 Okay, so for the past couple of videos I have basically been telling you that a lot of the atoms, especially in the first three rows of the periodic table, have a preferred electrical charge. Lithium likes to be charged at plus one, oxygen, when it's charged likes to be charged at negative two, helium likes to have no electrical charge, et cetera, et cetera. And then I said, and I give you some rules, or I said you could memorize the edges of the periodic table and figure out what the charge of a lot of atoms like to be. And I said the middle of the periodic table, the rules kind of broke apart. And in the middle of the periodic table, there are a bunch of metals. And part of the reason why the rule breaks apart is that the metals, many metals, can actually have more than one type of electrical charge, depending on the environment that they're in. So the way this slide is titled, some metals break the rules. They can have more than one type of charge. As an example, iron, and the symbol for iron is a little bit weird, it's Fe. Iron basically comes in two charges. It can either be charged at plus two, or it can be charged at plus three. It depends on the environment that it's in, what charge it will be. But because of that, you have to tell people what type of iron ion you are working with. You can't just say, I'm working with an iron ion. Because people are going to say, well, which one do you mean? The one that's plus two, or the one that's plus three? You don't have to do that for a lot of the other elements that we were talking about before. If I said I have a lithium ion, according to the rules, you always know it's going to be plus one. If I say I have a fluoride ion, you know because of the rules, it's always going to be negative one. However, for a lot of the metals, you have to tell people which one you're working with. So if you were going to speak this one out loud to somebody, you would call it iron two. And obviously, if you were going, if you were working with this iron that's charged at plus three, and you were going to describe it out loud to someone, you would call it iron three, you know, you'd spell or you'd say the word three, and that tells people who have been trained with this which charge your iron has. There's another way of doing it, and that is to write the name of the element, and then in parentheses right next to it, you write the Roman numeral for whatever charge your atom has. So if I was working with iron that is charged at plus two, I could write the word iron, parentheses, and then the Roman numeral two. That is another way of telling people that I'm working with iron that has this charge. And if I wanted to describe this one to somebody, I might write iron, parentheses, Roman numeral of three. So if you ever see Roman numeral next to an element name, that is telling you what the positive charge of that particular element is. So if I write iron with a Roman numeral two, you should know that I mean this type of iron and not any other type. Here's a problem that you can think about for a minute. If I told you that I had some material, it's an ionic compound, so it's made of iron two and chloride ions, and I want to know what the formula is. Remember the symbol for iron is Fe, so it's going to be Fe some number of irons stuck to some number of chlorides. And you can pause the video and think about that on pausing. If I write the Roman numeral two, that means this iron has a charge of plus two. I'm telling you in the question that the chloride has a charge of negative one. So you have to play the game that we played in the previous video that basically says how many irons and how many chlorides do I need to stick to each other so that their electrical charges completely cancel each other. So if I had one iron at plus two and one chloride at minus one and I stuck them to each other, these do not cancel each other's charge out completely because this is plus two, this is only negative one. How many chlorides do I need to cancel out one of those irons? Well, I need two of these. So if I had a single iron with two chlorides, here's my second chloride stuck to it, this entire blob of three irons is going to have a total electrical charge of zero because this guy's plus two and these guys are both minus one. So I have a total charge of zero there. So if I wanted to figure out what the formula for iron two chloride was, I would say it's a single iron in my blob. So Fe, I could write the one, but most people don't. And how many chlorides? Well, there are two of them there. So it's going to be Cl2. So the formula for iron two chloride is FeCl2. And the same principle applies as what we did on the previous video. However, people have to tell you what the charge of the iron is for you to be able to finish the question or to be able to answer the question. You can pause and ask this question as well. What if it was actually iron three with chloride? So what's the formula for iron three chloride? Well, this time the iron comes with a charge of plus three. The chlorides are still stuck at minus one because that's pretty much the only type that they come in. And then you have to ask yourself, how many chlorides do I need to cancel out the charge of one iron that is charged at plus three? Well, I need three chlorides to do this because the chlorides only give me a charge of minus one. So if I wanted to make iron three chloride, it would be one iron and then one chlorine, two chlorines, three chlorines, each one charged at minus one. And the entire blob has an electrical charge of zero. So what's the formula for iron three chloride? Well, there's one iron in here, so we just write Fe, and there's three chlorides in here. So we write Cl3. And this is the formula for iron three chloride, and this is the formula for iron two chloride. Again, we're just following one rule, but now you have to be able to interpret this Roman numeral. Here's a different question, pausing whenever you like. It's a little bit backwards from what we just did. Copper, again, has a little bit of a weird symbol. It's Cu. Copper ions come as copper one or copper two. So what that means in English is that this copper has a charge of plus one. This copper here has a charge of plus two. The question is, if I told you I had a compound, I had some ionic compound, and the formula was CuCl2, which kind of copper ions do I have? Do I have copper ones or do I have copper twos? And you have to realize that the chlorides, each chlorine, comes with a charge of negative one. And how many do I have here? This is Cl2. I have two chlorides, so I've got two of them. And how many coppers do I have here? You have to be able to read the formula and realize that there's only one copper. And if I have one copper stuck with two chlorides, and the chlorides give me a charge of negative one and negative one, and the whole charge of my blob, the charge of my blob has to be zero, then this copper has to be charged at plus two, because it needs to cancel out the charge of the two chlorides. So now we can tell that CuCl2, the copper here, is actually copper two. It's not copper one. So that's just another way of seeing whether you understand the rule that we've been going over and seeing whether you can interpret these Roman numerals. So that's it for this video. I'm trying to keep the video shorter. We'll see if that continues successfully or not. But that's it for this video.