In many covalent reactions such as reaction between H2 and Cl2:
H2(g) + Cl2(g) à 2HCl(g)
the loss and gain of electrons could not be easily explained. In order to explain transference of electrons in either of the species in a more convenient way, the concept of oxidation number has been introduced.
Oxidation number (O.N.) of the element is defined as the residual charge which its atom has or appears to June when all other atoms from the molecule are assumed to be removed as ions by counting the shared electrons with more electronegative atom.
For example, in hydrogen chloride molecule, chlorine is more electronegative than hydrogen. Therefore, the shared pair is counted towards chlorine atom as shown below:
As a result of this, chlorine gets one extra electron and acquires a unit negative charge. Hence, oxidation number of chlorine is -1. On the other hand, hydrogen atom without electron has a unit positive charge. Hence, oxidation number of hydrogen in hydrogen chloride is + 1.
It may be noted that electrons shared between two similar atoms are divided equally between the sharing atoms. Hence in molecules like H2, Cl2, Br2 the oxidation number of element is zero.
RULES FOR ASSIGNING OXIDATION NUMBER TO AN ATOM
These rules have been formulated on the basis of the assumption that electrons in a covalent bond belong entirely to the more electronegative atom.
1. The oxidation number of the element in the free or elementary state is always zero irrespective of its allotropic form.
Oxidation number of helium in He = 0
Oxidation number of chlorine in Cl2 = 0
Oxidation number of sulphur in S8 = 0
Oxidation number of phosphorus in P4 = 0
2. The oxidation number of the element in monoatomic ion is equal to the charge on the ion. For example, in K+CI-, the oxidation number of K is + 1 while that of Cl is -1. In the similar way, oxidation number of all the alkali metals is + 1 while those of alkaline earth metals is +2 in their compounds.
3. The oxidation number of fluorine is always -1 in all its compounds. Other halogens (Cl, Brand I) also have an oxidation number of -1, when they occur as halide ions in their compounds. However, in oxoacids and oxoanions they have positive oxidation numbers.
4. Hydrogen is assigned oxidation number + 1 in all its compounds except in metal hydrides. In metal hydrides like NaH, MgH2 , CaH2, LiH, etc., the oxidation number of hydrogen is -1.
5. Oxygen is assigned oxidation number -2 in most of its compounds, however, in peroxides (which contain 0-0 linkage) like H2O 2, BaO2, Na2O2, etc., its oxidation number is -1. Similarly, the exception also occurs in compounds of fluorine and oxygen like OF2 (F-0-F) and 0 2F2 (F-0- 0 – F) in which the oxidation number of oxygen is +2 and+ 1 respectively.
6. In accordance with principle of conservation of charge, the algebraic sum of the oxidation numbers of all the atoms in molecule is zero. But in case of polyatomic ion the sum of oxidation numbers of all its atoms is equal to the charge on the ion.
7. In binary compounds of metal and non-metal, the metal atom has positive oxidation number while the non-metal atom has negative oxidation number. For example, O.N. of Kin KI is +1 but O.N. of l is – l.
8. In binary compounds of non-metals, the more electronegative atom has negative oxidation number, but less electronegative atom has positive oxidation number. For example, O.N. of Cl in CIF3 is positive (+3) while that in ICl is negative (-1).
By the application of above .rules, we can find the oxidation number of the desired element in a molecule or in an ion. It may be noted that if a molecule contains two or more atoms of same element, then oxidation number calculated by these rules is average oxidation number.
Let us apply the above rules to calculate the oxidation number of some elements.