Oxidation Number

Oxidation number is also called as oxidation states, it refers to the electrons lost or gain during chemical reaction.  To determine whether an atom undergoes oxidation or reduction in a chemical reaction, oxidation number is determined before and after reaction. Let us see for example the reaction of H2 and Cl2 in the formation of HCl:

In the above equation, in the reactant side H2 has an oxidation number of 0, Cl2 has also oxidation number 0 while in the product side H has +1 oxidation number and Cl has -1 oxidation number.
H showed an increase of oxidation number therefore it is the one that undergoes oxidation or loses an electron while Cl showed a decrease of oxidation number therefore it undergoes reduction or gains an electron.

Now in order to identify if an atom undergoes oxidation or reduction, we need to assign oxidation numbers to the different atoms in the reactant side and in the product side. There are rules to follow in assigning oxidation numbers.  Below are the rules:

1.  In free elements, each atom has an oxidation number equal to zero.  Example Na, K, Mg, Cu,  etc. Diatomic molecules also have zero oxidation number like O2, Cl2, F2, N2, Cl2, Br2, H2,  including other molecules like F4, S8.

2.  For monoatomic ions there oxidation number is equal to the charge on their atoms.  You can use periodic table in determining the oxidation  number or the different list of monoatomic ions in my previous blog.  Example elements in Group 1A ions  have +1 oxidation number, group 2A ions have +2 oxidation number.

3.  Oxidation number of oxygen is always -2 but in peroxide compounds oxygen has a charge of -1. Example in H2O (water), H has a charge of +1 and O has a charge of -2 and H2O2 (hydrogen peroxide), H has +1 charge and O as a charge of -1.

There are two atoms of oxygen having -1 charge, that is why peroxide has -2 oxidation number.

4,  Oxidation number of hydrogen is always +1 when paired with other nonmetals or polyatomic anions except for hydride. Example in HCl, hydrogen has an oxidation number of +1.  Hydride has -1 oxidation number, example CaH2, hydrogen here has an oxidation number of -1.

5. Flourine has -1 oxidation number in all compounds.  Other atoms in the halogen group ( Cl, Br, I) also has -1 oxidation number except when they are bonded with oxygen forming polyatomic ions, their positive oxidation numbers are used.  Example :

6.  In a neutral molecule, the total charge of the atom must be equal to zero.  In polyatomic ions the total charge is equal to the charge of the ion.

 Example of neutral molecule is MgCl2,  

      Mg has a charge of +2 and Cl has a charge of -1 x 2 = -2.  Therefore +2 - 2  = 0 or neutral.   

Example of polyatomic ion,

What is the charge of Sulfur?

     S  (-2x4)  =  -2

     S  (-8)  =  -2

     S  =  -2  +  8

     S  =  +6

Therefore the oxidation number of S in  is + 6.

TRY THIS:

Identify the oxidation number of each element of the following compounds and ions:

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