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

Oct 24, 2016

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, $N O$, is a neutral molecule, and thus the sum of the oxidation numbers must be zero. The oxidation number of $O$ is normally $- I I$, and it is here. The oxidation number of nitrogen must of course be $+ I I$, because the sum of the oxidation numbers must equal the neutral charge on the molecule. For $N {O}_{2}$, the oxidation number of $N$ must be $+ I V$. And of course, for ${N}_{2} {O}_{5}$ the oxidation number of nitrogen is?

For nitrous oxide, ${N}_{2} O$, the oxidation number of $O$ is still $- I I$. 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 $C {r}_{2} {O}_{7}^{2 -}$?