The redox couple
A redox reaction is always built up of two half reactions:
The equation of couple 1: 
red _{1} 

ox _{1} + electrons 
The equation of couple 2: 
ox _{2} + electrons 

red _{2} 
Total equation: 
red _{1} + ox _{2} 

ox _{1} + red _{2} 
The number of electrons involved must satisfy the rule:
ALWAYS THE SAME NUMBER OF ELECTRONS IS DONATED AS IS CAPTURED
Electrons do not get lost, are not just created from nothing, and so:
You must always confirm and certify the two half reactions in that way: the number of donated electrons must equal the number of captured electrons.
Example:
The equation of couple 1: 
Al 

Al^{3+} + 3e^{} 
 x 2 
The equation of couple 2: 
I_{2} + 2e^{} 

2I^{} 
 x 3 
Total equaition: 
2Al + 3I_{2} 

2Al^{3+} + 6I^{} 
The total equation is only reliable if the number of electrons in half reaction 1 equals the number of electrons in half reaction 2.
In case of insolubility of the salt Aluminium Iodide, there will be a secondary reaction: the precipitation of an insoluble salt.
Att.: This is not the case in the example; check that in table XI:
solubility of salts in water.