Reduction

In a reaction we know that atoms are transferred from one atom to another.Gain of electrons is called as reduction or loss of electrons is also called as reduction.in the reaction that forms ZnO from Zn and O2,The uncharged Zn atom cannot easily lose electron and be oxidised unless O2 gains the electrons and be reduced. In the reaction that converts NaCl to Na and Cl2,the chloride ions can lose electrons and be oxidised because the sodium ions are available to gain the electrons and be reduced.we can say that oxidation requires reduction and vice versa.electrons are not separated from elements or compounds, a substance cannot gain electrons and be reduced unless there is another substance that is able to transfer the electrons and be oxidised.

Reaction: 

2Zn(s) + O₂(g) → 2ZnO(g)                   

The oxidation half - reaction is

2Zn^- → 2Zn²⁺ + 2e^-            

The reduction half reaction is

O₂+4e → 2O^2-    

The zincBecause the zinc atoms lose the electrons that make it possible for the oxygen atoms to gain electrons and be reduced, the zinc is called the reducing agent. A reducing agent is a substance that loses electrons, making it possible for another substance to gain electrons and be reduced.The oxidized substance is always the reducing agent. Because the oxygen atoms gain electrons and make it possible for the zinc atoms to lose electrons and be oxidized, the oxygen is called the oxidizing agent. An oxidizing agent is a substance that gains electrons, making it possible for another substance to lose electrons and be oxidized. The reduced substance is always the oxidizing agent. . Thus,in the conversion of zinc oxide to metallic zinc, each zinc ion must gain two electrons. According to the modern definition, any chemical change in which an element gains electrons is called a reduction. (Yes, reduction means a gain of electrons.) Because this can be confusing, some people use a memory aid to remember what oxidation and reduction mean in terms of the electron transfer. One device is the phrase oil rig which stands for oxidation is loss (of electrons) and reduction is gain (of electrons). When an electric current passes through molten sodium chloride, the sodium ions, Na+, are converted to uncharged sodium atoms, and the chloride ions, Cl, are converted to uncharged chlorine molecules, Cl2. Because sodium ions gain one electron each, we say they are reduced. Chloride ions lose one electron each, so they are oxidized. 

Electric current

2NaCl(l) → 2Na(l) + cl₂(g)    

         oxidation:   2Cl^- → Cl₂ + 2e^- 

         reduction: 2Na⁺ +2e^- → 2Na

Electrons are rarely found unattached to atoms. Thus, for one element or compound.