Lewis Diagrams and VSEPR Models

hi it's mr. Andersen and this is chemistry essentials video 22 it's on Lewis diagrams and Vesper models and it's a powerful model if we can use these two together to make predictions about the structure of something like methane over here works great if we're looking at atoms that are bonded covalently and also in ions and so we could take methane for example look at its constituent parts so carbon and hydrogen we could figure out their Lewis dot diagrams if you don't know how to do that I'll put a little video up here so you can see that then you could create a lewis structure and finally we can create a Vesper model and once we have that we can learn all of these things about a molecule such as methane and so it works on molecules we start with Lewis diagrams and I'll give you a method for drawing Lewis diagrams what we can do is compare different diagrams that we come up with by looking at their formal charges we can look alternatives and lots of times we'll find multiple alternatives and then we have to show resonance another model is to use a Vesper model Vesper model is simply looking at where the pairs of electrons are and they're going to repel each other this is simply going to be Coulomb's law and so based on that repulsion we can create these three-dimensional models of what those Lewis diagrams look like and those so these two together is a pretty powerful model that you can use in chemistry what we can use is use that to predict geometry bond angles bond energy bond lengths and we can even put it polarity based on that as well lots of times we're going to have to extend it it's just a model it's not actually what the atoms look like and so we can do extensions on this based on overlap of bonds and also sometimes it's confusing when we get odd valence electrons and so I'll present another theory it's called molecular orbital Theory won't go into detail but will kind of explain where that would be useful and so first thing we have to do is to know how to draw Lewis structures there's lots of different methods on how to do this out there I couldn't find any good mnemonic devices and so we're going to use Vesper twice Vesper is going to be our model of repulsion but it's also an easy way to remember how you draw Lewis structures and so if we're drawing the Lewis structure the first thing you want to do is add up all of the valence electrons next thing you want to do is just sketch out the skeleton of where the atoms might be connected next you're going to add the electrons in general you should add it to the most electronegative atoms first and kind of work from the outside to the inside after you've done that we can look for pairs of bonds and then finally we're going to review the formal charges now I know this seems confusing but we'll work through a number of different molecules and I think it'll make sense so let's say we're starting with this this is hydrogen hydrogen cyanide if we're going to draw the Lewis structure of it the first thing we want to do is draw and add up the total number of valence electrons so to figure out valence electrons we're ignoring the metals you can see I've removed that d-block right here but what we've got is if you're in this first column then you're going to have one valence electron so hydrogen if you're beryllium you're going to have to if you're carbon you're going to have four and so we go up to this molecule and we're going to add up all the valence electrons and so that's one plus four plus five is ten so we start by Vivi adding the valence electron next thing we do is simply sketch out the skeleton an easy way to do that is to just sketch it out by drawing bonds between each of the atoms as they are in the molecule itself and so we've done that now since these are pairs of electrons we had to remove four of those electrons so now we're down to six next thing we do is we're going to add the electrons and so we should add that to the most electronegative or generally the one on the outside of the molecule we'll get to carbon in a second so I'm going to add that to satisfy the octet rule remember nitrogen wants eight electrons so it's going to have two four six and then it's sharing the one so it's going to have two electrons here so nitrogen at this point is totally happy but you should see that carbons not so happy and so we now we go to P we're going to look for pairs of bonds to try to make carbon a little bit happier so what we could do is we could share one of those electrons from nitrogen or one of those pairs and now nitrogen still satisfies the octet rule but carbons closer now it has six around it and so we could try that again and now we've got a lewis structure for hydrogen cyanide what's the last thing we're going to do is review the formal charges and a lot of people don't know what a formal charge is so we're going to start and go across from the left to the right with hydrogen how many electrons does hydrogen have how many valence electrons does it have well it has one how many have I assigned it in this model will have assigned it one electron remember its sharing one of its electrons with carbon but one of the electrons is really it and so we're going to have the one valence electron it has minus the one assigned and so it's going to have a formal charge of zero let's go to carbon how many valence electrons it's going to have four how many is it sharing well it's sharing one here one here one here one here so it's sharing 4 so it also has a formal charge of 0 let's go to nitrogen nitrogen has 5 valence electrons it's got these two just to itself but it's also sharing these 3 over here so it's got a formal charge of 0 if I got formal charges of 0 all the way across I'm happy that means that I've got a pretty good model so let's go on to another one let's look at carbon dioxide what do i do first i'm going to add up the valence electrons so it's 4 4 carbon plus 6 for each of the oxygen so i get 16 now i'm going to sketch out the skeleton so this is a quick tip that'll help you if you have an atom written by itself and then a number of other atoms after that generally this one's going to be in the middle and so when I sketch out the skeleton it's going to look like that with the carbon in the middle remember I've used four of my electrons so now I'm back to 12 valence electrons so I'm going to add electrons I can add them to the electronegative oxygen on the outside so how many have I added I've added all of my electrons and back down to zero but you can see that's carbons not happy so what I can do is I can do that pair switching right here and now I've got carbon it looks happier at this point now if we look at review the formal charges let's go from left to the right which we look at oxygen oxygen is going to have 6 how many does it have well you can see that it has 6 minus 1 so it's actually going to have a formal charge of negative 1 so that's kind of a not a great sign on my model so far if we look at carbon at 0 and if we look at oxygen it's going to have a formal charge of plus 1 because 6 valence electrons minus 1 2 3 4 5 and so it's going to have plus 1 and so could you put together this model in a different way so that I would have lower formal charges for sure what I could do is I could switch and get a pair on one side I could switch and get a pair on one side and now I got formal charges of 0 0 and 0 and so the is what it looks like now I should probably Jimmy this a little bit I should switch these pair of bonds over here and these over here but this is pretty close now let's go to ozone o3 so I'm going to add the valence electrons sketch out the skeleton let me add those electrons you can see I've got an extra one compared to when I did carbon dioxide and so now if we draw that I've got one structure like this could draw my formal charges so that's okay but can you visualize this that I could kind of invert it the other way and it could also go to get those formal charges remember formal charges the closer they are to zero the more stable kind of that structure is but these ones have the exact same formal charges and so what am I going to draw I'm going to draw resonance I'm going to draw this arrow in the middle this two-way arrow and that means that this structure or this structure are properly so we have to show both of those structures now let's get to the Vesper model so Vesper model is the valence shell electron pair repulsion what does that mean it's very simple it simply means that when you have pair of electrons they have this negative charge and they're going to push on other pairs of electrons and they're going to push on themselves and so think of it like a balloon if you were to hold one balloon it would look like this but if I were to give you two balloons they're going to move apart from each other if we hold it in the middle and so that tells us what the electrons are going to do and therefore what the structure is going to be so imagine if we have carbon dioxide like this it's going to be the same where of these pair electrons these pair of electrons your hand is kind of holding it in the middle and so what shape three-dimensional shape do we think it's going to have it's going to have this kind of a shape we call that linear linear is in a line I know it doesn't look three-dimensional but it will get there in just a second what's going to be the bond angle between the two we call that 180 degrees and this is a linear model so you can see now how Vesper allows us to start kind of visualizing these in 3-dimensional z– or in three dimensions one quick note if you're doing organic chemistry lots of times they'll call this SP hybridized at this point not going to go into what that means in AP chem it's not important but it is important that you understand the technology let's go to three electrons now excuse me three pair of electrons so we got three balloons it's going to organize itself like this what's something that looks like that may be nitrate and so if we're looking at nitrate like this kind of the nitrogen in the middle those electrons around the outside and so this is going to be that Vesper model it's going to be trigonal planar so it's going to have these three electrons coming out or these three atoms coming out it's going to be 120 degree angle between the two but it's also going to be flat at this point again quick organic chemistry no we call this sp2 hybridize so again we haven't got to this third structure yet we haven't got to this third dimension but we're quickly going to get there um let's say we're looking at something like ozone ozone you might think is going to be linear but if you look at this pair of electrons out here what kind of a structure are we going to get from ozone we're going to get a bent kind of a structure and now let's get to for a pair of electrons with four pair of electrons methane is that example I gave you at the beginning what's that going to look like well if you hold four balloons in your hand it's going to have this tetrahedral shape the bond angle is going to be 100 nine point five and it's going to really rise out of the paper itself we call this sp3 hybridized and that's kind of where it ends in AP chem your knowledge of all these bond angles and geometry know that we can also have more pairs of electrons and as we do we get to what's called if we let's say we're looking at PCL 4 that's going to be trigonal bi-pyramidal and so they're going to branch off like this what you really have is a trigonal planar here in the middle and then you're going to have a linear structure right down here through the middle why do we call it pyramidal if I were to connect those with lines you can see how we kind of two pyramids on top of each other octahedral is one are going to have six pair of electrons coming off again bond angles don't have to know it but you do have to know that we call this an octahedral at this point and so what are some extensions what are going to be some things that we have to build on this Vesper model well one thing we're gonna have to do is understand that not all the bonds are going to be the same single bonds like this are going to be different than double bonds like this and that's because the orbitals are going to start to overlap and so in a single bond what we have is called a sigma bond sigma bond is where they're sharing those electrons between the nuclei but as we start to add more electrons that are being shared what we're using is an overlap of orbitals and so we get called PI bonds and so pi bonds so this would be a sigma bond and then a PI bond or I could say this is a sigma and this is a PI bond those PI bonds as we add them are going to be less powerful but they're also going to kind of lock the molecule in itself so you get formation of isomers also sometimes the math doesn't add up in other words we're going to have a odd number of electrons so how do we add these pairs if we're adding one single electron so nitrogen dioxide is actually going to have a lewis structure like this but it's going to have one single electron out here so we call that a free radical and so there are extensions and so we can build a better model it's called the molecular orbital model or orbital Theory it's based on quantum theory and mathematics it's just a different model and we can answer a lot of these questions but it's probably a little too intense and a little too sophisticated for AP chem and so again did you learn the following to loot use Lewis geometry and Vesper models to predict geometry hybridization polarity if you did you did great try one on your own let's say I give you water could you draw the Lewis structure could you draw the Vesper model watch out for those electrons and I hope that was helpful

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