Reduction reactions-involve the gain of electrons by a substance in a chemical reaction (GER). The gaining of electrons will be on the left side of the equation.
Cu+ + 1e- ---> Cu The Cu+ has a charge of +1 and the Cu on the right side has a charge of zero. You need to add the electron on the left side to make the Cu+ just Cu (1-1=0)
Oxidation Reactions-the loss of electrons by a substance in a chemical reaction. The electrons will be on the right side of the equation.
Cr+3 ---> Cr+6 + 3e- You add the electrons to the right side to get the charge the same as the left side. 6+ + 3e- = 3. (LEO).