 Okay, so let's do this problem on oxidation numbers. So we have, well this is the one we solved in class, let's just quickly go through it. There are some elements that we automatically know what their oxidation numbers are, okay? Hydrogen, if it's in a compound is always going to be plus one. Let's just write those down over here. Hydrogen is always going to be plus one. Oxygen, if it's in a compound is always going to be minus two, okay? Those are the two that I specifically told you to remember individually. The other thing you want to remember is that if you look at your group one, group two, group three elements, all the ones that are going to make positively charged cations, right, normally, those are always going to stay at those same oxidation numbers. So sodium is always going to be plus one. Magnesium is always going to be plus two, okay? So group ones, let's just write that down now. Group ones, plus one. Group two, plus two. And group three a, or thirteen, plus three, okay? So with that information, you should be able to figure everything out, okay? So let's try this one first off. Remember, all these compounds, we want them to overall be a zero oxidation state, okay? So what do we know? H is plus one, right? So H plus one, and we only have one of them. So one times plus one for the whole compound. Unless the whole compound has a charge, but we haven't gotten to there yet, okay? Those are ions anyways, they're called complex ions, so it's actually a different thing. It's not really a compound. But anyways, notice none of these have an overall charge, okay? So then we're going to say plus, well we know oxygen, right, is negative two, and we have three of them. Three times negative two, like that. And then we have one bromine, right? So we're going to say one times X equals zero, like that, okay? So now we'll just add this thing up, right? One plus three times two, negative two is negative six, so one minus six, right? One times X is X, so plus X equals zero, right? So this is negative five plus X equals zero, right? And remember algebra, we can take this five, move it over to the other side. So X must equal plus five, right? So that's the oxidation number of bromine. Does that make sense? Well it's not charged, right? This is a compound, okay? So it's got kind of, if you wanted to think of it as being broken up into like individual atoms, yeah, bromine would, if you want to think of it that way, has a positive five charge. But it's not like that. This compound doesn't look like that. It looks like this. B-R-O-H. That's what it looks like. Yeah, that's what it looks like, well, with R. But it's very difficult for you guys to draw this compound yet, you know? You don't know how to build these types of things. In fact, this compound here is very difficult for you to build because you only know bromine to be able to make one bond, you know? But in some compounds, you know, these atoms will make weird bonds, okay? Bonding patterns. Yeah, so you get that five from taking the bonds here and taking those electrons away from bromine, actually, you know? So the thing is, but you can figure out what the oxidation state is here and then it kind of gives you clues to say how to build the thing or something like that, okay? So we're cool with bromine being plus five. Let's try this one, okay? Is that cool? You can do it, though. You can do it, you know? I have no doubt in my mind that you can do it. Yeah, sure. Well, let's just keep doing it, okay? So remember, this is ion here, okay? This is an ionic compound, okay? So calcium, it's a group two element, right? So it's going to be plus two, plus two. So one times plus two plus, well, how many nitrogens do we have? Two, because this whole thing is two, right? So we've got two nitrogens and three oxygens, right? So our X is going to be nitrogen, yeah. So if we want to put that next, two times X, like that, okay? Let's, I guess we should write up here, vr equals plus to keep that. Okay, and now we're going to go to oxygen. So it's two times three, right? So we've got six of them, plus six times minus two equals what? Zero, right? So now we'll just do what we've been doing. Two plus two X minus 12 equals zero, right? Two X minus 10, because 12 minus 12 plus two is 10, right? Equals zero. Two X equals 10, right? If we take that 10 over to the other side, and then we divide both sides by two, X equals plus five. We want to say plus because we want to make, in this I know normally we emphasize that it means plus if there's nothing there, but for charges we like to put the plus up there, okay? So in this case N would be plus five. Can you do it? You could do that, right? Yeah, now it's easy, right? It makes a world of difference when you come to office hours, I promise you. Why? Because I know how to explain this stuff, you know? And the thing is, I like to explain it to people who are willing to come and get help, you know? I really do. Make sure you do, yeah. I would always do that, yeah. Yeah, no, I'll bet you that's what it's doing. You guys think you could solve for what sulfur is? So I want you guys to solve for it. Don't look up here, I'll be solving for it myself, okay? Remember, sodium is a group one element, okay? I did. Okay. You're making me worried now. Okay. Okay, so let's go over it now. So remember, sodium is a group one element. So remember, sodium is a group one element. So group one elements are plus one. You've got two of them, two times plus one. Then the thing we're looking for is sulfur. We've only got one of them, so that's going to be our X, right? Then plus four times negative two, because negative two is oxygen, we've got four of them, equals zero, right? Because the whole compound has to equal zero. Two times plus one is two. One times X is X. Four times negative two is negative eight. Zero equals zero, right? So we're going to combine the two and the negative eight to get negative six. X minus six equals zero. We take that six over to the other side. X equals plus six. So in this case, sulfur's oxidation state is plus six. Okay. Remember what you told me next time you tell your kids. I've already told you. You better be nice to them, especially given me a hard time about it. No. Okay. CL2. You're nervous around your kids? Is that what you're saying? No. Whatever. Don't start that. You're so smart. No, that's the thing, you know? I think you're so smart. I love it. I love it. You know what I love is to watch you be like, oh, it's so easy, you know? I love it, you know? Okay, what about this one? What's the oxidation number of CL? Remember, it's one of the rules. If you want to go back and look at your rules. Go back and look at the rules for oxidation. Keep going. I know. Okay, so I think it's... Up here. There. No. Up here. This one. Right? Where's the rule one... Leo. That's not it. Okay. Rule number one, the oxidation number of any uncombined element is zero. Right? So is this element combined with another type of element? No. No, so the oxidation number is zero. Okay? Let's try this one, just kind of to understand what we're talking about. What's the oxidation number of hydrogen in H2? But since it's elemental form, it's zero. Okay? Any element. Any element? If it's not combined with another element? Zero. Zero. Okay, let's try one more. But don't make it confusing on you. Let's try this one. Right? What's the oxidation number of oxygen in this compound? Zero. Zero. Or in this molecule, I should say. It's not a compound. So a compound is the combination of more than one type of element. Okay? A molecule can be like Cl2, where there's not two types of elements. Right? Just one element. Okay? Let's do this last one here, pH3. What's the oxidation number of P? So how would you do it? So P is going to be your X, right? Just like you said. And then what are we going to do? Plus three times plus one. Why is it plus one? Because H in a compound is always plus one. Right? Equals zero. Right? X three times one is three. Equals zero. X equals negative three. Right? So P in this case, negative three. Can you do those things by yourself, do you think? Yeah, I think so too. Okay? Would we cool with that one?