| 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). |
