molecular geometry going to be the topic of this lesson and we'll start with a discussion of vesper theory which is kind of the foundation for our different molecular geometries and then we're going to go through all the different electron domain geometries and molecular geometries for two three four five and six electron domains that most of you need to memorize and then also show you how you go from a lewis structure and know which corresponds to it my name is chad and welcome to chad's prep where my goal is to take the stress out of learning science now in addition to high school and college science prep we also do mcat dat and oat prep you can find those courses at chadsprep.com now this lesson is part of my new general chemistry playlist it's an entire year of general chemistry i'm releasing several lessons a week throughout the school year so if you want to be notified every time i post one subscribe to the channel click the bell notification so let's dive in here and we'll start again with that discussion of vesper theory and if you look at it it really should be the separa theory but that just kind of sounds dumb so we say vesper theory but it's valence shell electron pair repulsion and so the idea is that electrons are negatively charged and so whether we have bonding or non-bonding electrons around some sort of central atom in a compound they want to spread out as far as possible to minimize the repulsion between them so the further they are the less repulsion they're going to experience and that's going to keep their energy low which we associate with stability so this is kind of how we explain the different shapes is we're going to look at the number of different electron groups around an atom and we'll call them electron domains and these electron domains are either going to be an atom we're bonded to and it turns out it won't matter if it's a single a double or a triple bond but just an atom that the central atom is bonded to or a non-bonding pair of electrons and those groups will spread out as far apart as possible and we're going to deal with two electron groups three electron groups four electron groups five electron groups and six electron groups which again from here on out we'll call electron domains so and you're on the hook for understanding the geometries associated from two to six electron domains so we'll start with two electron domains and when you've got two electron domains for them to spread out as far apart around that central atom as possible they're just going to be on opposite sides and they're going to be 180 degrees apart so here we'd refer to this bond angle as being 180 degrees apart from bond to bond 180 degrees and we refer to this molecular geometry as linear now for two electron domains it turns out that molecular geometry and electron domain geometry they're going to be the same thing so but we're going to start with electron domain geometries and then we'll move on and talk about a special class of molecular geometries that are kind of derived from there so with two electron domains it's going to be linear for the electron domain geometry and it turns out also for the molecular geometry we'll see the distinction between those for larger numbers of electron domains now for three electron domains it turns out the farthest you can put three things apart it turns out it is a planar structure it's a two-dimensional structure it's like in the plane of the board here and essentially all you have to do is take 360 degrees around the circle and divide it into three parts so you get three equal bond angles and 360 over three is a hundred and twenty degrees and if you look these kind of form the corners of a triangle and so they they kind of name the shapes they try to make sense out of them based on where the outside atoms all are and so with the three corners of a triangle and in a single plane a two-dimensional shape they call this trigonal planar all right four is where things get a little bit complicated because it's no longer two-dimensional if it was two-dimensional if it was two-dimensional we just take 360 degrees again and divide it by four and we get bond angles of 90 degrees but it turns out by adopting a three-dimensional geometry by instead of occupying a single plane we're going to spread out into three-dimensional space here we can split spread those angles out from 90 up to 109.5 it turns out and so if you look here so to draw a three-dimensional shape on a two-dimensional surface we have some conventions here and we call this a wedged bond right here and it's supposed to represent an atom here that's coming out of the board towards you that's why i've drawn the the atom b here very large and then we've got this dashed bond right here which is supposed to represent something going into the board away from you that's why i've drawn this really small because it's further away from you than the other atoms here and stuff so if you take a look at this shape here you can kind of see that these three form the bottom of a pyramid and they all kind of go up towards the top of this pyramid and it turns out this pyramid shape is called tetrahedral so and this pyramid is a tetrahedron now you should realize that again all the angles here are 109.5 degrees not 90 degrees and it doesn't matter which of these two i chose i chose these two because they're in the plane and it's easiest to see but i could have chosen these two the angle between these two bonds is 109.5 the angle between these two is 109.5 the angle between these two is 109.5 the angle between these two is 109.5 all the angles of any two of the bonds are 109.5 degrees so in the the the temptation is to look at this lewis kind of structure and think oh if they're next to each other they're 90 and if they're opposite from each other they're 180 but that's not true the three-dimensional shape of this tetrahedral here is that all the bond angles are 109.5 period there's no 90 and 180 so it doesn't matter which two you choose they're all 109.5 okay so this is up to four electron domains so we can also deal with both five and six electron domains but that's going to require a an expanded octet and so some of you aren't gonna be on the hook for the for these but that's gonna be the vast minority of you the majority you are gonna be on the hook for five and six electron domains as well and so if we see the way this works so you're going to have three of the five forming a triangle around that central atom kind of in this horizontal plane so this is hard for me to draw because i am artistically challenged but that's kind of a triangle formed around this in this horizontal plane around that central atom and then you're going to have one above and one straight below and what you'll find is that these three that form the triangle form a pyramid with the top one and so that would be a triangle-based pyramid and then they also form a pyramid that's inverted with the bottom one as well and so what you end up with one pyramid on the top facing right side up and one pyramid sandwiched up against it facing down and so it's two triangle based pyramids one right side up one upside down but we end up calling this therefore trigonal i can't spell apparently by pyramidal or you might hear people say bipyramidal i really couldn't tell you which one's correct but trigonal bipyramidal or bipyramidal same diff cool and again it makes sense based on the shape here with two triangle-based pyramids that we'd call it such so then we move on to six electron domains here and it's going to be somewhat similar to this and one straight up one straight down and again these four across the middle here form a perfect square that's once again in this horizontal plane so and then you have one straight up and one straight down cool now it would be nice if they called this like square by pyramidal to be consistent with what they did here but they don't so it turns out we refer to this as being octahedral so and it turns out that comes from the name of the shape so octahedral actually means eight faces and if you kind of look all these so we formed a triangular face right here we'll form another triangular face out here we'll form another triangular face out here and then there's one on the back side so there's on this top pyramid there's four triangular faces and then on the bottom pyramid you'd end up with another four triangular faces and so it has eight triangular faces hence eight faces being the name here octahedral so don't get fooled by octa here there's only six electron domains when it's octahedral now bond angles for these expanded octets are a little bit complicated because they're not all the same so but it turns out octahedral's a little easier to deal with because if you take any two that are next to each other so they're 90 degrees apart so notice like in this plane we're just splitting these four things up around 360 in that horizontal plane and they're all 90 degrees apart but if you pick two that are opposite each other or top and bottom or any two that are opposite it's 180 and so we end up with two sets of bond angles of 90 and 180. so you pick any two adjacent ones 90 pick any two opposite ones 180. it gets even a little more complicated for the trigonal bipyramidal so i saved it for last year so but these three that are in the triangular plane right in the middle here you pick any two of those and you're going to get bond angles of 120. so however they're in the horizontal plane and this guy's in the vertical plane and so if you pick any one of these three with him it's going to actually form a right angle and be 90 degrees and again he's got a right angle formed with this one and this one as well because again there in the horizontal plane he's in the vertical plane but then you could also say well relative to the one that's immediately opposite him that would be 180 degrees and so most of the time you see this defined as three sets of bond angles 90 degrees 120 degrees and 180 degrees and these are the five electron domain geometries you're on the hook for now it turns out we can actually go past six electron domains there are our different structures that do exist with more just not going to be on the hook form in a typical general chemistry class at all so uh we'll stop at six that means you guys got to know five different electron domain geometries linear trigonal planar tetrahedral trigonal bipyramidal and octahedral which apparently i erased an a here cool not only are you on the hook for knowing their names you also got to know the bond angle so again 180 degrees 120 degrees 109.5 which you really really need to get this one down this one's asked really commonly 90 120 180 and then 90 and 180 these are the five foundational electron domain geometries a little bit we're going to talk about molecular geometries and they're all going to be derived from these lovely shapes and what we'll find out is that if you start replacing any of the atoms and not really in this one but i guess in the rest if you start replacing any of these atoms with non-bonding pairs of electrons instead we'll still give it the same electron domain geometry name but the molecular geometry which is really based on where atoms are located not where nonbonding electrons are located and since we'll be missing an atom and have a non-bonding pair instead the molecular geometry is going to get a different name and so if around the central atom you start putting lone pairs that doesn't change again as long as you just count the total number of electron domains 2 three four five and six the electron domain geometries you know those five but the molecular geometries are going to get a little more complicated we will find out though is that if all your electron domains are bonding domains then your molecular geometry is going to have exactly the same name as the electron domain geometry but if you start replacing any of these atoms with non-bonding pairs of electrons your molecular geometry will get a different name than the electron domain geometry let's have some fun and take a look all right so we're going to start with two electron domains and i've drawn three different lewis structures up here and i just want to give some variety here but we really don't have any options for two electron domains it's just going to be linear no matter what you do and it's linear here it's linear here and it's linear here and so what you should realize that with two electron domains your electron domain geometry is called linear but your molecular geometry is also going to be called linear so these are going to be in a straight line here and so notice in beryllium's case here we've just got a single bond on both sides and that would be considered two electron domains because brilliance bonded to two atoms and has no lone pairs so if we look at the central atom here carbons bond to the two atoms and notice it doesn't matter that we've got a single bond on one side and a triple bond on the other side it's not the number of bonds necessarily but the number of atoms the central atoms bonded to and it's bonded to two atoms and that central atom has no lone pairs themselves so that's two electron domains and again that means the electron domain geometry is linear and they're both bonding domains so that means the molecular geometry here is linear as well and then finally in the case of carbon dioxide again it's just bonded to two atoms they're double bonds this time but that again whether it's single double triple that's just a single electron domain counting an atom you're bonded to and so it's bonded to the two atoms and then carbon has no lone pairs for a total of two electron domains once again that means the electron domain geometry is linear and the molecular geometry is also going to be linear so i just want to show you some variety that you know single bonds double bonds triple bonds all count as one electron domain each so it really is just the number of atoms the central atom is bonded to not necessarily the number of bonds that it's making cool we'll start to see some variety though when we go to larger numbers of electron domains let's take a look so now let's take a look at three electron domains and we said this earlier when you've got three electron domains your electron domain geometry or edg for short is trigonal planar so and i've got three examples here and so in this first one the central atom is boron and it's bonded to three other atoms and has no lone pairs a total of three electron domains so for the second one here the carbon here is bonded this is the central atom he's bonded to three other atoms and has no lone pairs so once again and then finally this last one that was going to be a little bit new here so the central atom sulfur is only bonded to two atoms but then has a non-bonding pair of electrons and once again that's still three electron domains so all of these would be defined as having an electron domain geometry as trigonal planar so what we'll find out is that for the first two since all three electron domains are bonding the molecular geometries have the same name but when your central atom has any lone pairs and has a non-bonding domain so if any of those electron domains are non-bonding molecular geometry is going to get a different name and we'll see why here so in this case our molecular geometry sometimes called the mg for short in this case trigonal planar the second case here also again trigonal planar same as the electron domain geometry again because all three electron domains are bonding in both cases and sometimes we'll go ahead and redraw the lewis structure lewis makes everything look like it's either 90 degrees or 180 degrees looks like these two are 90 degrees apart it looks like these two are 180 degrees apart well the truth is no knowing that it's now trigonal planar we'd know that all the bond angles are 120 degrees and so we might draw a structure better representing the molecular structure from here on out so but again lewis says oh everything's 90 or 180 the way i draw them so but the truth is again we have to now factor in some different things we know about the molecular geometries and electron domain geometries based on the number of domains now in this second one here we might do the same thing and spread this out a little bit to represent that trigonal planar structure but one thing you should know is here we've got a double bond to the oxygen so that means there's four electrons there not just two like there is here and here and so there's more repulsion going on from four electrons than there is from two and what's going to happen is because we're going to repel these two bonds down further according to vesper theory here that this angle right here is going to be a little greater than 120 degrees and same thing with this one here a little greater than 120 degrees and then that's going to force these two to be or these two to be closer and this angle to be a little less than 120 degrees and so you should know that pi electrons when you have like a oh i don't i don't want to say pi electrons we'll learn about that a little bit when you've got a double or triple bond it's going to typically lead to greater repulsions in a structure like this and so the bond angles aren't going to be exactly 120 in this case we'll also find out that lone pairs here in a sec are going to have a similar effect so we'll take a look at so2 here and so for so2 if we were to draw a similar representation to kind of take into account the shape here because these options aren't 180 degrees apart turns out they're roughly 120 degrees apart based on what we know about the trigonal planar electron domain cool now it turns out that lone pairs tend to lead to a greater repulsion than bonding electrons uh and in this case this bond right here this or this angle between the bonds is probably not exactly 120 and truth is it's probably slightly less then 120 degrees now you might be like well chad there's double bonds there what about what you just said well it's true but it turns out lone pairs usually have a greater repulsion and so i couldn't tell you what the exact bond angle here is but it's probably ever so slightly less than 120 and one thing you should know when i say slightly less than 120 it might be like 118 so it's not a huge difference usually a couple of degrees kind of a thing now if we look at molecular geometry and we haven't put a name on it here but what i can tell you is that the molecular geometry here is not going to be called trigonal planar so because we don't even form a you know a triangle in the same way from the outside atoms in this case if we look at the shape when you've only got three atoms in your total in your structure you've got two options it's either going to be all straight in a row which we learned to call linear or they're not going to be in a row which case we'd call it bent and so in this case this molecular geometry is called bent and the idea again is that you know when we had three atoms three outside atoms forming a triangle trigonal planar so but now when one of those atoms is not there again for molecular geometry we look at where the atoms line up not where lone pairs of electrons line up and these three atoms again are either linear or they're bent and in this case they're definitely not 180 degrees apart linear so they are bent we'll see this gets even a little more complicated as we get larger and larger numbers of electron domains so let's take a look at four electron domains all right so with four electron domains your electron domain geometry is tetrahedral and all three of these examples have four electron domains in the case of ch4 here they are all bonding domains in the case of nh3 there's three of them that are bonding domains and then one nonbonding domain and then in the case of water here h2o we've got two bonding domains and then two non-bonding domains and so for the first one here because all four electron domains are bonding the molecular geometry is going to get the same name as the electron domain geometry and be called tetrahedral so and you'll find out especially if you take organic chemistry that we get you know pretty involved in drawing accurate three-dimensional portrayals of these molecules and so if we kind of match this up with what we did earlier you could kind of draw something along the lines of that cool so but most you aren't going to be on the hook for something like this when we start getting three dimensional here you're probably just going to go with lewis but again one thing i really want to point out and focus on again is that if i say what is the bond angle between these two hydrogens or at least the bonds for those hydrogens it's 109.5 and if i say what's the bond angle between these two don't say 180 again all the angles are 109.5 regardless of what mr lewis says mr lewis makes everything either look 90 or 180 but you have to take what you've learned now about electron domain geometry and molecular geometry to know what the real bond angles are what's the angle right here 109.5 what's the real angle right here 109.5 okay so now we're going to put a lone pair on there and that's going to change a couple of things so one again it doesn't change the electron domain geometry four domains it's still tetrahedral but it is going to change the molecular geometry and so first thing we'll do is we'll draw the shape here try and represent it and we're going to have a hydrogen here here and here and then the lone pair of electrons and if you connect all the atoms here what you're going to find out is that you still got a triangle base but then kind of the only other atom is the nitrogen so going up to that nitrogen well if it was going up to another hydrogen like you didn't here you'd have a nice you know full pyramid if you will and it'd be a tetrahedron but now it's kind of a flattened pyramid because instead of capping up here to another hydrogen it's just going up to the nitrogen which is much shorter in the one dimension and so this is not a tetrahedron a tetrahedron is the same in all dimensions and so in this case it turns out that the molecular geometry is going to be called trigonal pyramidal or pyramidal again depending on who you talk to and so it's a triangle based pyramid but again it's not a perfect tetrahedron because it's not the same in all dimensions it's kind of like a flattened pyramid if you will and again we call that trigonal pyramidal and so this is ammonia it turns out nh3 and if if somebody says hey what is the electron domain geometry of nh3 you're supposed to say tetrahedral but if somebody says what's the molecular geometry you're supposed to say trigonal pyramidal you're also supposed to know again that the lone pair gives greater repulsions to the other electron domains than they give to each other and so as a result this is going to kind of flatten these angles down a little bit and so this angle right here instead of being 109.5 is going to be ever so slightly less than 109.5 and it turns out it's actually right around 107 degrees so you don't actually have to know that it's 107. the only reason i put it up there is that again i want you to know that when i say less than 109.5 i don't mean like 50 or 90. i mean like two and a half degrees like 107. so just a couple of degrees this this difference but you should know that it's not exactly 109.5 they're exactly 109.5 here in ch4 but here the bond angles are just slightly less than 109.5 and again you're not on the hook for knowing it's 107 you're on the hook for knowing it's just slightly less than 109.5 moving on to water here so in water here a lot of students look at this and they think oh those hydrogens again are 180 degrees apart well again they're not all the angles no matter what two atoms you choose and we only have two atoms it's roughly 109.5 but once again it's not going to be exactly 109.5 because the lone pairs are going to change things you might be like well chad aren't those lone pairs opposite each other well again those lone pairs are roughly only 109.5 degrees apart regardless of what lewis makes them look like if we actually draw a better representation of the geometry in water often see us represent it like this and this angle again right here below between the bonds that bond angle is going to be less than 109.5 degrees and again just slightly less and it turns out if you cared it's actually like 104.5 degrees which again you're not typically getting on the hook for but what i wanted to show you though is that now that i've got two lone pairs there's even more repulsion it's going to push those hydrants closer together so that now it's even another two and a half degrees lower at 104.5 degrees and so big thing take away is that you're supposed to realize that when you put lone pairs on the central atom it's going to lower the bond angles usually by just a little bit so sometimes you get a question that says which of the following has bond angles of exactly 109.5 well he does and nh3 and water don't so but if i said which of these three has bond angles that are just slightly less than 109.5 well now nh3 and water do whereas ch4 does not cool so that's four electron domains let's go now to our expanded octets and take a look at five electron domains so with five electron domains now reminder that the electron domain geometry is called trigonal bipyramidal or bipyramidal and now we've really got four different options for molecular geometry so we can have all five electron domains be bonding we can have four bonding and one nonbonding we can have three bonding and two nonbonding or we can have two bonding and three non-bonding so and there's something funky it turns out about the trigonal bipyramidal shape but where you put the lone pairs is going to matter not all five positions it turns out going around are equivalent and so we'll remember something about that so with this first one life is not so bad the molecular geometry here so because all five domains are bonding it should get the same name as the electron domain geometry and that is totally true and so our molecular geometry is still trigonal bipyramidal cool and the angles are either 90 120 or 180 just like normal now here's the deal so if you recall these three kind of form a triangle in the horizontal plane and then you have these pointing straight up and down well it turns out they get special names for the ones that point straight up and down they are referred to as being axial and so if you look at like your you know if you know your anatomy your axial skeleton is the skeleton that runs right up the vertical of you includes like your spine and your skull and stuff like that so so these two are referred to as the axial positions they're 180 degrees apart so but then you've got these three so right here in the middle that kind of go around the equator of the molecule if you refer if you realize and so they're often referred to as being equatorial well if you recall we said that lone pairs of electrons give off greater repulsion than bonding electrons and so when we get to starting to put lone pairs around the central atom where they go actually matters with these two different positions and so uh it turns out they're going to preferentially go in these equatorial positions so that we can minimize the repulsions so they're going to experience less repulsion when we put them equatorial than us if we put them axial and so you've got to know and it's going to affect the molecular geometry and kind of the shape it actually adopts then when we start putting lone pairs in they're going to preferentially start adopting these three positions not the axials and so notice once we go to one lone pair it had to just go in one of these three positions to two lone pairs again it had to go into two of these three positions and then with three lone pairs all three equatorial positions have the lone pairs the axials still get the atoms so super important that we know that and this is only something to worry about for trigonal bipyramidal electron domain geometry so five electron domains now visualizing this here so if you kind of take this right here and turn it sideways so you'll find out that these two again are 120 degrees apart but then these are going to form like a table top surface so these would be like the legs of the table and this would be like a table top surface looking like a saw horse or a seesaw depending on who you talk to and so it goes by both names you can call it seesaw or sawhorse cool so you could kind of envision it turn it sideways and you put one kid on this and one kid on this end and they're just teeter-tottering back and forth legacy so if you will or it's a sawhorse kind of that tabletop surface with a couple of legs although most the sawhorses i've deal with have two sets of legs but whatever goes by both names that is the molecular geometry associated with having five total domains but four are bonding and one is non-bonding okay moving on to the next one here again now two of the equatorial positions have lone pairs and if you look here we know that these two chlorines in the axial positions are roughly 180 degrees apart and then this guy's in the horizontal plane so if these are in the vertical plane and he's in the horizontal they should be 9 degrees apart here and 90 degrees apart here and again these are 180 apart from each other and if i turn this sideways i'd say they form the perfect letter t and that's exactly what we call this we call this t shaped so again the electron domain geometry for all of these is trigonal bipyramidal but the molecular geometry is going to get a different name if there are lone pairs on that central atom so again trigonal bipyramidal when all of your electron domains are bonding so with one non-bonding four bonding seesaw or sawhorse with three bonding two non-bonding t-shaped and then this last one here so this will be important i'll loot back to this in a later lesson in this chapter so for this one here all three equatorial positions um are lone pairs and so we just have these two fluorines and the xenon and in this case with three atoms we said this earlier with three atoms they're either in a straight line we call it linear or they're not in a straight line and we call it bent well in this case these in the two axial positions they really are 180 degrees apart and if they really are 180 degrees apart then this is going to be called linear and so this is the second electron domain that we're seeing called linear here so earlier we saw that there were two different molecular geometries that were called bent one with three total electron domains and one with four total electron domains and now we're seeing that there's two different molecular geometries called linear as well that are possible okay so that's five electron domains and now let's get ready and move on to six electron domains so now we've got six electron domains and our electron domain geometry is going to be octahedral for each of these and uh we're either gonna deal with all six electron domains being bonding and again if all your electron domains are bonding then your molecular geometry gets the exact same name as the electron domain geometry so in this case sf6 here is going to be octahedral so however in these other two if5 here which has one lone pair in the iodine and then xcf4 which has two lone pairs on the xenon so they both still have six total electron domains but here five bonding one nonbonding here four bonding two non-bonding and the molecular geometries are definitely gonna have a different name than the electron domain geometry so these are not going to be called octahedral for the molecular geometry and it turns out if you take a look at this one right here so and again it turns out with octahedral this is different than trigonal bipyramidal turns out all the positions are equivalent there's no like axial and equatorial because if you just turn this thing 90 degrees then what would be axial would now be equatorial and vice versa and all the positions are equivalent so this is different than trigonal bipyramidal in that case so if you put on one lone pair it doesn't really matter where you put it so i just chose to put it down here i could have put it up here or in any one of these positions as well but it's easiest to see the shape here so you can kind of see that again these are the four that are kind of in that horizontal plane forming a square and then they kind of form a pyramid all towards this one up here and so it's a square based pyramid and so the molecular geometry is actually called square pyramidal or again square pyramidal depending on who you talk to okay now when you go to two lone pairs the big thing you do need to remember though is that those lone pairs experience the greatest repulsion and so you don't want to put them only 90 degrees apart you want to make sure you put them 180 degrees apart so going back to the basic shape i chose to put top and bottom but i could have chose these two to be the lone pairs or could have chose these two to be the lone pairs and it would be the same thing regardless but you'll definitely most commonly see it drawn this way but you definitely just have to make sure those lone pairs are drawn on opposite sides 180 degrees apart and so the four outside atoms that are left just form a perfect square and this is actually a two-dimensional structure for the atoms it's just a square all within a single plane and so we call it square planar cool and we have now done an example of every single molecular geometry you are on the hook for so you need to memorize all of them so you need to be able to look at a lewis structure and count the number of electron domains and right off the bat that should tell you the electron domain geometry and then you should be able to look and say okay well how many are bonding how many are nonbonding and therefore identify it further and get the correct molecular geometry as well so definitely some memorization here you're on the hook typically for all of it they could give you any sort of lewis structure and then ask you the molecular geometry they could also take this a step further they could also just give you the chemical formula like xcf4 and then expect you to draw the lewis structure so that you can determine the molecular geometry or the electron domain geometry and so you want to get really good at these lewis structures from the last chapter so because oftentimes they're going to be the first thing you have to do in answering a question on molecular geometry as well now if you found this lesson helpful then hit that like button and if you like puppies then you should hit that 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