 Hello my name is Brad Langdill and I want to talk to you about how to assign some oxidation numbers to compounds and molecules. The thing to remember here, well you've got rules for assigning oxidation numbers and the scope of the video is not to really explain what the rules are. I'm guessing that if you're in my class you already know what those rules are or you've at least seen them. You can find them at LDindustries.ca in the notes oxidation numbers. If you're not one of my kids you probably have a bunch of rules from a textbook or from your teacher that maybe you've seen, get those out, read through them and I'm going to show you how to apply the rules today. I'm not going to tell you what the rules are, I'm going to show you how to apply them. So here's a bunch of examples, let's go through and find oxidation numbers for each atom in each compound. So for starters here's the first one, it's just sulfur, so S8 is an element on its own. It has an oxidation number of zero. Any time you have an element by itself its oxidation number is zero. That one's really easy, moving on to the next one. Sulfur dioxide. Here we have oxygen and there's two of them. So what I'd like to do is I'll put a little two, that's indicating there's two oxygens and one of the things from those rules that you learned is that oxygen has an oxidation number of negative two. You also learned from your oxidation number of rules that a compound or molecule in this case that has a net charge of zero, this one's neutral, has no charge, also has to have its oxidation numbers out of to zero. So I have negative four from the two oxygens, which means that sulfur must have a positive four oxidation number. Positive four, negative four adds up to zero. Let's try the next one here. Now we have a compound that does have a charge. So I'm going to do the same thing as before. I've got three of the oxygens, they're worth negative two each, that's negative six from the oxygens. Now I want to have an overall charge of negative two. So for sulfur, I need to have in total plus four. Positive four plus negative six gets negative two. But check it out, each of those sulfurs isn't going to be positive four. So what I'm going to do now is kind of write it like this. Go backwards from what I did over here, from what I did here. I'm going to write that as two times positive two. So the oxidation number for sulfur, for a single sulfur in this compound is positive two. There's two of those sulfurs, each of them are positive two adds up to positive four. Oxidation number for oxygen is negative two. There's three of them, they add up to negative six. Negative four, negative six is negative two. Okay, so sulfur here was positive two, oxygen was negative two. Let's take a look at the next one here. More sulfur, more oxygen. And now we see hydrogen for the first time. Hydrogen is assigned to always, almost always an oxidation number of positive one. Oxygen, again let's squeeze that in there. There's four times negative two. So I have negative eight in total and I have positive two in total. I need a neutral compound here in the sulfuric acid. So I'm going to have to make sulfur positive six. Now we have everything add up to zero. All right, so sulfur as you can see, like a lot of compounds can have a lot of different oxidation numbers. Okay, what about this one here? A little bit weird. We can start off like we did with the oxygen, no problem there. We have negative eight from the oxygen. But what about the sulfur? I can't figure that out because I don't know what magnesium is. So you might want to think of this as an ion. Magnesium as an ion, if it was by itself, would be magnesium two plus. That means that it's going to have an oxidation state of plus two. You might also remember a rule that says that if you have a group one or group two metal ion, their oxidation numbers are the same as their charges. So magnesium is group two. It gets a charge of two plus and an oxidation number of positive two. Okay, now it's not too bad to figure out what sulfur is going to be. So sulfur is going to have a positive six charge. Positive six plus two is positive eight balances out the negative eight. Let's try the sulfate ion. I have four oxygens. Each of them were worth negative two. So total of negative eight. I want to have a leftover charge of negative two. So sulfur, again, is negative six, pardon me, positive six for its oxidation number or its oxidation state. It's actually the same as it was in the last example. And I put this one in here to show you that really putting in a metal ion in front of that doesn't actually change the oxidation numbers any. And then we got one more down here. Now let's look at H2SO3. Pretty similar. Let's see what happens to sulfur. It's going to change its oxidation number. So three times negative two is negative six. We've got two times positive one is positive two. It still has to be neutral. So sulfur is going to have to pan out to positive four. That way it adds up to zero. So there's some simple oxidation numbers there. Didn't get into a whole lot of the harder examples but hopefully that gets you off in the right track. If you're looking for more examples, more help, check out the website www.ldindustries.ca.