Covalency, Oxidation State and Oxidation Number

Covalency of an element is defined as a number indicating its combining capacity. For example, it is represents the number of hydrogen atoms which can combine with a given atom. It also represents the number of single bonds which an atom can form. It is also defined as the number of electrons its atom is able to share. In any case covalency is a pure number and has no plus or minus sign associated with it.
Oxidation number is defined as the charge which an atom appears to have when electrons are counted. It is positive or negative. For example, in ammonia the covalency of nitrogen is three but its oxidation number is -3.
 In ionic compounds the oxidation state of an element is the same as the charge on the ion formed from an atom of the element. For example, in potassium bromide potassium is said to be in the +1 oxidation state and bromine in -1 oxidation state. Is ionizes as
                                    KBr = K+ + Br
In other words, oxidation numbers of potassium and bromide are +1 and -1 respectively.
Oxidation state of Aluminium in Al2O3 is +3 and the total oxidation number of two aluminium atoms is +6. Thus oxidation number of two aluminium atoms is +6. Thus oxidation state of an element is its oxidation number pre-atom
 There may actually be a difference between the magnitude of covalency and the oxidation number.

Example :

    CH4 (Methane)                            CH3Cl (Methylchloride)        
   CH2Cl2 (Methylene chloride)      CHCl3 (Chloroform)         
    CCl4 (Carbon tetrachloride)
In each case one atom of carbon shares a total of 4 pairs of electrons with other atoms. Carbon atom is, therefore, tetracovalent in each case.
Oxidation number for carbon in CH4, CH3Cl, CH2Cl2, CHCl3 and CCl is -4, -2, 0, +2 and +4 respectively.
Thus, while covalency of carbon remains constant (=4) in each case, its oxidation number varies from -4 and +4.

Oxidation and Reduction in term of Oxidation Numbers

The term oxidation refers to any chemical change involving increase in oxidation number where as the term reduction applies to any chemical change involving decrease in oxidation number.
 Consider the following chemical changes:
                        2H2 + O2 = 2H2O
Here, oxidation number of hydrogen changes from 0(H2) to +1 (+H2O). It is therefore, a case of oxidation of hydrogen.
The oxidation number of oxygen decreases from 0 (O2) to -2 (H2O). It is therefore, a case of reduction of oxygen.
In the same reaction, oxidation number of hydrogen increases and that of oxygen decreases, i.e. hydrogen undergoes oxidation while oxygen undergoes reduction. Thus oxidation and reduction occur together.
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