this program is now a continuation of a look at from our Lewis Dot Diagram into what determines the shape of a molecule I'm going to start with by sharing with you a sketch of what the scientists at least believe the aspirin molecule looks like you'll see sections of it are flat sections of it resemble small triangles other sections like this one here appear to come out of the page what's the theory that lies behind this orientation and shape of our molecule well that theory is called the vasper theory that vasper theory is based upon the idea that only the veent shell electrons are involved that's why we've been practicing Lewis Dot diagrams because they only involve those that are in the veence Shell here is a Lewis Dot Diagram for a molecule that we've drawn a bit earlier this here is our what we called the nitrate molecule and we should of course put little square br ET around that now we're only interested in the veence electrons hence these diagrams when they're drawn only use the veence electrons now electrons act in pairs that's why you see a lot of them paired together and sometimes those pairs will act in groups let me show you an example of what I mean by that so here we have an electron pair here we have an electron group and here we have an electron pair we call these collectively under one term we call them electron domains so this particular molecule has three electron domains around the nitrogen these domains repel each other that's what this statement says so these three regions are going to try to push themselves apart as far as possible if they did that they would take up this shape they by getting as far away from each other as possible this shape is called trigonal planer and we would expect that the angle between these lobes or these domains would probably be somewhere around 120° this isn't the only shape that can take up there are some other common shapes so I'm going to start by sort of introducing a few of those right now sometimes you'll find four domains around that Central atom four domains make a three-dimensional shape which we refer to as a tetrahedral and tetrahedrals tend to have angles somewhere around 109.5° another common shape that you'll come across where you have two regions around the central atom see if I can bring one of those over this we refer to as a linear shape in a linear shape we have angles between our regions of 18 180° so that's what's meant by this statement number three that the electrons repel each other and they will then take on these particular orientations lastly we come up with a concept that lone pair repel more than bonded pair let's take a look at what's meant by that idea here I have a picture of the a molecule with four electron domains and two of those domains are bonded to other atoms shown here we refer to this as a bonded domain and we refer to these that aren't connected to atoms as lone domains it turns out that these lone domains are capable of repelling more so they can push down a little bit harder on these bonded domains and cause this angle to be somewhat distorted or changed from in this case the traditional 109 let's look at a few examples of how to put this Theory into place I'm going to begin with with methane and down below here I've given you the total number of electrons that are present in the methane molecule I'm going to begin by putting the electrons into my picture there's eight of them and I start off at the bonding sites here we see a situation where we have four electron domains four electron domains suggest we form a tetrahedral shape so I'll put carbon in the middle up coming out the side this is supposed to be a wedge coming out of the plane of the paper and this one going back in so this is how our electron domains will arrange themselves as far apart from each other as possible in a tetrahedral like shape now it turns out that every single one of these domains has a hydrogen hooked onto it so as a result I end up with this shape a tetra itself with Bond angles of 109 between everything this sequence of working out the diagrams I'm going to follow in all of my examples a Lewis Dot Diagram an idea of how the electron domains arrange themselves and finally the shape of my molecule all right all right let's let's take a look at ammonia it also has eight electrons so again I'll start by putting them at the bonding sites I still have two left so I'll put them out here so this would be an example of a substance with a lone pair of electrons now the shape that this will form with four regions around it will be very similar to what we saw before nitrogen will have a tetra Al arrangement for its electron domains so electron domains there there there and there there and there three of these domains are bonded so we could put hydrogens let's say here here and here and as a result we end up with this shape and here the lone pair up top so the electrons are arranged in a tetrahedral but this molecule is called trigonal pyramidal or pyramidal shape now our bond angles in a tetrahedral they're 109 however the presence of this unbonded pair squeezes down on that 109° so we end up with something that tends to be less than 109° for our angles in this molecule let's take a look at water again the Lewis Dot Diagram so we'll start with that picture eight electrons so the first place we put them is our bonding sites I still have four left over so I'll give them to that oxygen in the middle and again we have four regions so that oxygen will have an orientation of its electron domains also in a tetrahedral like shape however this molecule will have two unbonded pairs and two pairs that are bonded this shape is called a bent or vshape again we have a distortion of the angle because of these lone pairs that can push down on that angle and again we have a situation where we would expect the bond angles to be less than 109° ozone has a formula 03 again we'll quickly do the Lewis Dot Diagram for 18 electrons we'll put two there two there complete the octets of everybody on the outside I've used up 16 electrons I have two left I give them to the atom in the center now in this particular case that oxygen in the center isn't quite satisfied so I'm going to move two electrons in to satisfy that so a look at this indicates we have three electron regions or three electron domains you might recall that forms a a uh flat triangle trigonal planer so the oxygen in the middle 120 degre between our items and I'll put a two lines here to kind of represent that Bond and we have our electrons located as such now it turns out that two of these regions are bonded say it's this one and this one this also leads to a bent shape of our molecule now as far as our Bond angles are concerned we would expect 120 in a flat triangle however this unbonded pair up top repels a little bit more and as a result we tend to get Bond Bond angles that are less than 120° and here's the last example I want to take a look at carbon dioxide so again we'll quickly do the Lewis Dot Diagram with 16 electrons in the picture bonding sights complete the outside the carbon in the center short four electrons so I'm going to move two pairs in help that out so I'm going to remove that pair and that pair and let's bring them up into here now this substance has two bonding regions I've got a bonding region there or I should say two electron domains and an electron domain there so two electron domains would suggest I'm going to form a linear shape so carbon is going to have essentially two domains around it there's four electrons on that domain and four electrons in that particular domain it's going to make a linear shape both of these ends are hooked up to other atoms and so it would tend to form the linear shape that we associate with the carbon dioxide molecule so to finish a little summary always begin with a Lewis Dot Diagram to understand how many electron domains you have around your central atom once you have knowledge of those numbers of electron domains think about how those domains are going to get as far away from each other as possible and the angles they would have and finally think about some of those pairs of Rons or domains being bonded and some of those domains not being bonded and how that will affect and distort our angle or the shape of our molecule so you need to go through thinking about all three processes to be able to come up with the correct shape thanks for watching