Valency Vs Oxidation Number

Valency	Oxidation Number
It is the combining capacity of an atom. It is expressed as the number of hydrogen atoms or double the number of oxygen atoms with which an atom of the element combines.	It is the residual charge which an atom has or appears to have when all the atoms are removed as ions.
Since it refers to combining capacity, it is a whole number only. As such it does not carry any plus or minus sign. For example, in NH3, the valency of nitrogen is 3 and that of hydrogen is 1.	Since it refers to charge, it can be positive, negative or zero. For example, in NH3, nitrogen has oxidation number of -3 and that of hydrogen is +1.
Valency of an element cannot be zero (except for noble gases).	Oxidation number of an element can be zero. For example, oxidation number of C in CH2Cl2 is zero.
Since atoms always combine in whole numbers, valency of an element is always a whole number.	Oxidation number may have fractional values. For example, oxidation number of S in Na2S4O6 is +2.5.
In general, elements like C, N and S exhibit constant valency. Thus, valency of N in all its compounds is three.	Even the elements C, N and S exhibit variable valency. For example, the oxidation number of N (given in brackets) in its compounds vary from +5 to -3 as: N2O5(+5), NO2(+4), N2O3(+3), NO(+2), N2O(+1), N2(0), N2H4 (-2) and NH3(-3).