Question

what is the rules of oxidation no. of an atom?

Answer 1

The oxidation number of an atom is zero in a neutral substance that contains atoms of only one element.

Answer 2

Rule 1: The oxidation number of a monatomic (one-atom) ion is the same as the charge on the ion, for example:
Rule 2: The sum of all oxidation numbers in a neutral compound is zero. The sum of all oxidation numbersin a polyatomic (many-atom) ion is equal to the charge on the ion.