Drawing Lewis Structures is something we learned how to do in Gen Chem. Nothing has changed since then, but now since we know about bonding preferences, that makes it a lot easier.
Lewis Structure Rules
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concept
How to use Organic Chemistry to make Lewis Structures easier.
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um, remember that Lewis structures were ways to describe molecules that everything was drawn explicitly. So that means that all the bonds, all the lone pairs, all the atoms were drawn out Perfectly explicit. Least you could see everything on the structure. Okay. And back in the day you learned this set of rules and Jen Kim that you're supposed to remember about computing theoretical number of valence electrons, then subtracting electrons and stuff like that. But if you already have a knowledge of the octet rule and if you already have knowledge of bonding preferences, it turns out that we could do almost all of these loose structures just on that knowledge. Okay, so I'm gonna go through these rules just for the sake of reminding you. But I'm also going to show you that that now, in organic chemistry, we have an easier method to do this based on the new information that you've learned about bonding preferences. Okay, so I'm just gonna go over these rules really fast. Remember that the first thing that you always have to do was, if you're given a molecular formula and you're trying to convert that into a Louis structure and trying to figure out what does the Louis structure look like? You always start with the atom that has the highest bonding preference in the middle. Okay, Now, for those of you may be in your class, you may have learned it somewhat different. You may have learned that you start with the most electoral positive, Adam. Okay, that's fine. Both of them work. I just want to make sure that you always start off with electro positive. Just means the one that is furthest away from flooring. Alright, So in that case, you would just start off with that one. Or you could start with the highest bonding perfect meaning, the one that makes the most bond. So, for example, if I were to compare nitrogen versus oxygen, which would win nitrogen because nitrogen like stuff three bonds and oxygen like stuff to does that make sense. So you start off with the highest bonding preference in the middle, and then you propose what's called a signal bond framework. Okay? Now, I want us to pay attention to this word proposed. Okay? Proposed moons, that this is gonna take some trial and error. You're not gonna know it usually on the very first try. So what I suggest is don't don't stress out about this. Like, just go ahead and write. Maybe the stupidest thing you can think of at the beginning. Just the one that's, like, maybe the most obvious that that's wrong. And then figure out how to fix it. That's the important thing. Hopefully, as we get better in less and less tries were able to get the right structure. Okay, Now, just one more point. If it turns out to Adams, have the same bonding preference. So, for example, sulfur, I mean yeah, sure. Sulfur and oxygen. Okay. They're both in the same group, so sulfur and oxygen like to both have to bonds. Okay, well, then you would just place the bigger one in the centers. That means sulfur would be oxygen. Okay, then what you do is you complete all the architects using lone pairs. That means that once you draw this signal, bon framework, you're gonna fill everything in with just lone pairs everywhere to complete everyone's octet. So those things gonna be loaded with dots, it's gonna look like it got hit with a shotgun. Alright, then what we do is that there's this complicated, not complicated, but just this math is to do where you have tow, calculate the theoretical valence electrons, then calculate the actual numbers of electrons that you see on the page. Then you would subtract the two numbers and get what's called the electron difference. And then, if you had a positive electron difference, then you would create double bonds, meaning that if you have too many dots on the page, you start making double bonds to get rid of dots. Okay? And if you had too few dots, then you start adding more lone pairs in order to satisfy the actual theoretical valence. Alright, so that's the rules. But what I want to tell you is that steps three through five now that we know or go on. Now that we know about bonding preferences and and formal charges, you could actually just use formal charges and bonding preferences to figure all that out. So you were not going to keep doing this math anymore.
*You might have learned a slightly different version, but this is close enough, you get the point.
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example
Drawing the Lewis Structure for N2H4.
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um, example would be a question that says, find the Louis structure for the compound end to age four. I don't know what that looks like, so let's just go ahead and jump into it. The first thing I do is I take the four atoms, and I say, which one has the highest bonding preference? Obviously one of the nitrogen. Okay, So what I do is I take my biggest 1/1 of the highest bonding preference, and they put it in the middle, and then I propose a sigma pawn framework. Okay, Notice that even put random here because I'm trying to emphasize that you're not supposed to just know this from the first shot. Ah, lot of students feel confused because they're like, How am I supposed to know this? This is trial and error. So what I did here was I literally just made, like, the dumbest version that I could. I just put a bunch of atoms around the end. Okay, so I said, Okay, one of the H is could go this to this end. The other h could go here. The other h could go here, and then one of the ends can go there. Okay. What I'm basically doing is I'm just trying to get the first the first Adam in the middle toe Phillips octet, and then any other atoms, I'm attaching them somewhere else. So basically attached four atoms that end because the most city can handle. I had one hydrogen left over. Then I was like, Hey, you know, I'll just put it on that end and see what happens. Okay, so now if I go to my second rule, it says fill in the octet with lone pairs so that first nitrogen that's in the middle fills its octet, has four bonds. But the nitrogen that I put on the side does not. This one would need to Lone pairs. That's what I did in this drawing. I went ahead and added those two lone pairs. And now what I did is I go ahead and calculate formal charge. This is the part that is different from Gen. Kim and Jim. Kim usually wouldn't do this, But in Oracle, I always want you to calculate formal charge. Now that we do that, what we find is that I'm gonna have a positive charge on the end because the end wants to have five electrons, but it only has four. And then I have a negative charge on that end because that end has six. Is that cool? So far, this is not very good. I can probably do better than this. So what I'm gonna do is if I get Teoh a structure that has lots of charges or lots of adjacent charges. I have two options. Either I can rearrange the signal. Bon framework. Meaning I can rearrange where these where these single bonds are. Or I can add pi bonds to remove excess lone pairs. Okay, so in this case, I have two different options. I could try to make a double bond, but I would run into a big problem if I made a double bond right? The way it is drawn right now. The problem is that let's say that I tried to put a dull bond here to get rid of these electrons. Then what would happen is that that nitrogen would wind up breaking its octet. This one right here would now have 10 electrons. So there's actually nowhere that I could make a double bond. That makes sense. Okay, there's actually nowhere that I could make double bond That wouldn't violate someone's octet. So that means that I have to change the way change around the signal on framework. So what I'm thinking is this This nitrogen wants more electrons. This nitrogen wants less electrons and wants less electrons the way that I could make that happen, possibly by moving a bond. If I could move one of these h is over here, Then what that would do is I would take away one lone pair and replace it with a stick. And remember that a stick only counts as one, whereas a lone pair accounts is to So let me redraw this looking like this and and each each each h. Okay, Now, I still have to make sure that everything fulfills its octet. So I'm gonna have to add lone pairs. So I'm gonna need a lone pair here and a lone pair there now, like, go ahead and a re calculate formal charges and see if this is any better. So the formal charge of this nitrogen would now be zero because of the fact that it's got three bonds on a lone pair, which is the way nitrogen wants to be. And this one is also zero. So that's it. We found a good Louis structure. Okay, so obviously this one is kind of easy, but I'm just showing you that is the right Lewis structure for this molecule. And we got there through trial and error. We didn't just immediately know what the answer Waas, alright.
In this example, we were left with a separation of charges (+ next to -), so we rearranged the σ-framework to see if we could remove them.
This is a process of trial and error. Don’t be afraid to draw the wrong structure at first!
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Problem
Draw the Lewis structure of the molecule
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Draw the Lewis structure of the molecule
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Problem
Draw the Lewis structure of the molecule
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Like the new method? Prefer the old one instead? Feel free to use whichever method you like more. As long as you get the right answer on the exam, that’s all I care about.