 Okay, compared to the monster video that you may have listened to previously to this, this is going to be a pretty short video. This is a summary of what we have been talking about and what you should know. You should know that electrons in an atom can sit, and I put sit in quotes, in different energy levels. The lowest energy level is usually considered to be where the electrons are closest to the center of the atom, the second lowest energy level, the second closest, etc., etc. You should know how many electrons each energy level can hold. First level can hold two, second and third can hold eight. You don't need to know beyond that. You should know how the electron energy levels are filled up. In other words, you fill up the lowest energy level first. You fill up level one first. If you don't have any more space but you need to put more electrons in your atom, you start filling up the second lowest, then the third lowest, etc., etc. You should know what a valence electron is. That is an electron in the outermost energy level. You should know this rule that we were going over and over again for a half hour. The preferences that atoms will have with respect to the number of electrons they have, in other words, atoms prefer not to have partially filled outermost energy levels. If they're given the opportunity, they will do something to fix that situation, either by stealing electrons, sharing electrons with other atoms, or giving electrons away. You can use that information to figure out what charge different atoms have under many different circumstances. At this point, I want to shift gears a little bit and talk about something a little bit different. I want to talk about the fact that drawing electrons and drawing atoms the way that I've been drawing them is a little bit of a pain. You don't want to do this if you can avoid it. You don't want to draw these elaborate pictures. So it turns out that somebody, his name was Gilbert Lewis, he lived about 100 years ago, invented a simplified way of drawing important or what are usually considered important electrons. This technique that I'm going to show you is called drawing electron dot symbols. Sometimes it's named after the guy who invented it. Sometimes it's called drawing Lewis structures. So either of those things may be used to describe this. It's a simpler way of showing valence electrons. So the idea here is that nobody really cares, or most people will not care so much about the electrons that are in the innermost shells. It turns out that a lot of times the most important electrons are the electrons that are in the outermost energy level. So the ones that I'm circling here, they are in the outermost energy level. They are the valence electrons. And it turns out that the valence electrons control the way the atoms attach to each other. So a lot of times people want to ignore the inner electrons. And this is what Lewis structures do. It's a simpler way of just showing valence electrons. The way that you do it is instead of drawing this elaborate thing here, you just write the symbol for your atoms. So I think this was a fluorine atom, so I would write F. And then you would put dots around your symbol, so let me redraw the F, but only for the valence electrons. So one dot for each valence electron. In this case, how many valence electrons do I have? One, two, three, four, five, six, seven, eight. I have eight. So I would draw it like this. And usually you draw them north, south, east and west of the symbol, and you draw them in pairs if you have to, or you can draw them alone as a single electron if you have to as well. So this particular fluorine atom has eight valence electrons. It has two additional electrons in the first energy level, but those aren't valence, so we don't draw them. And we draw one dot for each valence electron. This guy has eight valence, so we draw eight dots. Two to the north, two to the west, two to the east, and two to the south. So you probably realize that this is an easier way of telling people how many valence electrons your fluorine atom has instead of drawing this much more detailed picture. So if I was going to draw a neutral hydrogen atom, I know that hydrogen atoms don't prefer to be like this. A neutral hydrogen atom has one valence electron, so I would just draw one dot. It doesn't really matter if you draw it to the north, to the west, to the south, or to the east. What I want you to realize is if you see one dot written next to a symbol, that is telling you that your atom has one valence electron. If you see eight dots, like we drew for the fluorine, that means that that fluorine atom has eight valence electrons. Pretty simple, right? If you had a neutral helium atom, it would have two dots, and in this case we draw them together as a pair. Again, it could be to the north, to the west, or to the south. There are some rules about whether you draw these electrons, these dots in pairs, or whether you draw them alone. So you could say, well, why can't I draw helium like this with one dot to the west and one dot to the east? Don't worry about those rules. We're not going to go over them. They're not terribly important. And all I want you to realize is that if you see some kind of drawing like this with these dots around an atom, that's telling you how many valence electrons it has. It's not telling you how many total electrons it has, just how many valence. Again, these are sometimes known as Lewis structures, and these are just some examples of Lewis structures. So this is an oxygen atom. How many valence electrons does it have? Well, it's got six, because there are six dots written around the symbol. This is a neon atom with eight valence electrons. This is a carbon atom with four valence, sodium with one, and a chlorine with seven valence. So that's all I want you to know about Lewis structures or electron dot symbols, is that you look at them, you count the dots, and that tells you how many valence electrons you have. So that's it for this video.