What is the oxidation number of each oxygen atom in ozone?

Answer 1

Oxidation numbers are an invention of chemists; they are simply an accounting scheme that helps chemists to keep track of things in a redox reaction.

There are several different ways to assign oxidation numbers in ozone.

The only fixed point that all would agree on is that the sum of oxidation numbers on the three atoms must come to zero.

Argument 1: All 3 atoms are oxygen, so they all have the same electronegativity, so the oxidation numbers must be 0,0, and 0

Argument 2: When writing the formula of ozone and trying to obey the rules of valence, you have to use formal charge. The formula could be written

O=O(+)-O(-)

In this case the oxidation numbers must be assigned as 0,+1, and -1

Argument 3: The formulation of argument 2 is correct BUT both "end" oxygen atoms are in symmetrically identical environments, and the formula written above is just one of two resonance structures. The symmetry of the molecule must be recognised, and so the oxidation numbers are -1/2 , +1, and -1/2

You can find professional chemists who will argue quite strongly for any one of these three assignments. None of them is really "right" or "wrong". It is all a matter of how you want to do your accounting. (But just sit back and nod wisely if you find a chemist who is arguing passionately for a particular assignment).