Rules for Assigning Oxidation number

1.) All elements in their free state (uncombined with other elements) have an oxidation number of zero.        

C,  Na,  Ca,  Mg,  H_2,  O_2,  CL ₂,  N ₂

2.)    H is +1, except in metal hydrides, where it is -1

HNO₃ -> N= +1 ,    NaH -> H = -1,     CaH -> N = -1

3.)    O is -2 except in peroxides, where it is -1, and in OF₂, where it is +2.

4.)    The metallic element in an ionic compound has a positive oxidation no.

a.)    elements in Group 1-A are always +1

b.)    Elements in Group 2-A are always +2

c.)    Uranium is always +3

5.)    Fluorine is always negative one (-1)

6.)    In covalent compounds, the negative oxidation no. is assigned to the most electronegative atom.

7.)    The algebraic sum of the oxidation numbers of an element is assigned to the most electronegative atom.

8.)     The algebraic sum of the oxidation numbers of an element in a poluatomic ion is equal to the charge of the ion.