 Alright, as I mentioned in the previous video, we are done talking or introducing ionic compounds. Ionic compounds are compounds that are made of atoms that stick together because they have opposite electrical charges. Probably the best example of that is table salt made of sodium, which is charged at plus 1 chloride, which is charged at minus 1. They stick together, the attachment, the way they're stuck together is called an ionic bond. There's another major way for sticking atoms to each other. It's called covalent bonding, and we're going to talk about it now. There are other ways of getting atoms to stick to each other. We will talk about one other way as well at the end of this unit. So covalent compounds are compounds where two or more atoms, don't worry about non-metallic, two or more atoms share electrons with each other. So we're going to talk about sharing electrons now. And when you have two or more atoms sharing electrons, they make something that's called a covalent molecule. Okay, what do I mean by sharing? Let me give you an example with hydrogen. This is a hydrogen atom. It's supposed to be electrically neutral. It may not be obvious to you that it is. If you look up in the periodic table, hydrogen, you'll see that it has a single proton, so a single positive charge. If it's going to be neutral, it needs a single electron. So this little circle here is going to be my single electron. And the first energy level can hold two electrons. But because this is a neutral hydrogen, it only has one electron in the first energy level. And the second one, the one that the other position or the other seat that can hold an electron is empty. That's supposed to be an empty circle there. So this is a neutral hydrogen atom. If you remember from the rule that we had discussed in the previous unit, I said that atoms don't like to have partially filled electron energy levels. And so this is sort of an unstable or an unhappy hydrogen atom, not happy that way. We can add another hydrogen atom that is also unhappy to the mix. This hydrogen atom is also electrically neutral, has one positive charge because it's a hydrogen, has one electron in the first energy level. And this one here is an empty slot that could hold an electron, but it doesn't. So neither hydrogen is stable or if you want to think about it in human terms, neither one is happy. What they can do, though, is you can imagine them sort of making a deal. And the hydrogen on the left says, look, I'll give you, I'll let you use my electron to fill in that empty slot some of the time. And please, you return the favor and let me use your electron part of the time to fill the empty slot that I have. And if we do that, then we're both going to be more stable or in human terms happier. So that is what they do. Those are two hydrogen atoms. If they share their electrons, because they're sharing and they're sharing constantly, these two atoms are now stuck to each other. And this type of sticking together is called a covalent bond. Technically, covalent bond is two electrons being shared by two atoms. That is our definition of a covalent bond. And if you think about it, this hydrogen atom here on the left, now, at least part of the time, it has a completely filled first energy level. So it's more stable than it used to be. This hydrogen atom on the right, also now, at least part of the time, has a completely filled first energy level. So it is also more stable. So this is the way that hydrogen atoms prefer to be. If you just have a sea of hydrogen atoms, they will, if given the opportunity, they will stick to each other to obey the rule that we've been talking about for the past week or so. And because of that, individual hydrogen atoms are considered relatively unstable. Two hydrogen atoms stuck to each other by a covalent bond, because they're sharing electrons, is more stable. And if these are two hydrogen atoms stuck to each other, another way of writing it is to just write the formula H2. And if you remember, the number in the lower right of the symbol of an atom means how many are stuck to each other. Stuck to each other. But this formula here, it doesn't give you as much information as this picture over here to the left. The picture on the left, it's more complicated to draw, but it's telling you exactly what's going on between the atoms. They are sharing a pair of electrons, and that is called a covalent bond. Here, you can't really tell how they're stuck to each other. The other thing is that I am drawing a covalent bond between the two hydrogens by drawing these two little dots in between them. Those are supposed to be two valence electrons being shared. This is, and I said that this is a covalent bond. This is one way of drawing a covalent bond for people. However, it is considered a little bit tedious. And the more common way that you're going to see is that people substitute those two little dots with a single solid line. So if you see a single solid line between two atoms, that means one covalent bond. Or another way of saying it is that single solid line means two electrons being shared by two atoms. So you should be able to recognize both ways of drawing this, either with the solid line or with the two dots. Here's a different example of atoms sharing electrons with each other. This is a hydrogen atom again. It's supposed to be electrically neutral, so it has one electron. And this little spot here is an empty spot in the first electron energy level. So unstable or unhappy hydrogen. This is another unstable or unhappy hydrogen for the same reason. It has one electron, has one empty spot in its outermost electron energy level. And then we're adding a new character to the mix. This is a neutral oxygen atom. It has six valence electrons. So I'm not drawing the electrons in the inner energy levels. I'm only drawing the dots for the electrons in the outermost energy level. So there's six dots. And there's two empty slots, because if you remember, second energy level for electrons can hold a maximum of eight electrons. But we only have six in the outermost energy level for neutral oxygen, so there's two empty spots. Because of that, this oxygen is also somewhat unstable, unhappy. Apparently it's crying. This is a tear over here. And so how can we get all of these atoms to become more stable or more happy? It's pretty straightforward. Basically, this hydrogen says, look, I'll let you use my electron some of the time, but please return the favor. This hydrogen on the right does the same thing. It says, look, I'll let you borrow my electron for part of the time, but please let me borrow one of yours for part of the time. And if you do that, you end up with this molecule down at the bottom. Up at the top, these atoms were all detached from each other. They were all running around separate from each other. Here, because they're sharing electrons, they are stuck to each other, and they make a molecule. This hydrogen atom on the left has two electrons in its outermost energy level, so it's relatively stable. This hydrogen on the right, same situation, two electrons in the first energy level, which happens to be the outermost one for hydrogen, so it is also relatively stable. And this oxygen also has how many electrons? One, two, three, four, five, six, seven, eight. Eight electrons in its outermost energy level, so it is now also stable. So we've taken three unstable atoms and turned them into a stable molecule. And if you had to look at this and tell me what it is, I think many of you would realize that this is a cartoon of water. It's two hydrogens and a single oxygen. Now, the reason I bring this up is because you can play this game that we have been playing for the past week or so, saying that atoms are unstable if they have a partially filled outer electron energy level. And if I handed you a bunch of hydrogen atoms and a bunch of... Sorry, I handed you a bunch of Hs and a bunch of O's, and I said, look, make the simplest molecule that you can make with your Hs and O's that basically satisfies the rules that we've been talking about. If you did that, what you would come up with is water. And you would learn a fair amount about water. You would realize that the oxygen in water is in the middle. It's sandwiched between the two hydrogens, so that might not necessarily be obvious. And the attachment between the oxygens and the hydrogens is a single covalent bond. So another way of drawing water is to draw an O solid line H, O solid line the other H going in the other direction. This is another way of drawing the covalent bonds in water. So you can learn an awful lot about many fundamental molecules just by sort of applying this rule that we have been using over and over again in the past week. One thing that I want to point out is that water molecules, the atoms in water molecules are actually bent. They're not in a straight line. We're not going to talk about that. But when you see water a lot of times, just so if you're confused, I want you to realize that it's drawn bent this way instead of this way. This is less accurate. It's drawn this way to basically give a more accurate description of how the atoms are arranged in the molecule. So that's just sort of a little warning. So that's it for an introduction into covalent bonding, which is just another way of saying sharing electrons. We're going to do some fancier covalent bonding in the next video, but that's enough to digest for the moment.