Molecular Shapes and Electron Pair Geometries

Added: 2026-05-11

[00:00:03]all right guys so let's do a quick recap from Monday of drawing dot structures for molecular compounds remember these are going to be sharing electrons to fulfill the octet so looking at what we have here um the sulfite ion the SO3 and A2 I'll do that one first remember we have to figure out the number of valence electrons so and the number of valence electrons is based off of the column or what group of the periodic table that element is in sulfur is in group 6A so it has six valence electrons add to that oxygen is in that same group so it also has six valence electrons but there are three oxygens that accounts for 24 valence electrons but because this has that negative two charge remember the charge means that there is either more or less valence electrons and you always do the opposite of the sign so since it's a negative 2 charge that means that we gain two electrons so this structure is going to have 26. valence electrons when we draw this out we want sulfur whatever comes first tends to be in the middle unless it's hydrogen we're going to put our three oxygens around the outside we connect everything to the center atom so that's 2 4 6 out of our 26 electrons so we have 20 left then we said you have to fill the octet of all of the outer atoms so I'm looking at the oxygen on the left since it already has a bond that is two electrons it needs six more to have a full octet so we can add six electrons and we draw them in pairs so now we're down to 14 valence electrons left let's look at the oxygen on the bottom same thing it already has a bond it already has two electrons so it needs six more now we are down to eight valence electrons so let's fix the oxygen on the right that already has a bond which is two electrons so it needs six more now I have two valence electrons left so he said once you've filled octet for all the outer atoms anything that you have left over is going to kind of dump on the center atom and yet again draw these in pairs so here I've drawn all 26 valence electrons you want to make sure that everything is full with that octet a valence electrons so all three oxygens have eight electrons and since we added that pair of dots on sulfur it also has eight valence electrons the last thing we have to do because this has a charge is we need to put it in Brackets with the charge on the outside let's look at the next one we have scl2 for valence electrons sulfur we said was in group 6A so it has six valence electrons then we have chlorine which is in group 7A so it has seven valence electrons but there are two of them so 2 times 7 is 14. plus 6 is 20 valence electrons here and for our skeletal structure sulfur is going to go in the middle with the chlorines kind of hanging around on the outside we said we connect everything to the center so that's four of our 20 electrons we have 16 left we need to fill the octet of our outer atoms so the chlorine on the left since it already has a bond which is two it needs six more so now we have 10 electrons left and then the chlorine on the right same thing it already has two so it needs six more so we have four valence electrons left any time we have leftovers we're going to put them on the middle and yet again just make sure that you draw these in pairs it doesn't matter the location of the pairs though um so here I've drawn all 20 valence electrons everything has a full octet so I don't need any double or triple bonds and since it doesn't have a charge I don't need brackets let's look at the last one ammonia which is NH3 nitrogen has five valence electrons and hydrogen is in that group one so it only has one valence electron but there are three of them so this has a total of eight valence electrons for our Dot Structure nitrogen is going to go in the Middle with our hydrogens kind of hanging around on the outside we need to connect everything to the center so that's two four six I have two valence electrons left over when you make sure all of our outer atoms have a full octet now remember hydrogen is an exception where hydrogen only needs two so once a bond is attached to hydrogen it is full because it is an exception and can only have up to two you're never ever going to put dots on hydrogen so all three hydrogens are going to be full so my other two electrons I have are automatically going to go on the nitrogen yeah I'm going to come back to these drawings in a little bit um as examples for um structures that we're going to talk about today all right so we're going to continue today and we are going to add on to those dot structures that we drew on Monday um so we're going to look at bonding theories this looks at what the 3D model of these dot structures would look like um with bonding we end up with molecular orbitals these are going to be produced when two atoms combine and their atomic orbitals overlap so their SPD and F orbitals that we had electrons in when they overlap they kind of merge together to rearrange the shape of the overall molecule and molecular orbital does apply to a molecule as a whole so we're not going to be looking at the shape of the individual orbital anymore we're going to be looking at the shape of the molecule as a whole once they're bonded um and molecular orbital that contains a pair of shared electrons or a covalent bond is a bonding orbital so these bonding orbitals are the ones that overlap and are going to alter the shape of our overall molecule so to determine the shape of our overall molecules we're going to look at the Vesper Theory Vesper stands for valence shell electron pair repulsion Theory so according to the Vesper Theory since electrons are all negatively charged they are repelling one another they don't like to be close together so the repulsion between electron pairs causes molecular shapes to adjust so that valence shell electron pairs stay as far apart as possible so the individual orbitals and spaces where electrons can be housed will spread as far apart as possible depending on whether there's an atom there or if there's another electron pair there that it might be repelling against so each molecule is going to take a shape depending on how many bonds it forms and how many unpaired electrons it has and to determine the geometry shape of a molecule you first have to draw the Dot Structure so even if it doesn't tell you to draw the Dot Structure but it asks you what the shape or geometry is you need to draw it first because that's what you're going to use to gauge what the shape of that molecule would be so for molecular shape um remember you have to draw the Dot Structure first to determine molecular shape and in a little bit we're going to do electron pair geometry you have to have the Dot Structure first so molecular shape is the three-dimensional shape of the molecule according to how many atoms are attached to the central atom so you're going to look at how many atoms are attached to the center first whenever you're trying to identify the shape there are five shapes that you're responsible for knowing and identifying and we're going to go through each one of these to look at what Dot Structure what things does the shape need to have or the Dot Structure you have to be that certain shape so the five you have to know are linear bent trigonal planar trigonal pyramid and tetrahedral so those are the five shapes that you need to be able to identify so let's look at a quick kind of flow chart for how to identify these and I'm going to do a rough sketch of what some of these may look like just so you can kind of see and we'll go back in a moment and apply them and actually apply the shapes to the dot structures that we drew earlier today and on Monday so whenever you're trying to determine the shape according to Vesper you first want to see how many outer atoms are bonded to the center atom so how many atoms are around that Central atom if there are only two atoms attached to the center it is either going to be a linear or a bent shape those are only options when there are two atoms how you determine the difference between them is then you look and see are there any unshared electron pairs on the central atom so are there any dots on the central atom if there are no dots on the center atom and your Dot Structure looks something like that where there's two atoms on the outside and no dots on the center it is going to be a linear shape whereas if there are dots on the center so something like this or there might be more than one pair of dots on the center those would have a bent shape so linear does not have dots on the center bent does have dots on the center and we're going to talk about this a little bit more but the whole reason is those electron pairs on the center are repelling those attached atoms and pushing them as far apart as possible so it will alter the shape of it now if there's three atoms attached to the central atom it is going to be trigonal um and yet again you have to then ask yourself are there unshared electron pairs on the center atom so are there dots on the center atom if there are not dots on the center atom and they're just three atoms attached it is going to be trigonal planar so this I say that the central atom is plain it does not have dots so it's trigonal planar whereas if there are dots on the center atom those dots take up space and push all the atoms out of the way so it ends up being a trigonal pyramid shape and for our purposes if there are four atoms attached to the center is going to be a tetrahedral shape for us so this is kind of your thought process of when you're trying to identify the Vesper shape first count how many atoms are there and then are there dots on the center atom yes or no that will kind of help you identify these and yet again we'll look back at our examples from Monday and earlier today and identify their shapes a little bit all right and here you'll kind of see those shapes again this is just kind of looking at a model of them um similar to what you might have built in your molecular model lab you guys only need the first three rows the bottom two are those expanded octets but it does show you that the ones in green are the outer atoms red is our Central atom and if there is a blue oval that is a pair of dots on the center atom so you can kind of see how these arrange in three-dimensional space um so our linear is truly flat straight across um trigonal planar think of like a circle cut into thirds is trigonal planar um if one of those spaces is taken up by an electron pair where it's two atoms an electron pair that is that bent structure there's also a bent structure you'll see this in the third row at the far right there's a bent structure with two atoms and two electron pairs that I sketched out um if there are four atoms attached it's going to be tetrahedral and like I said if there are three atoms an electron pair it ends up being that trigonal pyramid um I do have this in our module as well if you need to refer back to it with our structures you also need to be aware of the bond angles so how much space is between those atoms that allows them to spread out as far as possible you guys don't have to know the bond angles for all of the shapes only these three um tetrahedral is probably the hardest one to remember this is when people tend to forget because we're not looking at a flat circle that is cut into Force we are looking at three-dimensional sphere that is cut into fourths um so the tetrahedral Bond angle that angle between every outer atom is going to end up being 109.5 that's the angle that you just have to memorize it's one of those that's kind of hard whereas linear you guys have done this in math if you have a straight line that angle is 180 degrees between those outer parts of the atom and then trigonal planar think of a flat circle split in the thirds and each kind of section of that Circle is going to be 120 degrees all right we're also going to have to identify the electron pair geometry for these um some of them or some dot structures have a shape and geometry that are identical but not always so electron pair geometries are determined by looking at how many electron groups are around the center atom or how many things around the center atom this is going to include how many atoms around the center atom plus How many pairs of electrons around the center atom so you're just going to add those numbers together and there are three different geometries you're going to have to be able to identify if there are two total electron groups around the center atom it is going to have a linear electron pair geometry if there are three total groups around the center atom is going to have a trigonal planar electron pair geometry and if there are four total groups around the central atom is going to have a tetrahedral electron pair geometry when I said let's go back and look at some of the dot structures that we did and we are going to find their shape geometry and also look at their bond angles if necessary um so one that we drew yesterday was our diatomic oxygen and it looks like this um here for the Vesper shape this does not follow the general rules that we talked about because there's only two atoms present there is not a central atom but if you think if you have two points that are connected they make a line so if there's only two atoms its shape is going to be linear however it's electron pair geometry let me it does not have one because it does not have a central atom for those groups to be around so anytime you have two atoms only its shape will be linear and it does not have electron pair of geometry since it does not have a central atom all right and if we look at carbon dioxide carbon dioxide had those two sets of double bonds four molecular shape there are two atoms attached to the center atom so it is either going to be linear or bent or our two options there are not dots on the carbon so remember when there are two atoms and no dots is going to have a linear molecular shape or the electron pair geometry it said that we can count electron groups around the center and I'm going to circle them that way you can visualize them a little bit and if you struggle with this I definitely suggest circling them so for electron pair geometry we said you count how many atoms are attached so there are two oxygens attached to the carbon so those are two electron groups there are no dots on the carbon so there's only a total of two electron groups so that's where this has an electron period geometry of linear so one thing that you will notice as we go through these if there are not dots on the center atom the molecular shape and electron pair geometry are going to end up being the same that is when there are no dots on the central atom all right so looking at the carbonate ion that we drew four molecular shape it does have three atoms around the central atom so it is going to be trigonal something but we need to signify is it trigonal planar or trigonal pyramid so remember we have to look at the center atom and ask ourselves are there dots on the central atom are there those unshared pair of electrons here there are no dots added onto carbon so carbon is playing so this is a trigonal planar shape um four electron pair geometry remember we said you count how many atoms around the center and I'm going to circle the different electron groups there's three atoms around the center atom no electron pairs around the center atom so here yet again whenever they're not dots on the center the shape and geometry are going to be the same so this also has a trigonal planar electron pair geometry if it were then to ask us for the bond angle since its trigonal planar there's going to be 120 degrees between each of those outer atoms so that could be kind of a secondary question that gets asked all right so let's look at our other examples we did yesterday we had water which had this Dot Structure with two hydrogens attached and then two electron pairs on the center for molecular shape remember we're going to ask ourselves how many atoms are attached to the center here there are two hydrogens attached to the center oxygen so two atoms attached to the center is either linear or bent here it does have dots on the center atom so this means that it is going to have a bent molecular shape four electron pair geometry since there are dots on the center atom the geometry is not going to match the shape so we said you count how many atoms are on the center so there are two atoms around the center atom and then count electron pairs on the center so there are also two electron pairs so this has four total electron groups around the center so four groups was a tetrahedral electron pair geometry looking at SO2 down below for shape there are two atoms attached to the center so it is either linear or bent there is a pair of electrons on the center atom mean that this also has a bent shape even though it looks slightly different than the Dot Structure above four electron pair geometry we need to count how many atoms are on the center atom so there are two atoms around the center and then count how many electron pairs there are there's one electron pair here so there are three total electron groups here so this has an electron pair of geometry that is trigonal planar so even though both of these are bent their geometries can be different because there's a different number of groups around the central atom all right so looking back at the examples that we did at the beginning of the video we have the sulfide ion the SO3 native two if we want to determine its shape and geometry so for shape it does have three atoms attached to the center so it is trigonal something but remember we have to signify is it planar or pyramid this does have a pair of dots on the center atom meaning that it's going to push all those other atoms out of the way so this is going to be a trigonal pyramid shape or geometry we need to count how many atoms around the center atom so there are three atoms around the center atom and an electron pair so there are four total groups around the center atom so this is a tetrahedral geometry the scl2 at the bottom is going to be the same as water that we did a moment ago where its shape will be bent because there are two atoms and there are dots on the center it's geometry there are two atoms and two electron pairs so it will have that tetrahedral geometry the NH3 very similar to the structure of the SO3 negative 2. for shape there are three atoms attached and there's a pair of dots on the center so this is going to be the trigonal pyramid and for the geometry there are three atoms and elect Trump hair around the center so it's going to have that tetrahedral geometry one that I did not have an example for is a tetrahedral shape but these are pretty straightforward an example would be like CH4 it would have the carbon and four hydrogens that would be a tetrahedral shape and since there are no dots on the center it would also be a tetrahedral geometry so you guys can actually go back and on your homework the dot structures that you were able to start on on Monday add the shape and geometry to those dot structures if it is an expanded octet you do not need to include them but if it is not an expanded octet for that Center atom only goes up to eight it needs a shaper geometry for it you can also go ahead and add these on your molecular model Labs based off of those dot structures um like I said if it helps to circle the groups do so but I would suggest doing it in like highlight or something where it doesn't look like part of your original Dot Structure um so that's why you notice I switched colors that way I could signify this um hopefully that helps and you can go ahead and get a jump start on this