The donation of electrons is called the oxidation and the gain of electrons is called the reduction. oxidation and reduction can further be explained by a knowledge of “Oxidation number”.
The oxidation state of an atom in its free or ground state is taken as zero. When the atom loses electrons its oxidation state increases and when the atom gains electrons its oxidation state decreases. The term oxidation-number represents the positive or negative character of the atom in a compound.
Oxidation number is defined as the charge an atom appears to have when electrons are assigned in accordance with the following arbitrary rules.
Electrons shared by two like atoms are divided equally between the two atoms. Electrons shared between two unlike atoms are assigned to the more electronegative atom of them.
Conventions Used in Assigning Oxidation Number or Oxidation State:
The oxidation number of an element in a free atomic state (Na, H, Cl, O, P etc) or in its poly-atomic state (graphite, H2, Cl2, O2 etc) is always zero.
The oxidation number of hydrogen is always +1 in its compounds. However, in metal hydrides like NaH, MgH2 etc. the oxidation number of hydrogen is -1 because metals are more electropositive than hydrogen.
O.N. of oxygen is always -2 in its compounds. However, in peroxides like H2O2, Na2O2, BaO2 etc. the oxidation number of oxygen is -1. In OF the oxidation number of oxygen is +2 because F is more electronegative than O.
O.N. of group IA element i.e. Li, Na, K etc is always +1 in their compounds.
O. N. of group IIA elements i.e. Be, Mg, Ca, Sr and Ba are always +2 in their compounds.
O. N. of F is always -1 in its compounds because it is most highly electronegative. Oxidation O. N. of other elements of group VIIA. (17) i.e. Cl, Br and I are also generally –1.
In an ion, the sum of the oxidation numbers of different atoms is equal to charge over the ion.
In a complex compound (involving co-ordination by ligands) it is more convenient to use oxidation number of group (ligand) as a whole instead of the oxidation number of individual atoms. For example, in HCN the oxidation number of CN- ion is –1. Here CN- as a whole is considered and not of individual C or N.
on the basis of the above standard oxidation numbers, which may be taken as rules, the oxidation, a number of a particular given atom in a compound can be determined.
Valency and Oxidation State:
Valency is a different term than oxidation number though sometimes the valency and the oxidation number of an element are same in a compound.
Valency of an element is given by the number of electrons it actually loses or gains or shares during the formation of a compound, Whereas oxidation number is just the apparent charge (not necessarily actual) over the atom when the electrons are counted according to the arbitrary rules given earlier.
In most of the cases, the valency of an element is constant whereas the oxidation state of an element may vary in its different compounds.
Valency and oxidation states of carbon in its different compounds give a good example of this:
In CH4, CH3Cl, CH2Cl2, CHCl3 and CCl4 the valency of carbon is always four (due to sharing of four electrons) but its oxidation number is – 4, -2, 0, +2 and +4 respectively.
Oxidation-Reduction in Terms of Oxidation Number:
On the basis of oxidation number a reaction involving the increase in oxidation number is called as oxidation while a reaction involving the decrease in oxidation number is called as reduction (Remember increase in O.N. means increase in positive O.N. or decrease in negative O.N., while decrease in O.N. means decrease in positive O.N. or increase in negative O.N.).
For example, in the reaction, 2Mg + O2 → 2MgO, The O.N. of Mg increase from 0 to +2.while the O.N. of O decreases from 0 to -2. Thus, magnesium is oxidised while oxygen is reduced.