Oxidation numbers describe the distribution of electrons between bonded atoms as if they were transferred, even though they are often shared in covalent bonds. Oxidation numbers are useful for understanding redox reactions where the oxidation states of atoms change, such as hydrogen being oxidized and fluorine being reduced when they react. The oxidation number of an atom is zero when it is alone in a neutral substance, and is calculated using rules such as hydrogen being +1 when bonded to nonmetals, and the sum of the oxidation numbers being zero in neutral compounds.

2. OXIDATION NUMBER

3.  Oxidation number are used to describe the
distribution of electrons among bonded atoms
 Covalent bond involves sharing of electrons, but
oxidation numbers show what the distribution
would be if the electrons were completely
transferred.

4.  Redox is a chemical reaction in which oxidation
states of atoms are changed.
 A good example is the reaction between hydrogen
and fluorine in which hydrogen is being oxidized
and fluorine is being reduced.
 This reaction involves both reduction as well as
oxidation process.

5.  The oxidation number of an atom is zero in a neutral
substance that contains atoms of only one element. Thus, the
atoms in O2, O3, P4, S8, and aluminum metal all have an
oxidation number of 0.
 The oxidation number of hydrogen is +1 when it is combined
with a nonmetal as in CH4, NH3, H2O, and HCl.
 The sum of the oxidation numbers in a neutral compound is
zero.
 H2O: 2(+1) + (-2) = 0

6.  The oxidation state of oxygen is -2. Find the
oxidation state of the other atoms in the
following ..
 H2O
 H2CO3
 HNO3
 H3PO4