulsion
  • When sodium chloride is 6:6-co-oridnated there are no repulsions so this is how it organises itself
  • When a structure is a 1:1 compound e.g. NaCl or CsCl it crystallises depending on the radius ratio of the positive and negative ions - if the radius of the positive ion is bigger than 73% of that of the negative ion then 8:8-co-ordination is possible - less than that (down to 41%) then you get 6:6-co-ordination
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The physical properties of sodium chloride

  • High melting and boiling point - there are strong electrostatic attractions between the +ve and -ve ions so it takes a lot of thermal energy to overcome them
  • All ionic substances are like this, the differences between the substances will depend on:

1) The number of charges on the ions - Mg has same structure as NaCl but has a higher melting and boiling point as the 2+ and 2- ions attract each other more strongly than the 1+ and 1-

2) The sizes of the ions - The smaller they are the closer together and so the electrostatic attractions are greater  - The attractions are less between the bigger ions and so less thermal energy is needed to separae them

  • Sodium chloride crystals are brittle - if stress is applied, ions of the same charge are brought side-by-side and so the crystal repels itslef to pieces
  • Sodium chloride is soluble in water - Many are soluble in water. It depends on whether there are big enough attractions between the water molecules and the ions to overcome the attractions between the ions themselves - positie ions are attracted to the lone pairs on water molecules and dative covalent bonds may form - water molecules form hydrogen bonds with negative ions
  • Sodium chloride is insoluble in organic solvents - the attractions between the solvent and molecules and the ions aren't big enough to overcome the attractions holding the crystal together
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The electrical behaviour of sodium chloride

  • Solid NaCl doesn't conduct electricity because there are no electrons which are free to move
  • When it melts it undergoes electrolysis which involves conduction of electricity because of the movement and discharge of ions (sea of electrons)
  • In this process sodium and chlorine are produced - chemical change
  • The positive sodium moves towards the -ve charged electrode (cathode) - each sodium ion picks up an electron from the electrode to form a sodium atoms - these float to the top of the melt as molten sodium metal - if doing it in open air it will catch fire and burn with an orange flame)  (
  • The movement of electrons from the cathode onto the sodium ions leaves spaces on the cathode - the power source moves electrons along the wire in the external circuit to fill those spaces - this flow of electrons would be seen as the electric current (external circuit would be the rest of the curcuit minus the molten NaCl)
  • Chloride ions are attracted to the +ve electrode (anode) - each chloride ion loses an electron to the anode to form an atom - the pair up to make chlorine molecules - chlorine gas is produced (
  • The new electrons deposited on the anode are pumped off around the external circuit by the power source - they end up on the cathode where they will be transferred to sodium ions
  • Molten sodium conducts electricity because of the movement of the ions and the discharge of the ions at the electrodes (both of these have to happen to get electrons flowing in the external circuit)
  • In solid sodium chloride the ion movement can't happen - any possibility of any current flow in the circuit is stopped
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