Question

How do I find the oxidation numbers of individual elements in an ion or molecule?

Answer 1

The sum of the oxidation numbers ALWAYS equal the charge on the ion, OR on the molecule....

Explanation:

And of course, if it is a neutral molecule, the sum of the constituent elemental oxidation numbers is ZERO.

Nitrogen oxide, `NO`, is a neutral molecule, and thus the sum of the oxidation numbers must be zero. The oxidation number of `O` is normally `-II`, and it is here. The oxidation number of nitrogen must of course be `+II`, because the sum of the oxidation numbers must equal the neutral charge on the molecule. For `NO_2`, the oxidation number of `N` must be `+IV`. And of course, for `N_2O_5` the oxidation number of nitrogen is?

For nitrous oxide, `N_2O`, the oxidation number of `O` is still `-II`. And for this neutral molecule, the oxidation number of `N` is `+I`, i.e. `1+1-2=0` as required.

Given all this, can you tell me the oxidation number of the metal in `Cr_2O_7^(2-)`?