In order to understand electronic concept of oxidation and reduction let us study the reaction of magnesium with oxygen. When magnesium is burnt in oxygen it gets oxidised to magnesium oxide (MgO). In the formation of magnesium oxide, two electrons from magnesium atom are transferred to oxygen atom.
The process of transference of electrons is described as redox process. Let us define oxidation and reduction in terms of electrons.
Oxidation is a process in which an atom or a group of atoms taking part in chemical reaction loses one or more electrons. The loss of electrons results in the increase of positive charge or decrease of negative charge of the species.
The species which undergo the loss of electrons during the reactions are called reducing agents or reductants. Cl-, Fe2+ and Cu are reducing agents in the above examples.
Reduction is a process in which an atom or a group of atoms taking part in chemical reaction gains ·one or more electrons. The gain of electrons results in the decrease of positive charge or increase of negative charge of the species. For example,
The species which undergo gain of electrons during the reactions are called oxidising agents or oxidants. In the above examples, Ag+, Fe3+ ions, Br2 molecule are oxidising agents.
SIMULTANEOUS OCCURRENCE OF OXIDATION AND REDUCTION
Since oxidation involves loss of electrons and reduction involves gain of electrons, it is evident that if one substance loses electrons, another substance at the same time must gain electrons because electrons cannot be the products in any chemical change. This means that in any process, oxidation can occur only if reduction is also taking place side by side and vice versa. Thus, neither oxidation, nor reduction can occur alone. Both the processes are complementary like give and take and proceed simultaneously. That is why chemical reactions involving reduction-oxidation are called redox reactions. In fact, during the redox reaction there is a transference of electrons from the reducing agent to the oxidising agent as shown below:
For example, consider a reaction between zinc and copper ions
In this reaction zinc atoms lose electrons and are oxidised to zinc ions (Zn2+) whereas cupric ions (Cu2+) gain electrons and are reduced to copper atoms. Thus, cupric ions act as oxidising agent and zinc atoms act as reducing agent. In fact the oxidising agent gets reduced while reducing agent gets oxidised during the redox reactions.