# How to differentiate the notation of a compounds charge and an anions oxidations state?

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?

• Because charge is written like that? – Mithoron Jan 11 '16 at 19:00
• So you never write oxidation state in the upper right corner? – LandonZeKepitelOfGreytBritn Jan 11 '16 at 19:03
• en.wikipedia.org/wiki/Oxidation_state – Mithoron Jan 11 '16 at 19:13
• Oxidation state: first the sign then the number: -2. Charge: first the number then the sign: 2-. – EJC Jan 11 '16 at 20:20
• @Marko Roman numerals are used in formulas and names, so there's no mistake. – Mithoron Jan 12 '16 at 0:04