 So let's look at an example. So in this case, we're trying to figure out where the chromium is being oxidized or reduced in this case So we're going to look at the find the oxidation number of chromium over here and over here So we'll do the same process this time. We've got 2x because we've got two chromiums. So 2x 7 oxygens, so 7 times minus 2 That equals minus 2. That's our charge there So 2x minus 14 equals minus 2 So add 14 to both sides. So 2x equals plus 12 x equals 12 on 6 equals plus 6 So in this case chromium over here, the oxidation number of chromium is plus 6 If we have a look on this side, we're going to go back to one of our rules from a little while ago If we go back, the oxidation number of a monatomic ion equals the charge of that ion So the charge of the chromium ion here is plus 3. So the oxidation number is simply plus 3 Okay, so we've gone from plus 6 to plus 3. That means the oxidation number has decreased So if we go back here, the oxidation number has decreased. That means the chromium has been reduced in that case So in this case, it's reduced Okay To one final example, we'll look at sulfur in this case So sulfur dioxide going to sulfate ion. Is it being oxidized or reduced? So over here, we've got x plus 2 times minus 2 equals 0 because there's no charge up here So x minus 4 equals 0. So x equals plus 4. Remember to always write the charge down Over this side, we've got x plus 4 times minus 2 equals minus 2 because we've got a charge up here So x minus 8 equals minus 2. So x equals plus 6. So add 8 to both sides and we get plus 6 So if we write those up here, so we've got plus 4 here and plus 6 here Go back to our rule. If the oxidation number Increases then this the element is being oxidized. So in this case We're going from plus 4 to plus 6. So that means it's being oxidized. It's getting bigger All right So that's a look at oxidation numbers and oxidation states and how to find them