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AP.Chem:
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Video transcript

this next set of videos we're going to predict the shapes of molecules and ions by using VSEPR which is an acronym for valence shell electron pair repulsion and really all this means is that electrons being negatively charged will repel each other like charges repel and so when those electrons around a central atom repel each other they're going to force the molecule or ion into a particular shape and so the first step for predicting the shape of a molecular ion is to draw the dot structure to show your valence electrons and so let's go ahead and draw a dot structure for B EC l2 so you find beryllium on the periodic table it's in group two so two valence electrons chlorine is in group seven all right and we have two of them so two times seven is 14 and 14 plus two gives us a total of 16 valence electrons that we need to account for in our dot structure so you put the less electronegative atom in the center so beryllium that goes in the center we know it is surrounded by two chlorines so we show beryllium bonded to two chlorines here and we just represented four valence electrons so here's two valence electrons and here's another two for a total of four so instead of 16 right we just showed four so now we're down to 12 valence electrons that we need to account for so 16 minus 4 is 12 we're going to put those leftover electrons on our terminal atoms which are our chlorines and chlorine is going to follow the octet rule all right chlorine chlorine is already surrounded by two valence electrons so each chlorine needs six more so go ahead and put six more valence electrons on each chlorine and since I just represented 12 more electrons there now we're down to zero valence electrons so this dot structure has all of our electrons in it and some of you might think well why don't you keep going why don't you why don't you show some of those lone pairs of electrons on chlorine moving in to share them with the beryllium to give it an octet of electrons and the reason you don't is because of formal charge so let's go ahead and assign a formal charge to the central brilliant Atem here so remember each of our covalent bonds consists of two electrons so I'll go ahead and put that in all right and if we want to find formal charge I first think about the number of valence electrons in the free atom and that would be 2 4 4 beryllium so we have 2 electrons in the free atom and then we think about the bonded atom here so when I look at the covalent bond I give one of those electrons to chlorine and one of those electrons to beryllium and I did the same thing for this bond over here and so you can see that it is surrounded by 2 valence electrons 2 minus 2 gives us a formal charge of 0 and so that's one way to think about why you would stop here for the dot structure so it has only two valence electrons so even though it's in period 2 right it doesn't necessarily fall I have to follow the octet rule just has to have less than 8 electrons and so again formal charge helps you understand why you can stop here for your dot structure let me go ahead and redraw our molecule so you can see it a little bit better and we'll go ahead and move on to the next step so let me go ahead and put in my lone pairs of electrons around my chlorine here so we have our dot structure all right next we're going to count the number of electron clouds that surround the central atom and I like to use the term electron cloud you'll see many different terms for this in different textbooks you'll see charge clouds electron groups electron domains and they have slightly different definitions depending on which textbook you look in and really the term for me the term of electron cloud helps to describe the idea of valence electrons in bonds and in lone pairs of electrons occupying these electron clouds and you can think about them as regions of electron density and since electrons repel each other right those regions of electron density those clouds want to be as far apart from each other as they possibly can and so let's go ahead and analyze our molecule here so surrounding the central atom so we can see that here are some bonding electrons right here surrounding our central atom so we could think about those as being an electron cloud and then over here we have another we have another electron cloud so we have two electron clouds for this molecule and those electron clouds are furthest apart when they point in opposite directions and so the geometry or the shape of the electron clouds around the central atom they're pointing in opposite directions it's going to give you a linear shape here so this molecule is actually linear because we don't have any lone pairs to worry about here so we're going to go ahead and predict the geometry of the molecule as being linear and if that's linear that we can say the bond angle right so the angle between the chlorine the beryllium and the other chlorine is 180 degrees so just a straight line alright so that's how to use VSEPR to predict the shape let's do another example so co2 so carbon dioxide so we start off by drawing the dot structure for co2 all right carbon has four valence electrons oxygen has six and we have two of them so six times two gives us 12 12 plus 4 gives us 16 valence electrons to deal with for our dot structure the less electronegative atom goes in the center so carbon is bonded to oxygen so two oxygens like that we just represented four valence electrons right so two here and two here so that's four so 16 minus 4 gives us 12 valence electrons left those electrons are going to go on our terminal atoms which are oxygens that are going to follow the octet rule alright so each oxygen is surrounded by two electrons so therefore each oxygen needs six more valence electrons and go ahead and put in six more valence electrons on our oxygen now you might think we're done but of course we're not because carbon is going to follow the octet rule Carbon does not have a formal charge of zero in this dot structure so even though we've represented all of our valence electrons now we need to give carpet an octet we need to give carbon a formal charge of zero and we can do that by moving in this lone pair of electrons into here to share those electrons between the carbon and the oxygen and also with this lone pair of electrons so we move those in like that and now we can see that carbon is double bonded to our oxygens so now now our dot structure looks like this and each oxygen instead of having three lone pairs of electrons now each oxygen only has two lone pairs like that so there's our dot structure so let's go back up here to look at our steps for predicting the shape of this molecule all right so step one is done draw a dot structure to show the valence electrons next we're going to count the number of electron clouds surrounding our central atom so we go back down here and we find our central atom which is our carbon and we think about the regions of electron density that surround that so we can we can count this double bond as a region of electron density because we're not worried about how many electrons are there we're just worried about the fact that there is a region of electron density so that's one electron cloud and then over here we have another electron cloud so we have two regions of electron density we have two electron clouds here which you're going to repel each other all right so when we look at step three predict the geometry of the electron clouds alright predict the geometry of the electron clouds around the central atom well those electron clouds are going to are going to be opposite to each other and right they're going to point in opposite directions so once again they're going to force this molecule into a linear shape right so this carbon dioxide molecule is also linear with a 180 degree bond angle so once again we don't have any lone pairs of electrons on our central atom so we can don't really have to worry about that and we can go ahead and predict the geometry as being linear so that's how that's how to approach it draw the dot structure think about electron clouds and think about the shapes of your molecules in the next video we will look at how to approach three electron clouds
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