Oxidation number is calculated by assuming the unknown ON as x and applying the rules of oxidation number for the rest of the atom/molecules bonded with the unknown atom.

For all the chlorine atoms we assume its oxidation state as x, and for atoms Na, K = +1, O = -2. Thus by thumb rule of oxidation, the algebraic sum of oxidation numbers of all the atoms/molecules in a compound is equal to zero or to the charge on that compound for an ion.

The Cl₂O₇ structure is interesting to learn, as there are two atoms of chlorine atom with different kinds of O ─ O, Cl ─ Cl. Its structure is given as:

Ascending order of compounds w.r.t their oxidation number is:

NaCl (-1), Cl₂(0), Cl₂O(+1), KClO₂(+3), ClO₂(+4), NaClO₃(+5), ClO₃(+6), Cl₂O₇=NaClO₄(+7).

Among the above compounds the oxidation number +2 is not present.