An oxidation reaction results in the loss of electrons and an increase in oxidation number. A reduction reaction results in the gain of electrons and a decrease in oxidation number. They always occur together, and the net reaction is called a redox reaction. You can remember which is which with “oil rig”—Oxidation Is Loss; Reduction Is Gain. In a redox reaction, the reactant atom that gets oxidized is the reducing agent, and the one that gets reduced is the oxidizing agent.
The reaction between solid zinc and aqueous copper(II) nitrate is a single displacement reaction:
Zn(s) + Cu(NO3)2(aq) → Cu(s) + Zn(NO3)2(aq).
There are two half-reactions within this, and when we add them together, we get the redox reaction:
Notice that the electron terms cancel out, and the final redox reaction clearly shows two electrons transferring from zinc to copper.