Ionic Equations  
In writing a chemical equation to describe an ionic reaction, we have several options. Consider this reaction.  When 100 mL of a 1.0 M cadmiun nitrate, Cd(NO3)2, is added to 100 mL of a 1.0 M solution of sodium sulphide, Na2S, a bright orange-yellow solid precipitates as the two solutions mix.  The solid can be shown to be cadmium sulphide, CdS and if we filter the mixture and evaporate the filtrate, crystals of sodium nitrate are left behind.  From this equation we can write the chemical reaction:
 
          Cd(NO3)2(aq)  +  Na2S(aq)  ------>   CdS(s)   +    2 NaNO3(aq)
 
This is a molecular reaction because we have written it as complete molecules.
 
A more accurate description of a reaction as it actually takes place is provided by an ionic equation.  In an ionic equation the formula of any strong electrolytes are written in dissociated form to show that the solute exists in the solution in the form of separated ions.   On the other hand, the formulas of solids and weak electrolytes are written in molecular form. For a solid, this shows that the ions are not free to move away from each other, but instead are forced to stay together within the crystals of the solid.   For a weak electrolyte, writing the molecular formula shows that this is the predominant form of the solute in solution.  The ionic equation for the reaction that we've been discussing is
 
       Cd2+(aq)  + NO3-(aq)  +  2 Na+(aq)  +  S2-(aq)  ---->   CdS(s)   +   2 Na+(aq)  +  2 NO3-(aq)
 
To obtain the ionic equation we have divided each of the soluble, ionic compounds into the ions that are released when the salts dissolve, but we have left intact the formula of the solid.
 
An ionic equation, like any other equation, should be balanced, and this one is.  It satisfies, two criteria for a balanced ionic equation.
 
1.  Material balance:  All atoms on one side of the arrow must appear somewhere on the other side.
 
2.  Electrical balance: The net charge on the left equals the net charge on the right.
 

Net Ionic Equations
If you pay careful attention to the above reaction you'll see that two of the ions apear in exactly the same way on both sides of the equation.  They are in this case the nitrate, and sodium ions. This tells us that during the reaction, these ions are left unaffected. They are present before the reaction starts and they are still there after the reaction is over.  They do not actually participate in the formation of the CdS.  They are called spectator ions.   They are there only to carry the cadmium and sulphide ions.  A net ionic reaction is a reaction written without the spectator ions being shown. A typical net reaction is shown below.
 
                 Cd2+(aq)   +  S2-(aq)  ---->   CdS(s)

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