Octet Rule

Alright, guys. Now what I want to do is introduce probably the most important rule in all of chemistry. And that's the octet rule, So let's just jump straight into it. So the octet rule is based on the idea that all atoms on the periodic table eventually want to reach what's called the noble gas configuration. Okay, The noble gas configuration just means that they look like noble gasses in terms of the amount of electrons that they have. And just you guys remember the noble gasses. Are these pink colored, um, Adams right there? Okay, The noble gasses are the most stable atoms in the entire periodic table. So you can think of all the other atoms as just trying to get to that red state in terms of the the stability of the noble gasses. Okay. And actually, this tendency to gain or lose electrons in order to reach that configuration is what's known as the octet rule. So when I say octet rule, all I'm really talking about is how can this Adam become like a noble gas in terms of electrons? Okay,

So it turns out that in order to prove that noble gasses are so stable, we can actually use the molecular orbital theory that I was teaching you guys before. So let's go ahead before I talk about the periodic table. More. Let's go ahead and go down to this um to this linear combination really quick. And I'm gonna show you guys why noble gasses are so stable. Um by the way, think about it this way. If adam is stable, is it going to want to form a bond or not form a bond? The answer is that it doesn't want to form a bond. The reason is because if it's stable by itself, it doesn't need anyone else. That's like a person that's already financially stable. Right? You have your own like pimp pass apartment. You're not gonna want to get a roommate, right? You're already like bawling. So why would a noble gas get a roommate? And that's exactly the thing. It's not going to let me show you why. So in this case, I'm gonna use helium, which if you notice helium is one of the noble gasses and I'm gonna show you guys how it's a little bit different. So, remember that these were our atomic orbital's I'm still using one S. A. One S. B. Because these are still one s orbital. Remember that one s orbital can hold two electrons? How many electrons does each of these helium atoms have? Well, according to the atomic number, what's the atomic number? Two. So should have two electrons. So one S A. Should have two electrons. One S B Should have two electrons. Alright, so both of these atomic orbital is already going to be full right off the bat and that's what I've drawn right here. Those are the two um Adam's just non bonding. If they're non bonding they would just interact like that, they wouldn't interact and they'd just be full already. Now, what happens if we try to make these interact with a bonding or constructive um interference? Okay, what's gonna happen is that two of these electrons are gonna jump down to a lower energy state? Okay, and they're gonna fill the sigma orbital, I mean the signal but they're gonna fill the signal molecular orbital. Okay, so that's good. The problem is that I still have two electrons left that need to go in another molecular orbital because you can't just combine some of them, you have to combine all of them. So then it's gonna have to jump up to the next energy state because remember that Pauli exclusion principle says you can only fit two in age and then after all principle says that you need to jump up to the next energy level. So what that means is that two of these electrons are also going to have to go into the anti bonding orbital, do you think that's going to be stable? Not at all. That's going to cancel out the stability that was gained from the bonding orbital. So basically the bonding orbital is going to be canceled out by the full anti bonding orbital. Does that make sense? This is really bad. So it turns out that um I think back in the 90s they actually did make finally they got helium to bond. Okay, it was it took forever. You know how much energy it saved? 0.1 kg joules per mole? Alright, so what that shows you is that really there is no basically no benefit to helium interacting through a bonding orbital? Yeah, through through a bonding molecular orbital. So it's not going to. And what you find is that if you do find helium in a balloon or in outer space or ever, helium will be found with just one in one atom. It won't be found as Ehe two. It will just be found as helium. Alright, so it just shows you how in real life they don't like to bond together. All right.

So remember we said that organic chemistry is the chemistry of life. So we're not gonna see heavy metals involved in organic reactions for the most part or in biological systems. Because of this, the most important roles of the periodic table are rows one and two. And sometimes on occasion you might see world three being involved. A vast majority. The reactions will deal with the elements in these three rows. Now of course like plants, you wouldn't see a plant composed of atoms of lead or platinum or mercury. We will come into contact with reactions of course dealing with oxygen and nitrogen and sulfur, things like that. Now, just a crash course in reading this periodic table, we can see that the periodic table has about 118 elements. Now, I haven't filled out everything in terms of this graph. Well remember this bottom row here, it has been completely filled in with conventional names for the elements that are here here and here. But again, we're not really concerned with that. We're really focusing on the first three rows. Now when we talk about rose, remember another name for a row is a period. So we have roll one which contains hydrogen and helium road to or period to which is lithium lithium. All the way to neon Row three or period three is sodium all the way to Argon. Besides rose or periods. We have groups. So our groups represent our columns. Or fancy term families. Now we'll tend to say groups or families or groups or columns and we're talking about each one of these groups. So a group one A. Here is hydrogen all the way down to F. R. Group two a. All the way to group eight over here. Now, we're not concerning ourselves with the elements found here in the pit. These are your transition metals. Uh They themselves they're not group A elements, their transition metals, they're kind of weird. This would actually be one, be here and to be here And then it would it would come back over here as three B. So transition metals are a bit weird. Just realize that in or go, we only care mainly about the first three rows or periods because those are our nonmetal for the most part, elements, they'll be involved in a lot of reaction, you're gonna see throughout this course. Okay, so just keep that in mind. Remember groups are your columns periods. Are your rose? We care mainly about the first three rows of the periodic table.

So now I just want to go over some really, really quick rules. And these are gonna be the rules that we need for the octet rule. Okay, so Adams can satisfy their octet by forming chemical bonds or by possessing lone pairs. So what that basically means is that remember that I said Adams want to either lose electrons or gain electrons in order to fit the noble gas configuration. Right? Well if you are losing electrons that means you're gonna be sharing electrons with other atoms. Okay, so that would be a bond. If you are gaining electrons that means you're gonna be taking on more electrons. So you might have like a lone pair on the atom. Okay, those electrons are called octet electrons. So keep in mind when octet electron is Alright, so basically let's talk about the first row elements hydrogen, helium and lithium. Okay, hydrogen helium and lithium are so small that they're usually only gonna have just that one s orbital and it can only have two electrons. Okay. Or they're gonna so basically what that means is that they're gonna prefer prefer to only have two octet electrons so that they can become like helium. Okay, so hydrogen is gonna want to gain an electron, lithium is gonna want to lose an electron so that they can both be like helium and they're both gonna want basically all of these are going to want to have to octet electrons cool so far. This is also known in some books as the duet rule. Okay, but the duet rule is the same thing as the octet rule. As long as you think of noble gas, it's the same thing. All right. So then we have our second row elements. This is first row or first period you could think of second row elements are carbon, nitrogen, oxygen flooring. These are all gonna prefer to possess eight octet electrons. That's why we use the name octet rule. Common. Now, it turns out there is an exception to that. The atoms that are smaller than carbon are going to have are gonna prefer to possess less than eight electrons. The reason is because they don't have that many orbital's so it's actually difficult for them to accommodate eight electrons. So B E. Or beryllium is gonna prefer to have four and boron is gonna prefer to have six. These are just things you need to know. You just need to memorize that. Then finally, we have our third row or third period elements that are gonna be able to form what's called expanded octet. Okay, this is what an expanded octet is. We talked about s orbital's and we talked about p orbital's but it turns out that phosphorus and sulfur are so big that the p orbital are gonna get filled up. Do you know what comes after the peas? The D orbital's We're not even gonna talk about D orbital's in this class. What you should know is that D orbital's can also hold extra electrons. So what's going to happen is that phosphorus and sulfur can choose to hold more than eight electrons if they want to. So phosphorus is going to be able to hold 10 electrons, And sulfur is going to be able to hold 12 electrons.