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Which species can act as an oxidizing agent but not as a reducing agent?

(A) $\ce{Cl2}$
(B) $\ce{Cl-}$
(C) $\ce{ClO2-}$
(D) $\ce{ClO4-}$

I thought the answer was A because if $\ce{Cl}$ is reduced, it becomes very stable with a full octet and therefore cannot be the reducing agent; however, according to the 2007 Chemistry Olympiad answer key, the answer is D. Why is that?

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    $\begingroup$ There is a very simple and well-known reaction in which Cl2 acts as a reducing agent. Then again, I believe perchlorate also would act that way if confronted with elemental fluorine or something. The question is badly posed. I guess the implication was that chlorine is already in its topmost oxidation state, hence... $\endgroup$
    – Ivan Neretin
    Dec 23, 2016 at 5:44

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For a species to act as a reducing agent, it should have the ability to get oxidised. Any element in its highest oxidation state, cannot oxidise anymore, and can hence not act as a reducing agent. $\ce{ClO4-}$ has $\ce{Cl}$ in $+7$ oxidation state, and cannot have a higher oxidation state, and hence cannot act as a reducing agent.

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