To calculate the oxidation state of an element in a molecule you need not always know the structure of the molecule. There are certain set of rules used to assign oxidation states in polyatomic molecules:

Rules For Assigning Oxidation Number :

(i) Oxidation number of free elements or atoms is zero.

(ii) Oxidation number of allotropes is zero.

(iii) Oxidation number of atoms in homo-nuclear molecules is zero.

(iv) Oxidation number of mono-atomic ions is equal to the algebric charge on them.

(v) Oxidation number of `F` in compounds is `- 1`.

(vi) Oxidation number of `H` in its compounds is `+ 1` except in metal hydrides where it is `- 1`.

(vii) Oxidation number of `O` is `- 2` in its compounds, but in `F_2O` it is `+ 2` and in peroxides it is `- 1` and `- 0.5` in `KO_2`.

(viii) Oxidation number of alkali metals in their compounds is `+ 1`.

(ix) Oxidation number of alkaline earth metals in their compounds is `+ 2`.

(x) Oxidation number of an ion is equal to its charge.

(xi) Oxidation number of a molecule as a whole is zero.

(xii) The sum of oxidation number of all the atoms in a molecule should be zero and in an ion equal to its charge.