To determine the oxidation number of an atom(s) in a molecule or an ion, start with the known oxidation numbers and the rules summarized below:



Keep in mind that the summary is zero for neutral molecules and is equal to the charge for ions.

For the dichromate ion, Cr2O72- the element with a standard oxidation state is oxygen which is -2, and we need to determine the oxidation state of the chromium. 

Rule 8 – The oxidation of oxygen is –2:

Assign x for the oxidation state of Cr and set up an equation. Remember, for ions, the sum of the oxidation numbers multiplied by their subscripts must be equal to the charge of the ion:

2x + 7(-2) = -2,  x = +6


Further Reading


More examples of the oxidation state in this multiple-choice quiz:


Oxidation State and Redox Reactions Quiz


Reactions in Aqueous Solutions Quiz

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