# What is the oxidation number of Cl in Ba(ClO2)2?

Oxidation numnber of $C l$ in chlorite? Well the parent ion is $C l {O}_{2}^{-}$. (How did I know this?) So oxidation number of $C l$ is $+ I I I$.
We have $C l {O}_{2}^{-}$; the individual oxidation numbers of the elements $\left(2 \times O + C l\right)$ must sum (by definition) to the charge on the chlorite ion, which is $- 1$. Oxygen gets the charge when we break the $C l - O$ bonds, so it is $- I I$ (oxygen is more electronegative than $C l$). So $- 2 - 2 + C {l}_{O N} = - 1$. This gives $C l$ in chlorite a formal oxidation number of $+ I I I$. What is the oxidation number of $C l$ in perchlorate anion, $C l {O}_{4}^{-}$? Please post your answer here.
Note that the oxidation number of $O$ in is compounds is normally $- I I$, as here. Elemental oxygen is $0$, BUT peroxide, ${O}_{2}^{2 -}$, is $- I$. See here.