- Electrons shared between two unlike atoms are counted with more electronegative atom. For example, the electron pair shared between H and Cl in H+1:Cl-1 is counted with more electronegative Cl. As a result of it hydrogen having lost share in the electron pair appears to have +1 charge and chlorine appears to have -1 charge. Hence oxidation numbers of H and Cl are +1 and -1 respectively.
- Electrons shared between two like atoms are divided equally between the two sharing atoms. For example, in hydrogen molecule, H:H, the electron pair is equally shared between the two atoms. Thus both the atoms appears to have no charge i.e. oxidation number of hydrogen is zero in hydrogen molecule.
In the molecule of water given in the margin, the two electron pairs shared between oxygen and the two hydrogen atoms are counted with the more electronegative atom. Hence in water, oxidation number of each H is +1 and that of the O atom is -2.
With the help of above rules, we can calculate the oxidation number of an atom present in a molecule or complex ion.