What is the oxidation state of chlorine in #ClO_2#?
The oxidation number of Cl in ClO₂ is +4.
You can learn how to calculate oxidation states at
For your problem, the important rules are
Oxygen usually has an oxidation number of -2. Exceptions include molecules and polyatomic ions that contain O-O bonds, such as O2, O3, H2O2, and the O22- ion.
The sum of the oxidation numbers in a neutral compound is zero.
Thus, in ClO₂, the oxidation number of O is -2 (Rule 1)
For two O atoms, the total oxidation number is -4.
So the oxidation number of Cl must be +4. This makes the sum of all the oxidation numbers equal to zero (Rule 2).