Just because an atom satisfies its octet doesn’t mean that it is stable. We also have to consider valence electrons. Sounds familiar? Let’s look into what those are.
What is a valence electron?
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So now let's talk about probably one of the most helpful topics in this entire chapter, and that's the topic of bonding preferences. Now this is something that I remember when I was in undergrad and I was taking or go. I was really confused about because my professor, a lot of times would just assume that I understood how Adam's worked and how atoms bonded. I remember my first would be like, Oh, nitrogen forms three bonds and oxygen with a positive charge forms three bonds. And I thought I was supposed to memorize all this stuff. It was really confusing. It turns out that there's really simple logic that we can use to figure out exactly how many bonds every atom wants toe have. And once you have this down, you're never gonna forget it. So let's go ahead and move just right along. So bonding preferences are based on the concept of Octa electrons, but also based on another type of electrons. Let's talk about it. It turns out that there are actual several ways to combine octet electrons in order to satisfy the octet rule. For an Adam Okay and Valence electrons, this is a new word valence. Electrons are the names that we give to the Octa electrons that the atom actually owns. Okay, so remember that we said that Adams can choose to share electrons and bonds or they can choose toe, have electrons, this lone pairs. Both of them are gonna count the same in terms of octet electrons because they're surrounding there, you know, part of the of the shell, But they're actually gonna count differently in terms of the Valence counts. Let's talk about that. The number of the valence electrons is going to determine which of the octet that you could make. If there's several versions of octet, it's it's gonna turn which the optics is the most stable. And this is the really basic role. What we're going to say is that in Adam is gonna own every lone electron that it has, and it's gonna own Onley one electron for re bond, that it has a really nice, easy way to say this is that it's gonna own every dot one electron for every dot, and it's also gonna own one electron for every stick. So I'm just gonna say dot equals one and also stick equals one. Okay, that's just another way to say it right
Every dot = 1 valence electron, and every stick = 1 valence electron. (Octet is different, see above.)
Valence Electrons and Stability
What is the difference between valence and octet electrons?
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So let's go ahead and do this kind of worked example, and you guys can help. Help me figure out, um, like the differences and stability for these. All right, so the first thing I want you guys to do, we just talked about the octet rule. So what I want you to has to do is to figure out how maney octet, electrons each of these carbons would have. So go ahead and start off from the left right there and go ahead and pick out how many. How many opt in elections? Does it have eight case? Let's write that in. Now. What I want you to do is look at the next one and say, Okay. Well, how many electrons does that one have? Let's move to the right and you're gonna find there is that you also have eight. It's different, though. It looks a little different, but I have to from the lone pair and two from each of these bonds. So it's still eight. Now, we're gonna keep going on. What you're gonna find is that all of these fulfill the octet rule. Okay? They all give carbon eight electrons. Does that makes him so far, even though they look radically different, but they still follow the architectural. So now here's my next question. Do you think that these air all equally stable? Do you think that they all they could all exist in the same way? And it turns out No, definitely not. It turns out one of these is way more stable than all the other ones. Okay. And the reason has to do with Valence electrons. Okay. The way that we count Valence electrons was remember, we count dots and sticks. So what I want you guys to do now is count. What is the Valence? Electrons for all of these. So let's start off with the one on the left. How maney sticks. Does it have four? How many dots does it have? Zero. So we have four Valence electrons. All right, the next one. How many dots does it have to? How many sticks does it have? 32 plus three is five. And you could keep going on. What you're gonna find is that this one has six valence electrons, and this one has seven valence electrons. All right, so what we find is that the octet rule is being satisfied with all of these, But they have very different amounts of Valence electrons. Okay,
All of these carbons satisfy their octet, but only one of them is actually stable. The others suck. How can we tell which one will be stable?
The Periodic Table Predicts Bonding Preference
Understanding the hydrogen isotopes.
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It turns out that if you're octet is if you're oxide is filled with all these atoms, the way you determine the one that's most stable is by figuring out what row that Adam is in. And it turns out that for each of the second row elements, the element is gonna prefer to own or it's gonna prefer toe have a violence determined by its group number. Okay, so what that means is that if an atom is in group four like carbon, then how maney valence electrons is it gonna wanna have for So this is gonna be the most stable arrangement of carbon by far, whereas all these other ones are gonna get progressively worse and worse. In fact, some of these don't even exist because there will be so crazy, hard to create, they'd be so unstable. Does that make sense? So now what I wanna do is extend this because you're like, Oh, well, Johnny, everyone knows Carbon has four bonds. Maybe you learn that in Gen come. But now I want to extend this to all the other atoms so that you guys will know for the second row what they look like and what they're always gonna want to look like in their neutral bonding preference. All right, so let's go through this really quick. As you can see, I've listed out the different atoms, kind of in order of where they are in the periodic table, and we're just gonna go ahead and start off with hydrogen. Now, I understand that hydrogen is not in the second row, but it's way more common than lithium. So we're gonna just work with hydrogen. We're gonna replace that one. Okay, so hydrogen is in group one A. And that means that how many electrons doesn't wanna have in its architect? Do you guys remember two? Okay, so that means that how can and Adam have an octet? Well, you can either have lone pairs or could have bonds. That means the hydrogen. It can exist in two different forms. I could have a hydrogen that has a bond that would give it its two electrons. Who would fill it? Octet. But I could also have a hydrogen that has a lone pair. Do you guys see that? Would both of these fulfill the octet? Yes. They would both fulfill the octet. All right, so Now, my question is, are both of these equally stable? The way that I find that out is I look at the group number. Well, what's the group number one? A. So how maney valence electrons does the first hydrogen have? It has one. Because I only count one for each stick. Okay, The second one, how many does it have to switch was going to be the most stable. This is gonna be the way that hydrogen looks. It will not look like this. And that's why every time you see hydrogen, it's always attached with just one stick with one bond. Because that's going to be the one that satisfies not only the octet, but also the valence of having one stick or one valence electron. Does that make sense? Cool. So now what I want is to do down here Now that we figured that out is really just right. What we have like, let's write our results. How Maney Bonds does hydrogen like tohave one. How many lone parents is it like to have zero guys Cool it, that let's move on. So let's go to beryllium. So, beryllium, do you remember how maney octet electrons beryllium likes tohave Quattro. All right. Except four. So there's gonna be actually three different ways that we could do this. I'll draw all three. Just that you guys can kind of realize what that is. We could have brilliant. With two sticks. We could have brilliant with a stick in alone pair. Or we could have beryllium with two lone pairs. Are you guys getting that? All of these versions would satisfy the octet rule equally. Okay, But the way I determine the ones that that's most stable is I look at the group number. What's the group number two? Okay, So if I'm in group to which of these is gonna be the favored arrangement, you guys got it. It's brilliant with the two sticks, Okay, Because that's the only one that is gonna not only fulfill the octet, but also fulfill the valence. So that means that beryllium in its bonding preference, likes toe. Have how many bonds? How many lone pairs? Zero. Okay, so hopefully that's making sense. Let's move on to the next one. All right, so now we are at Boron. Do you remember how many electrons octet, electrons, boron likes toe have this is from your octet rule. Like we talked about it. Six. Okay, How maney valence electrons does it like tohave three. So can you guys predict what I'm gonna want to do here? I think I heard it. What you're gonna want is boron with three bonds, okay? And the reason is because that's not only going to fulfill your octet of six, but it's also would fulfill your balance of three. Cool. So that means that now it's gonna one of three bonds and zero lone pairs. Are you guys kind of seeing a pattern here? None of these want lone pairs and their escalating. Alright, So we already got two carbon. And just by looking at this pattern, what should the next number be? Well, it should be that I have four bonds and zero in pairs, and when you draw it out, it actually does come out to that. This carbon wants to have eight electrons. Remember that carbon, nitrogen, oxygen and flooring all once have eight in case they're all going to stay at eight. And it wants to have four valence electrons because it's in group four. So that actually is theory. Change mint. So Now let's continue to apply this pattern. So let's go into nitrogen. Okay. So, nitrogen, how many electrons does nitrogen want to have? You want to have eight? If we were just to apply this pattern, don't draw this yet, but I'm just showing you What I would do is I would draw five here and a zero here, and I would say nature and wants that five bonds and zero lone pairs. Is there a problem with that? And I'm making a mistake. I'm actually making a huge mistake, all right. The reason is because if nitrogen had five bonds, would it still satisfy the octet rule? No, because remember, each bond counts is too. So if nitrogen had five bonds, it would have 10 electrons, and it would break the octet role. And it would even be worse state. It would be even worse off. It would be super unstable. So that means that somehow I need to get nitrogen to have five valence electrons. But still only eight are electrons. So that means that out of these electrons out of these eight electrons, I need to get nitrogen toe own mawr of the electrons. What? How can we get its own more by adding lone pairs. Okay, so hopefully what you guys maybe came to on your own is that nitrogen. What I'm gonna want to do is three bonds and one lone pair. Because if I could do that, then what I'm gonna get is I have eight electrons, total eight, and then I have every stick counts as one, 123 and every dot counts is 145 Okay, so now when I go down to my pattern, it's actually gonna be that I want three bonds and one lone pair. Okay? And now this is going to start a new pattern. Okay, so for oxygen, oxygen still wants a electrons, but now wants to own six of them. It wants even more because it's in group six. Okay, So can you guys think of our arrangement that would work for that? Yeah. You guys are smart. You guys know what's going on now? So it would be to bonds and two lone pairs. Okay, because now the more lone pairs I have, the more owning. So that means that now, this would count as six Valence electrons, eight octet. So this would be to bonds and two lone pairs was conceived. The pattern here we can guess what the last one is going to be in. That pattern actually does hold flooring would wanna have You could see the bonds keep decreasing, so flooring would have won bond and it would have three lone pairs. And that actually makes sense. Because if you give flooring one bond and three lone pairs, what you get is an octet of eight, but a valence of seven. OK, now, guys, I really can't emphasize the value of this of understanding this enough because this thes buying preferences are what a bunch of other topics are built off of. For example, bond line structures, formal charges. All this stuff has to do with your knowledge of being able to know these preferences. Okay, also, just gonna help you draw compounds later
We can predict how many valence electrons each atom wants to have by looking up its Group Number on the periodic table.
In our example above, carbon is in Group 4A, meaning that it wants to possess 4 valence electrons, making the first structure the most stable.
Now you don't have to guess what an atom looks like in its neutral state! Simply use the periodic table to look up this information.