# How to determine peroxy oxygen?

Which of the following compound doesn't contain peroxy oxygen?

1. $\ce{CrO5}$
2. $\ce{Cr2O3}$
3. $\ce{Na2O2}$
4. $\ce{BaO2}$

I think peroxy oxygen is peroxide, am I right?. I have read in this answer that there is no direct way to recognise peroxide oxygen, and I think that's the reason no one has taught me that in the school.

I am familiar with many peroxides like $\ce{Na2O2}$, $\ce{BaO2}$, etc. But I not able to recognise between 1 and 2.

Is there any way for this type of question to recognise peroxide oxygen? And if there is no way to determine then really I need to remember all, because we can't use X-ray crystallography in exam or while studying high school chemistry!

Answer given is 2. $\ce{Cr2O3}$

• try to draw strutural formula using highest valency for the element. It is pretty straighforward way to get the answer by elimination. Commented Oct 12, 2014 at 6:59
• If you first assume that the oxygen has an oxidation state of $-2$ but then the metal ends up with something suspiciously high, you might want to rethink that oxygen. Which is more plausible for CrO5 — Cr(X) or peroxide linkages? Commented Nov 13, 2023 at 21:47

For $\ce{CrO5}$, if all the oxygen have a charge of -2, then Cr would have to have a charge of +10, which is impossible, therefore you know that some of the oxygen must have a charge of -1(peroxy). On the other hand, for $\ce{Cr2O3}$, Cr has a charge of +3, which is possible (Cr(III) salts are plenty)
• "If this is greater than -2, then there has to be a peroxyl oxygen involved." I'm probably missing something, but couldn't the oxygen be a superoxide instead? In a superoxide $\ce{O_2-}$, each $\ce{O}$ atom has an average oxidation state of $-0.5$, whereas in peroxide each oxygen has an oxidation state of $-1$ only... Commented Mar 1, 2018 at 14:42