this video is an introduction to Vesper Theory Vesper theory is a set of rules that we can use to look at a two-dimensional leis structure of a molecule and figure out what the molecule would look like in three dimensions like this cuz molecules are like actually things in real life so they'd have three-dimensional structures that are often more complex than we can draw in two Dimensions here so let's start take a look at some Lew structures and figure out what the 3D shapes of those molecules would be here's our first example burum D chloride we got a burum atom it's a central atom surrounded by chlorin on either side note that burum here is an exception to the octet rule which means that it's happy to have fewer than eight electrons in its veence shell when burum is making two bonds like it does here with chlorine it has only four electrons in its veence shell and it's perfectly happy with that so just keep it in mind but it doesn't have any important bearing on what the vasper shape is so a little bit about the Vesper rules here Vesper stands for veence Shell electron pair repulsion which is a really fancy way of saying that electrons or pairs of electrons want to push away away from each other and want to be as far away as possible from each other and that kind of makes sense cuz electrons have negative charges so opposite charges repel and obviously these things are going to want to be far away from each other let's look at what bearing this has on the three-dimensional shape of a molecule so in buril here where are these veent shell electrons that want to push away from each other well the veence shell electrons are in these bonds I've often said that you can think about coal bonds as if they're hands from the atoms with electron Pairs and that these hands are connected because they're both holding on they're both sharing this pair of electrons so we could draw burum D chloride like this where we have a hand from the burum a hand from the chlorine coming together to hold to share this pair of electrons so this just reinforces the idea that there is a pair of electrons in each one of these bonds that shared between the atoms okay so as we said from Vesper these electrons want to be as far away from each other as possible they want to repel so how is this going to influence the 3D Shape of this molecule how can these B bonds arrange each other in arrange themselves in 3D so that they are as far away from each other as possible the three-dimensional shape of burum D chloride is going to look like this we've got a burum here in the middle and then we have these two chlorians on either side and all three atoms form a line they're all in a row here a straight row we call this a linear molecule which means line so that kind of makes sense and let's look at the angles here the angles between these two bonds going to be 180° so 180° between these two bonds is how the electrons that are in these two bonds it's how they can be as far away from each other as possible so we can say that this linear shape that we have here is the way that two things are going to surround elv around a central atom Central atom is here then we've got these two things that are two bonds and the two bonds are as far from each other as possible in this linear shape now in buril D chloride I'm talking about single bonds here but it actually doesn't matter whether we got double bonds or triple bonds for example CO2 has a shape like this where there's a double bond here and a double bond here but there are electrons in both of these bonds and so each one of these double bonds they just count as a bond so for CO2 I still consider it as just two things around a central atom so CO2 it's going to have this linear shape here too this will be the carbon and these will be the two oxygens they'll be 180° apart so double bonds don't worry it's just two things one to around a central atom okay just to drive this point home trip bonds it's the same thing got a triple bond here a single Bond here I just consider this to be two things around a central atom one 2 so hcn is going to have this linear shape as well with these two Ang these two bonds being 180° apart so we always get a linear molecule 180° whenever we have just two things surrounding a central atom now let's take take a look at some molecules where we have three things that are surrounding a central atom here in BF3 I have a central atom surrounded by three bonds to other atoms and in this case Boron like buril before is an exception to the octop rule here Boron when it's making three bonds has six veence electrons it's totally happy with that so as I said earlier when we were talking about burum is that we can think about these bonds between the Boron and the Florine here as hands that are sharing an electron pair and the electron pairs in each one of these bonds push against each other and they want to be far away so when we have three things the electron pairs in these three bonds how do we arrange these so that they are as far away from each other as possible in 3D the molecule is going to look like this I'm going to have these three atoms 1 2 3 surrounding a central atom and I'm going to get the shape called trigonal planer the trigonal comes from the fact that there are three one two three things that's just what trigonal means and planer because take a look at this these atoms are all arranged in this plane all right they're all in a straight plane here now what are the angles between the atoms in a trigonal planer shape they are all 120° so the angle between here and here is 120 here and here and here and here so that is what BF3 would look like in three dimensions now just as before it doesn't matter whether we're talking about double bonds single bonds triple bonds it's all the same okay so in ch2o here I have three things surround rounding the central atom I got a double bond a single Bond and a single Bond but it's still just three things that want to be as far from each other as possible so that means that ch2o here is going to have the same shape in three dimensions as BF3 does it's going to be a trigonal planer molecule with 180° between each pair of bonds now next thing we're going to do is we're going to look at some molecules that have three things around them okay but these three things are not all bonds here's an example of this S2 okay it's got three things around this Central atom it's got a bond here that's one thing a double bond here that's two things but then it's got this unshared electron pair up here these three things all have electrons in them so they all want to push away from each other so what shape is SO2 going to have in three dimensions if you think it's linear in the three of these atoms are all lined up in a row that's not right because you're not taking this unshared electron pair into account to figure out the shape of this let's go back to this trigonal planer molecule okay in this trigonal planer molecule we had three things around a central atom it's just they were all other atoms okay so this is how you arrange three things around a central atom to be far away from each other now in SO2 we're going to get a shape that's very similar except it's that one of these atoms from the trigonal planer shape is going to have been replaced by an unshared electron pair but otherwise look at how similar they are okay it's atom atom atom atom it's just this atom here has been replaced by this unshared electron pair but these at atoms are still in the same place because this unshared electron pair pushes the atoms away from each other just like this atom did okay so they're based on the same shape three things around a central atom it's just one of these from the trigonal planer shape has been replaced by an unshared electron pair okay so this molecule here we call this a bent molecule because instead of being in a straight line the atoms are arranged in this kind of bent shape looks like someone just grabbed it and bent it like that so what are the angles going to look like in the bent molecule well in the trigonal planer molecule over here when you had three atoms the angles between any two Bonds were 120° in the bent molecule though it turns out that this unshared electron pair here pushes harder against these two atoms than the atom up here would okay and so that means that the angle between these two bonds is going to be a little bit less than 120 because the atoms are getting pushed closer together it's going to be less than 120 it's going to be more like about 116° between the two of these just once again because this unshared electron pair is pushing harder than this atom so instead of 120 they're pushed harder pushed closer together and it's more like 116 but here's a point we get the bent molecule when one of these three atoms from the trigonal planer is replaced by a lone electron pair so always keep your eye on these lone electron pairs because they have a very significant impact on the shape that a molecule is going to end up having now let's move on to some molecules that have four things around the central atom CH4 here has four things around a central atom and they are all bonds to other atoms so each of these bonds contain a pair of shared electrons and that means that the bonds all want to push against each other and be as far from each other as possible this molecule CH4 is going to have this shape in three dimensions okay this is called a tetrahedral shape and it's how you arrange four bonds as far away from each other in 3D as possible okay tetrahedral and in the tetrahedral molecule there are 109.5° between any two bonds that are next to each other in this molecule so 109.5 here 109.5 and so on so four things four things around a central atom you get a tetrahedral shape with 109.5 degrees between Each Bond now NH3 here also has four things around a central atom but not all of them are bonds to other atoms okay so we have 1 2 three bonds and then a fourth thing that's alone electron pair so what's its shape going to look like in three dimensions I'm going to go back to this tetrahedral shape for just a minute because this is how we arrange four things around a central atom when they're all other atoms okay but in this shape they're not all other atoms okay so NH3 is going to end up having this shape which is called a trigonal pyramidal shape look at how similar it is to the tetrahedral shape okay I'm sort of showing them on their sides here it's just that the atom that was up here when we had four atoms around the central atom has been replaced by an unshared electron pair here okay so we've got three atoms three atoms are the same between this and this and it's just this atom has been replaced by an unshared electron pair so this has a shape that we call trigonal pyramidal and we call it trigonal pyramidal because if you look at it from its side it kind of looks like a pyramid okay got these three atoms pointing down all right now for angles in I don't know quite I would to put this I put it up here I guess for atoms in the trigonal pyramidal for angles in the trigonal pyramidal mod molecule we'll remember that in the tetrahedral we have 109.5° between all of the bonds but a trigonal Pam midle just like we saw with a bent molecule the unshared electron pair here pushes a little harder against these two bonds than an atom would and so that means that the angle between these bonds is pushed a little tighter and so it's smaller than 109.5 for a trigonal pyramidal molecule like NH3 the bond angle is more like 107° little less than 109.5° so four things if you have four things around a central atom but three of them are bonds and one of them is a lone electron pair you end up with a shape it's called trigonal pidal that looks like this okay one more example and then we're done with done with a Vesper video here's the last molecule we're going to look at water H2O okay this thing has four things around a central atom two of them are bonds one two and two of them are lone electron pairs one two so what's its 3D shape going to look like how can we arrange these four things as far away from each other as possible in three dimensions as before I'm going to look back at my tetr eral molecule which shows how I arrange four atoms or four bonds as far away from each other as possible in 3D for H2O though only two of the four things are bonds the other two are lone electron pairs okay so that means that I'm going to end up with a shape like this okay I've got my two atoms down here hydrogen and hydrogen but then I've got my two lone electron pairs up here look at how this is similar to the tetrahedral molecule if I look at them from the side okay it's got I've got atom atom and atom and atom okay it's just these two atoms from the tetrahedral molecule have been replaced by these two lone electron pairs from the water molecule these unshared electron pairs on the oxygen here okay so look at that from the top how they have very similar structures it's just these two are missing and they've been replaced by the lone electron pairs we say that this molecule has a bent shape because these Mo these atoms here are not in a straight line but they're bent like this now what what are the angles here let's look at the tetrahedral again which had 109.5 then in the trigonal pyramidal when we had one electron pair it pushed the bonds a little bit closer together so we had about 107° between them a little less than 109.5 now when we have bent instead of one unshared electron pair like in the trigonal planer I have two lone electron Pairs and so the combination of those two is going to push the atoms even a little bit closer so in a bent molecule like water the angle between them is going to be 105 degrees about 105 degrees Which is less than 107 Which is less than 109.5 so the more unshared electron pairs you add the tighter the two or more atoms get pushed together okay so if you got two bonds and two lone electron pairs around a central atom you're going to have this bent shape now there's just one thing that I want to say about this bent shape okay there are two ways that we can get a molecule with a Ben shape but they're different okay we can get a bench shape when we have three things around a central atom and one of them is an unshared electron pair then we get something like SO2 where we have something that's a little less than 120° between these another way to get a bench shape is when we have four things around a central atom but two of them are lone electron Pairs and in that case because we have four things everything's a little tighter we have an angle of 105 degrees that's a little less than 109.5 so you just finished watching this Vesper video where should you go from here well the first thing that it's important to know is that there's a difference between watching the video and actually being able to look at a leis structure and figure out what the 3D Shape of that molecule would be so the first video that you should watch is this Vesper practice problems video where we'll go through a whole bunch of Lewis structures and go through the steps to figure out what the three-dimensional shapes will be so that's definitely the next thing you should watch now there are some common mistakes that students often make when they're learning Vesper so I made a video on that called Vesper common mistakes watch that after you've done the practice problems to make sure that you're not falling into any of the common traps that tend to trip people up when they're learning Vesper now maybe this is all of the stuff that you have to learn for Vesper and this is everything up to molecules that have four things around a central atom but depending on what you have to know you might have to know molecules where there are five things like this around the central atom or where there are six things like this around a central atom so I made some other videos on this sort of stuff okay I made the video a video on the trigonal bipyramidal family which are all of the molecules that have five things around a central atom and then there's another video on the octahedral family which are the molecules that all have six things around a central atom and now finally after you've watched the video on the trigonal bipyramidal and the octahedral you can do the Vesper practice problems uh for these Advanced structures where you where you'll go over the uh the molecule shapes I talk about in this video in this video so this uh this might look a lot this might look like a lot but if you go through it it should really give you a solid foundation with this threedimensional V asper stuff