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I have had the case a few times. Being given something of this form:

$$\ce{Cr2O7^2-}$$

How do I know whether the $2-$ is referring to the oxidation of the oxygen anion or the charge of the whole compound?

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    $\begingroup$ Because charge is written like that? $\endgroup$ – Mithoron Jan 11 '16 at 19:00
  • $\begingroup$ So you never write oxidation state in the upper right corner? $\endgroup$ – LandonZeKepitelOfGreytBritn Jan 11 '16 at 19:03
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    $\begingroup$ en.wikipedia.org/wiki/Oxidation_state $\endgroup$ – Mithoron Jan 11 '16 at 19:13
  • $\begingroup$ Oxidation state: first the sign then the number: -2. Charge: first the number then the sign: 2-. $\endgroup$ – RBW Jan 11 '16 at 20:20
  • $\begingroup$ @Marko Roman numerals are used in formulas and names, so there's no mistake. $\endgroup$ – Mithoron Jan 12 '16 at 0:04
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make sure to read up about that topic. The concept of charge and oxidation state are closely related but aren't the same.

Usually oxidation state is written with roman numbers (that's how I learned it) and are not explicitely written like in the example you showed.

Charges on the other hand are always written (if mentionned) on the upper right side of the anion.

I am far from being an expert, but thinks this might be an answer to your question.

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  • $\begingroup$ Superscripted Roman numerals are (I think?) a somewhat newer notation, but are in my experience becoming quite common. $\endgroup$ – hBy2Py Jan 11 '16 at 21:23

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