 Say for part two of our ionic bonding video, we'll talk about Lewis dot diagrams and how to use them to illustrate ionic bonding. First of all, Lewis dot diagram is an illustration that shows the valence electrons in an atom. And what we do is we draw the symbol of the atom and then we put dots around it to represent each of the valence electrons. But it's important that we put the dots around it in the correct way. The way that I teach it in class is that you have to put one dot on each side and then once you've filled all four sides, you can go back and double it up. So basically, if you think about it this way, you have a symbol for the element. You've got a top, a bottom, a left and a right. Those are the places where you can put the dots. And again, the rule would be you put one dot on each side first and then you're allowed to go back and double them up until you reach the desired number of electrons. When we end at eight, group 18 has eight valence electrons, so you would never have more than eight dots. So let's just draw a couple of these Lewis dot diagrams first. Let's start with the Lewis dot diagram for phosphorus. So again, I just start with the symbol P and then look at my periodic table. Phosphorus is in group 15 and that tells me there are five valence electrons. So if I do one dot on each side, then I can start doubling things up. So one, two, three, four, five. The key is we have to have the fewest number of pairs and by doing it that way, we're guaranteeing that that happens. It doesn't really matter where you start. You can start on the top, bottom, left or right. It really doesn't matter. Do go around in order. I tend to go clockwise. And after you've done enough of these, you'll get an idea of where you need your single dots to be and you'll begin adjusting on your own. Let's do the Lewis dot diagram for calcium. Calcium is in group two on the periodic table, so it has two valence electrons. Again, minimum number of pairs. We start on the top and work our way around. They'll both be singles and that's how you do it. Now, when we're talking about ionic bond and we've got these metals and with non-medals, we have to remember what they're gonna have to do to become stable. Because this is a metal, it's gonna have to lose those two dots. So my calcium's gonna lose all the dots it has. For the non-metal, the goal is to get eight dots. So my non-metal here is short three. We need to pair up the rest of those electrons. We need to get three more dots. So we get eight. The way we're gonna use this Lewis structure, these Lewis dot diagrams illustrate monies by showing the transfer of electrons, by drawing these atoms in, putting arrows in to show how the electrons move from one place to the other. And the rule is we just draw them out until all of our metal atoms have lost all the dots they need to lose and all of our non-medals have all eight dots that they need to have. So generally speaking, we just start out with one of each and see what happens. So I've got two dots on this calcium that I need to get rid of and they're gonna go towards the single dots in the phosphorus. So one of them's gonna go there to pair up with that one and one's gonna go there to pair up with that one. We transferred our electrons, we look at our metal and we try to make sure that it's happy. And again, to be happy, it has to lose all its dots and that would make the calcium very happy. Then we have to look at our non-metal and see if it's happy. And again, it needs to have eight. It needs to pair up all the lone electrons. At this point, we would have one, two, three, four, five, six, seven. We're still one short. Whenever that happens, we're one short on what our non-metal needs, we bring in another atom of our metal, like so. And that new atom can provide that last electron that the phosphorus needs. And again, we reassess. We look back and say, okay, are my metal atoms happy? Well, that one's happy because it lost both of its electrons. But this one is not because it only lost one. It still has one more to get rid of and this phosphorus is full. It's taken all it can take. So when that happens, we just bring in another phosphorus. And that new phosphorus atom can take that electron that the calcium needs to get rid of. And again, we reassess. Are my metals happy? That one's lost both. That one's lost both. My metals are happy. And then we assess our non-metals. This one has got the three it needs to be stable, but this one's only got one. Again, when that happens, when your non-metal needs more electrons, get yourself another metal. We can move this one here. We can move that one there. And again, reassess. That calcium's lost everything. That calcium's lost everything. All my metal atoms are happy. This phosphorus has gained three, it's happy. This phosphorus gained three, it's happy when you've reached the point where every atom is satisfied, you're done. You have made what you need to make. You've drawn out that ionic bonding. Now we look at what we've got here. We can write down what the formula is. We've already come up with formula metals. Go first. I have three calciums. So CA3. And I have two phosphorus. P2. These cannot be reduced, so that would be the formula. CA3 P2. And using this to illustrate ionic bonding, using this to predict chemical formulas, we can do all kinds of stuff with it. It's a very important tool in chemistry. If we do one more, I'll show you how you can work around that whole balancing back and forth thing should you want to. Let's combine these two. Let's combine together magnesium and chlorine. And again, we're gonna start by figuring out what the Lewis dot diagram should look like. Magnesium, like calcium, is in group two. So one dot, two dots, first two valence electrons. And chlorine is in group 17 with seven valence electrons. So one, two, three, four, five. Five, six, and seven. Three pairs in a single. And again, we think about what they've gotta do. These have to lose the two electrons that they have. So anytime we're dealing with a magnesium in our illustration, we're gonna have to make sure it loses all of its dots. And we got seven over here around the chlorine, so it's gonna need one more so we can get eight dots. Now, if you don't wanna go through the whole back and forth, trying to figure it out that way, you can go ahead and figure out the chemical formula first. Using the crisscross method. Magnesium is in group two. So its oxidation number is plus two. And again, that's because of the ionic bonding thing. It has two valence electrons. It needs to lose those two valence electrons. When it does, it'll have 12 protons. And it'll have 10 electrons. Positive 12 plus negative 10 is positive two. Chlorine is in group 17, and group 17 is a negative one. Again, it goes to the whole ionic bonding thing. It's got seven valence electrons. It needs to gain one more. When it does, it'll have 17 protons and 18 electrons. Positive 17 plus negative 18 is negative one. When we do the crisscross, we take the numbers and all of the numbers, we drop the signs. And we swap them. We check to make sure they can't be reduced. And one out of two is already in slowest terms. We rewrite the formula without the one, MgCl2. So this is telling us what we gotta draw. We have to draw one magnesium, and we've gotta draw two chlorine. And if our formula is correct, then it should work out perfectly. So again, the metals gotta lose them all. It can give that to that chlorine. And then that chlorine's filled, and it can't take any more. And we move the other one down there. And that chlorine is filled. It's got no more room. The metal has lost both of its dots. And one, two, three, four, five, six, seven, and the new one would be eight. One, two, three, four, five, six, seven, and the new one would be eight. Everything's got eight electrons, so it's good. So you can either go from the drawing to the formula, or use your crisscross method to go from the formula to the drawing.