How to Find Oxidation Numbers for an Ion

To find the correct oxidations state for each element in the ion, we use a few rules and some simple math.

First, for the ion in this video, since since it has an overall charge of -1 we could say that the oxidation numbers need to add up to charge on the ion (-1).

We write the oxidation number (O.N.) for elements that we know and use these to figure out oxidation number for Cl.

----------
GENERAL RULES
Free elements have an oxidation state of zero (e.g. Na, Fe, H2, O2, S8).
In an ion the all Oxidation numbers must add up to the charge on the ion.
In a neutral compound all Oxidation Numbers must add up to zero.
Group 1 = +1
Group 2 = +2
Hydrogen with Non-Metals = +1
Hydrogen with Metals (or Boron) = -1
Fluorine = -1
Oxygen = -2 (except in H2O2 or with Fluorine)
Group 17(7A) = -1 except with Oxygen and other halogens lower in the group
----------
We know that Oxygen usually is -2 with a few exceptions. When Oxygen is in a peroxide, like H2O2 (Hydrogen peroxide), it has a charge of -1. When it is bonded to Fluorine (F) it has an oxidation number of +2.

Here it is bonded to Cl so the oxidation number on Oxygen is -2. Using this information we can figure out the oxidation number for the element Cl in ClO3-.