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 P2O5 the element with a standard oxidation state is oxygen which is -2, and we need to determine the oxidation state of the phosphorus. 

Rule 8 – The oxidation of oxygen is -2:

Assign x for the oxidation state of P and set up an equation:

5(-2) + 2x = 0, x = +5


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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