Oxidation number or Oxidation state
(1) Definition: Charge on an atom produced by donating or accepting electrons is called oxidation number or oxidation state. It is the number of effective charges on an atom.
(2) Valency and oxidation number: Valency and oxidation number concepts are different. In some cases (mainly in the case of electrovalent compounds), valency and oxidation number are the same but in other cases they may have different values. Points of difference between the two have been tabulated below
O.N. is the charge (real or imaginary) present on the atom of the element when it is in combination. It may have plus or minus sign.
It is the combining capacity of the element. No plus or minus sign is attached to it.
O.N. of an element may have different values. It depends on the nature of compound in which it is present.
Valency of an element is usually fixed.
O.N. of the element may be a whole number or fractional.
Valency is always a whole number.
O.N. of the element may be zero.
Valency of the element is never zero except of noble gases.