When an oxidation number is assigned to the element, it does not imply that the element in the compound acquires this as a charge, but rather that it is a convenient number to use for balancing chemical reactions. The guidelines for assigning oxidation states (numbers) are given below:

1. The oxidation state of any element such as Fe, H₂, O₂, P₄, S₈ is zero (0).

2. The oxidation state of oxygen in its compounds is -2, except for peroxides like H₂O₂, and Na₂O₂, in which the oxidation state for O is -1.

3. The oxidation state of hydrogen is +1 in its compounds, except for metal hydrides, such as NaH, LiH, etc., in which the oxidation state for H is -1.

4. The oxidation states of other elements are then assigned to make the algebraic sum of the oxidation states equal to the net charge on the molecule or ion.

5. The following elements usually have the same oxidation states in their compounds:
   • +1 for alkali metals - Li, Na, K, Rb, Cs;
   • +2 for alkaline earth metals - Be, Mg, Ca, Sr, Ba;
   • -1 for halogens except when they form compounds with oxygen or one another
   
   
   
   
   reference: http://www.science.uwaterloo.ca/~cchieh/cact/c123/oxidstat.html
   
   
   Miss Liang: I don't understand. What is the use of oxidation state and does it have anything to do with ions?