In covalent bond formation the electrons are not transferred as in ionic bond formation, but partial transfer of electronic charge takes place, known as electron shift.
The oxidation number method always assumes that there is a complete transfer of electrons from a less electronegative atom to a more electronegative atom.
Oxidation number is defined as is the charge or the apparent charge that an atom in a compound or ion would have, if all of the electrons in its bonds belonged entirely to the more electronegative atom. To determine the oxidation number of an element in a compound or an ion, certain rules have been formulated.
i) The oxidation number of atoms of elements in their elementary states and allotropic forms is zero.
Ex: Atom in H2
, S8, Na, Mg, Al has oxidation number zero.
ii) In neutral compounds the sum of oxidation number of all atoms is zero.
Ex: In H2
O the sum of oxidation numbers of hydrogen and oxygen atoms is zero.
iii) In case of the ions that are composed only of one atom the oxidation number is same as the charge on the ion.
Ex: The oxidation number of Sodium ion is ‘+1’, Magnesium ion is ‘+2’, Ferric ion is ‘+3’, Chloride ion is ‘-1’ and Oxide ion is ‘-2’.
But in complex and polyatomic ions, the algebraic sum of the oxidation numbers of all atoms in the ion is equal to the charge on the ion.
Ex: In the Carbonate ion, the algebraic sum of the oxidation numbers of the three Oxygen atoms and one Carbon atom is ‘-2’. The oxidation number of Carbon is ‘+4’, and oxidation number of Oxygen is ‘-2’, so Charge = 4 + 3(-2) = 4 - 6 = -2
iv) In their compounds all alkali metals have oxidation number ‘+1’ and all alkaline earth metals have oxidation state ‘+2’.
v) The oxidation number of Oxygen is -2 in most of its compounds except in two cases. a) In Peroxides like hydrogen peroxide and barium peroxide, each oxygen atom is assigned an oxidation number ‘-1’.
b) In Superoxides like potassium superoxide and rubidium superoxide each Oxygen atom is assigned an oxidation number ‘-1/2’.
vi) Hydrogen has +1 oxidation number when it combines with non-metals. And in metal hydrides, its oxidation number is ‘-1’.
vii) Fluorine has an oxidation number of ‘-1’ in all its compounds.
viii) The atoms of transition or d block elements exhibit several oxidation numbers.
Highest oxidation number of an element = number of valence electrons