We have in our earlier course defined oxidation as the addition of oxygen to an element or compound. The formation of sodium oxide, Na₂O from sodium and oxygen may be represented by the equation:

                                                              4Na + O₂   →   4 ( Na⁺) 2O^2-

This equation can also be split into two halves to show that this reaction is also an electron transfer reaction. Each sodium atom loses an electron to form sodium ion:

                                                               [ Na   →   Na⁺ + e^- ] x 4

And each oxygen molecule gains electrons to form oxide ions:

                                                               O₂ + 4e^-   →       2O^2-

on combining the two equation, the overall reaction is,

                                                4Na + O₂   →   4 ( Na⁺ ) 2O^2-  or   2Na₂O

In this reaction sodium undergoes loss of an electron. It is said to have been oxidized to sodium ion (Na+). An atom is said to have been oxidized if it loses electrons. Oxidation is a process which involves the loss of electrons. According to this definition of oxidation, many substances like oxygen, halogens and other non-metals can act as oxidizing agents, i.e., oxidize other substances. They are said to be oxidizing agents. Any substance that accepts electrons, for example, non-metals such as oxygen, sculpture and the halogen, can behave as oxidizing agents. if an aqueous solution of chlorines added to a cloudless aqueous solution of an iodide or if chlorine gas is passed through an iodide solution, the solution becomes brown due to the formation of iodine.

                                                        CI₂ + 2I^-  →   2CI^- + I₂

This means, chlorine oxidizes iodide ion to iodine.

The loss of electron during oxidation may result in :

(i)    increase of positive charge on atom or ion undergoing oxidation.
For example,

                                                         Na - e      →      Na⁺
                                                         AI - 3e       →     AI³⁺
                                                         Fe²⁺ - e     →    Fe³⁺
                                                         Sn²⁺ - 2e   →     Sn⁴⁺

(ii)    decrease of negative charge on atom or  ion undergoing oxidation.
For example,

                                                          2I^- - 2e   →    I₂
                                                          S^2- - 2e     →   S
                                                     MnO₄^2- - e   →   MnO₄^-
                                             [ Fe (CN )₆ ] ^4- - e   →   [ Fe (CN )₆ ] ^3-

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