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The sum of the oxidation numbers in atoms of a neutral compound is always 0, and the sum of the oxidation numbers in a polyatomic ion is equal to the ion's charge. So you would look at the charges of the individual atoms and that would be their oxidation numbers. Free elements, such as C by itself, always have an oxidation number of zero. HCl would be H (+1 oxidation number) and Cl (-1 oxidation number).
basically, if you have a transition metal as your central atom and you need to find its oxidation state, then you add up the oxidation numbers of all the atoms attached to it and subtract it from the total charge of the molecule.
I'll do an example so hopefully, it makes more sense. Take for instance, [Fe(CN)6]^(4-) --hexacyanoferrate (II) ion (note 6 is a subscript). Note that transition metals charge vary. I usually start by putting an X to represent "unknown" for my metal which is Fe. So, I have: X+(-1)6= -4. The -1 is the charge of CN and the -4 is the charge outside the . Now we solve for x which is -2 and that would be the oxidation number:)
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