Lewis diagrams for molecules

0:00- [Instructor] Let's draw Lewis dot structures for certain molecules, it's a lot of fun to do that, okay? Now the first thing we need to do to draw these structures is to identify the number of valence electrons, okay? And we've talked about these valence electrons in our previous videos, but this is such a central concept, so just let's quickly recap.

0:16Valence electrons represents the number of electrons in the outermost shell, okay? And if we bring up our periodic table, then the valence electrons for all the elements belonging to a particular group is given this way. So the first group will have the elements of, the first group will have one valence electron.

0:34The elements of the second group will have two electrons in its outermost shell, two valence electrons, and so on and so forth. When it comes to group 18, helium is an exception. It has two valence electrons, but the rest of them will have eight valence electrons, eight electrons, and its outermost shell, okay?

0:50And just to get a sneak peek of why this is the case, here are some structures of some of the elements. You can see hydrogen, one electron in its outermost shell. Lithium, one electron in its outermost shell. Sodium, which has total of 11 electrons, but one electron in its outermost shell.

1:07Same thing will continue. Okay, but what about the elements of these group? What about their valence electrons? Well, we don't have to worry about them, because we are drawing structures for molecules that are formed by covalent bonds. Meaning bonds formed by sharing of electrons.

1:21Now, metals, which I've drawn in red, okay? They usually do not participate in covalent bonds, okay? It's only the non-metals that usually do that. And the non-metals are in green. And you can see all the valence electrons for the non-metals we already have them.

1:35So we only got to worry about them and not worry too much about these, okay? All right, with that recap, let's now draw the Lewis structure for hydrogen. So the first step is how many valence electrons does hydrogen have? It has one valence electron. So let's write that, one valence electron.

1:51And now, hydrogen would love to have two electrons because it'll make it stable in its outermost shell. And how do I know that? Because if I look at helium, look, helium has two electrons and it doesn't bond with anybody. Because helium is stable, all noble gases are stable.

2:03That's how I know hydrogen will love to have two electrons in its outermost shell. Now that means it needs one more electron. And here's the thing, if it needs one more electron, it's gonna say, "Hey, I'm gonna share one electron." If you need one, you share one, okay?

2:17So hydrogen says, "Hey, I'm willing to share my one valence electron." So it's gonna share one electron. And if somebody else is willing to share their one valence electron, it's gonna form a nice bond with them, okay? That's why it forms a bond with itself. And so now we can draw the structure this way.

2:32So here's the hydrogen, and here is the valence electron. And it's ready to share that. Here's another hydrogen, here's the valence electron it's ready to share that. And look, they're gonna form a bond. This is called the covalent bond. And by sharing, each atom has access to two electrons, making them both stable.

2:53That's why they stay together, okay? Covalent bond. How do we draw the Lewis structure, the final structure? Well, we are just gonna represent this bond with a single line. So a single line represents a bond between two electrons. Okay? This is the Lewis dot structure for hydrogen.

3:09Okay, let's take another example. Let's take oxygen. If I go over here, oxygen has six valence electrons, that's where we start. Oxygen has six valence electrons. And how many does it need to attain stability? Well, if you look at neon, then you can see that it has eight electrons in its outermost shell, and I know neon is very stable so that helps me understand that eight electrons in the outermost shell gives you stability.

3:35In fact, that is the case for most other elements. For hydrogen it's two, but for most other elements, they require eight electrons in its outermost shell. We call this the octet rule, all right? So since oxygen requires eight electrons in it's outermost shell, it has six.

3:49So it needs two more. If you need to, you share two, that's the rule, okay? So it's going to share two electrons. And if somebody else is willing to share their two electrons, it's gonna form a bond with them. That's why it forms a bond with itself, okay? So how do we do that?

4:06Well, again, we're gonna draw two. So we're gonna draw the Lewis structure. We're gonna draw two oxygen atoms. And now since it has six valance electron, here's how I show the valence electrons, okay? One, two, three, four, five, six. Why do I draw it this way?

4:23Because just by drawing it this way, again, you can see just from the dot structure that there's one pair, there's a second pair, oh, there's no pair here, oh, there's no pair over here. These electrons are lonely, they want to get paired. And that's how I know that these two electrons are gonna participate in bonding.

4:39It's gonna be shared. So just by looking at the dot structure, I can see that it's going to share two electrons. That's another way to just confirm the same thing that we got over here, okay? So it's gonna share these two electrons. This oxygen atom is gonna do the same thing, one, two, three, four, five, six.

4:57It's gonna share these two, which means these two electrons and these two electrons can form a bond. Now, just to make it a little convenient for me to draw this, I'm gonna show the lonely electron here, I'm gonna show the pair over here. You can draw the pair wherever you want, okay?

5:09It just makes it so much convenient to draw bonds over here. So this means these two electrons are gonna form a bond over here. And these two electrons are gonna form a bond over here. And there you have it, that means oxygen is gonna form a double bond, and you can count the number of, because of this covalent bond, you can count the number of electrons each oxygen has access to now.

5:30If you look at this oxygen, it has one, two, three, four, five, six, seven, eight, it has eight electrons access to it. And this oxygen, one, two, three, four, five, six, seven, eight, it has also eight electrons access to. And so octet rule is satisfied. And so now how do we draw the final Lewis structure?

5:47Well, here's the oxygen atoms, one and two. So two bonds, two lines, and then we have to show the lone pairs. These are called lone pairs because hey do not participate in bonding, but it's important to show them. So here's the lone pairs. You can show the lone pairs wherever you want, okay?

6:03But do show them in the Lewis dot structure. And there you have it, that is a dot structure. All right, your turn, why don't you try drawing a Lewis dot structure for N2? Why don't you pause the video and give it a try? All right, here we go. Again, I look at nitrogen over here, and I know that nitrogen has five valence electrons.

6:21That's where you always start, number of valence electrons. And octet rule, remember, most elements follow the octet rule. So it requires eight electrons and it's outermost shell for stability. It has five, so it needs three more. If you need three, you share three.

6:37So it shares three electrons. And again, if it can find some other atom or elements that can share three electrons, it'll happily bond with them. That's why it forms a bond with itself. And so how are we gonna show that? Same way. So we're gonna draw the five valence electrons over here in the same way we wrote it over here.

6:56One, two, three, four, five. And again, you can see just by drawing the dot structure, we can see there's one pair over here. But there are three lonely electrons, which would love to participate in bonding. That's why it shares three electrons. Just by looking at the dot structure, you can confirm that.

7:12Same thing over here, one, two, three, four, five. So these three electrons can bond with these three. And again, just to make it a little convenient to draw. Okay, I'm gonna... Sorry, I'm gonna draw that pair over here, and I'm gonna draw that pair over here.

7:31All right, and of course, the more practice we get, the better we'll be at conveniently drawing them, okay? But anyways, now that we have these three electrons, they can form a bond with each other. And so bond number one, bond number two, bond number three. So nitrogen forms a triple bond with itself.

7:47And again, if you count the number of electrons, this nitrogen has one, two, three, four, five, six, seven, eight. And for this nitrogen, again, one, two, three, four, five, six, seven, eight. Octet rule is satisfied. So what does the Lewis dot structure, final structure look like?

8:08Well, I have three bonds, so it's gonna be a triple bond. And then do not forget the lone pairs. Never ever forget the lone pairs, okay? And this now explains why nitrogen loves to form three bonds. Either a triple bond or a double bond with one element and a single bond with another element, or three separate bonds.

8:25And oxygen, for example, loves to form two bonds. Double bonds with another atom or two single bonds. All right, let's kick things up a little bit. How about we draw one for water molecules? Hmm, things are getting interesting now. The idea is the same though.

8:41You start with the number of valence electrons for each one, and then think about how much they share and then try to form a bond. Now of course, because you have three elements over here, there could be a little bit confusion about how they're all connected. The general rule is the element which is on the leftmost side of the periodic table.

8:56Again, if here's a period table, look at the one that's on the leftmost side, which is least electronegative. Or you can also think of it as most metallic, because, you know, all metals are on the left side, right? So the one that is on the leftmost side will usually be the central atom, right?

9:13What about hydrogen? Hydrogen is the leftmost, but it is an exception. All right? So hydrogen will always be on this side. So except for hydrogen, any other elements that you have, you have to consider the one that's on the leftmost side, that will be your central atom.

9:25We'll take more examples, it'll make sense, but at least when it comes to water, since hydrogen will always be on the side, the oxygen will be in the center, so we can immediately draw the structure. So the oxygen will be in the center, the two hydrogens will be connected to it.

9:41And we already know their valence electrons and how much they're gonna share. So I'm gonna write one for oxygens. I'm gonna start with that same technique, one, two, three, four, five, six. So it's gonna share these two. And hydrogen just has one. And so you can kind of see these two are gonna form a bond with these two.

10:01And again, just for convenience, what I'm gonna do is I'm gonna take this and put that pair here so that I can nicely bond of these two. And there you have it. So this is gonna be bond number one, and this will be bond number two. And that's it, that's the structure.

10:20So dot structure. So it's gonna be oxygen, hydrogen, hydrogen, one bond here, one bond here. And do not forget the lone pairs. Okay, again, your turn. How about you draw one for NH3? Pause and try. All right, there are four elements over here, but again, because hydrogen will always be on the side, we know it's the nitrogen which is going to be the central atom.

10:53And the three hydrogen will be connected to it. So nitrogen will be in the center and the three hydrogen, well, we can draw the three hydrogens wherever we want. So lemme just draw them over here. And again, we'll start by drawing the central, the valence electrons on the central atom.

11:10So one, two, three, four, five. I know it has five valence electrons. So where do I draw them? I'm gonna draw the fifth one over here. You can see that, right? Now I can conveniently draw the bonds over here and one valence electron for hydrogen. It's way to share that.

11:26So one bond over here, one bond over here, and one bond over here. And this will be the lone pair. And again, you can check, hydrogen has access to two electrons, nitrogen has access to one, two, three, four, five, six, seven, eight. Okay, so what's the final structure going to be?

11:48Well, I have N and I have single bond H, single bond H, single bond H. Am I forgetting something? Yes, the lone pair. Do not forget the lone pairs. All right, final challenge. Carbon dioxide. So carbon is a new element for us now. Why don't you pause the video, look at the periodic table and try to draw the Lewis structure for this one?

12:12All right, since carbon is a new one, we need to think about carbon. Carbon has four valence electrons and it needs four more. And therefore, so lemme just write that down. Four valence electrons, it needs four more. If you need four, you share four. Okay, and now, now is an interesting one.

12:32I don't have hydrogens, I have carbon and oxygen. How do I know which is the element that comes in the center? But again, CO2 might be simpler, you might be able to guess it, but we're gonna look at which one is the leftmost element. And you can see carbon is the leftmost element on the periodic table.

12:45Oxygen is over here, carbon is over here. So carbon comes in the center. All right, so carbon will be in the center and oxygen will be attached to it. So I'm gonna draw two oxygen atoms over here. And now again, I can draw the Lewis structure for carbon. How do I do that? I'm gonna show the four valence electrons.

13:01One, two, three, four, and that's all there is. And look, they're all lonely, they're all four lonely and therefore they're gonna share all four of them to form pairs. And the same thing for oxygen as we've done before. One, two, three, four, five and six. Where should I draw five and six?

13:21Well, lemme see. I mean, we can draw it anywhere, right? So lemme draw the five and six here. Okay, same thing over here, one, two, three, four. I'm gonna draw the fifth and the sixth one over here. So we can bond this and we can bond this and we can kind of bond this and we can kind of bond this.

13:37It's a little crooked, but it's okay. We get the idea across. So this is one pair and then we can bond it over here and we can bond it over here. It's fine, it's fine. I know it looks a little weird, but we get the gist of it. Carbon is gonna form a double bond with this oxygen atom and the carbon is also gonna form a double bond with this oxygen atom.

13:57And let's count carbon has one, two, three, four, five, six, seven, eight. What about oxygen? One, two, three, four, five, six, seven, eight. And the same thing with this one. And so we can now draw a better Lewis final structure. So carbon, oxygen, oxygen. So two bonds.

14:17So double bond here, a double bond over here. Do not forget the lone pairs. So there are two lone pairs over here. Lemme draw them over here and here. And there are two lone pairs over here. Are there any lone pairs on the carbon? No, there are no lone pairs on the carbon.

14:33So there you have it. That was fun, right?