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AP.Chem:
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Video transcript

concept of steric number is very useful because it tells us the number of hybridized orbitals that we have so to find the steric number you add up the number of Sigma bonds or single bonds and to that you add the number of lone pairs of electrons so let's go ahead and do it for methane so if I want to find the steric number right steric number is equal to the number of Sigma bonds so I look at around my carbon here and I see 1 2 3 & 4 Sigma or single bonds so I have 4 Sigma bonds I have zero lone pairs of electrons around that carbon so 4 plus 0 gives me a steric number of 4 in the last video we saw that sp3 hybridized situation we get 4 hybrid orbitals and that's how many we need the steric number tells us we need 4 hybrid orbitals so we took 1s orbital and 3p orbitals and that gave us four sp3 hybrid orbitals so this carbon that must be sp3 hybridized so let's go ahead and draw that in here so this carbon is sp3 hybridized and the last video we also drew everything out right so we drew in those four sp3 hybrid orbitals for that carbon and we had one valence electron in each of those four sp3 hybrid orbitals and then hydrogen had one valence electron in an unhybridized s orbital so we drew in our hydrogen's right and the 1 valence electron like that this head-on overlap right this is this is of course a sigma bond so again we talked about this in the last video and so now that we have this picture of the methane molecule we can think about these electron pairs right so these electron pairs are going to repel each other right like charges repel and so the idea of VSEPR theory tells us these electron pairs are going to repel and try to get as far away from each other as they possibly can in space and that means that the arrangement of those electron pairs ends up being tetrahedral so let's go ahead and write that so we have a tetrahedral arrangement of electron pairs around around our carbon like that we think about the molecular geometry so that's like electron group geometry you want to think about the geometry of the entire molecule alright I could think about drawing in those those electrons those bonding electrons like that all right so we have a wedge coming out at us in space a dash going away from us in space and then these lines mean in the plane of the page and so we can go ahead and draw in our hydrogen's and this is just one way to represent the methane molecule which attempts to show the geometry of the entire molecule so the arrangement of the atoms turns out to also be tetrahedral so let's go ahead and write that so tetrahedral and let's see if we can see that four-sided figure so a tetrahedron is a four-sided figure so we can think about this being one face right and then let's go ahead and draw a second face if I draw a line back here that gives us four faces to our tetrahedron all right so our electron group geometry is tetrahedral the molecular geometry of methane is tetrahedral and then we also have a bond angle let me go ahead and draw that in so a bond angle this this hydrogen carbon hydrogen bond angle here is approximately 109 point five degrees all right let's go ahead and do the same type type of analysis for a different molecule here so let's do it for ammonia next all right so we have nh3 if I want to find the steric number right the steric number is equal to the number of Sigma bonds that's one two three so three sigma bonds plus number of lone pairs of electrons so I have one lone pair of electrons here so three plus one gives me a steric number of four so I need four hybridized orbitals and once again when I have when I need four hybridized orbitals I know that this nitrogen must be sp3 hybridized because sp3 hybridization gives us four hybrid orbitals and so let's go ahead and draw those four hybrid orbitals we would have nitrogen and let's go ahead and draw in all four of those so one two three and four those are the four hybrid orbitals when you're drawing the dot structure for nitrogen right you would have one electron another electron another electron and then you'd have two and this one like that and then you go ahead and put in your hydrogen's right so once again each hydrogen has one electron and an unhybridized s orbital so we go ahead and draw in those hydrogen's all right so our overlap Orbital's so here's a sigma bond here's a sigma bond and here's a sigma bond so three sigma bonds in ammonia and then we have this lone pair up here so the arrangement of these electron pairs so is just we talked about before right so we have this tetrahedral arrangement of electron pairs or electronic group so VSEPR theory tells us that's how they're going to repel however that's not the shape of the molecule so if I go ahead and draw in another picture over here to talk about the molecular geometry I go ahead and draw in the bonding electrons like that and then I'll put in my nonbonding electrons up here this lone pair right here how is it an sp3 hybridized orbital so the arrangement of the atoms turns out not to be tetrahedral now has to do with this lone pair of electrons up here at the top so this lone pair of electrons is going to repel these bonding electrons more strongly than our pretend in our previous example and because it's going to repel those electrons a little bit more strongly you're not going to get a bond angle of 109.5 it's going to decrease the bond angle so let me go ahead and use the the same color we used before so this bond angle is not 109.5 it's goes down a little bit because of the extra repulsion so it turns out to be approximately 107 degrees and in terms of the shape of the molecule we don't say tetrahedral we say trigonal pyramidal so let me go ahead and write that here so the geometry of the ammonia molecule is trigonal pyramidal and let's analyze that a little bit so trigonal refers to the fact that nitrogen is bonded to three atoms here so nitrogen bonded to three hydrogen's that takes care of the trigonal part the pyramidal part comes in because when you're doing molecular geometry you ignore lone pairs of electrons so if you ignore that lone pair of electrons and just look at this nitrogen right at the top of like this pyramid right here so that's that's where the pyramidal term comes in so bonded to three other atoms like this this and this for our pyramid so trigonal pyramidal is the geometry of the ammonia molecule but the nitrogen is sp3 hybridized all right let's do one more example let's do water so first we calculate this Aric numbers so the steric number is equal to the number of Sigma bonds that's one two so two Sigma bonds plus numbers of lone pairs of electrons so here's a lone pair or here's a lone pair so we have two plus two which is equal to four so we need four hybridized orbitals as we've seen the previous two examples when you have when you need four hybridized orbitals that's an sp3 hybridization situation right you have four sp3 hybridized orbitals so this oxygen is sp3 hybridized so I'll go ahead and write that in here so oxygen is sp3 hybridized so we can draw that out showing oxygen all right with its with its four sp3 hybrid orbitals so there's four of them so I'm gonna go ahead and draw on all four in terms of in terms of electrons this orbital gets one this orbital gets one and these orbitals are going to get two like that all right so that takes care of oxygens six valence electrons when you're drawing in your hydrogen's all right so let's go ahead and put in the hydrogen here so once again each hydrogen with one electron and an unhybridized S orbital like that so when terms of overlap of bonds here's one sigma bond and here's another sigma bond right so that's our two sigma bonds for water once again the arrangement of these electron pairs is tetrahedral so VSEPR theory says the electrons repel and so the electron group geometry you could say is is tetrahedral but that's not that's not the geometry of the entire molecule because that's just thinking about electron groups and these hybrid orbitals the geometry of the molecule is different so we'll go ahead and draw that over here so we have our water molecule and draw our bonding electrons and now let's put in our nonbonding electrons like that alright so we have a different situation than then with ammonia with ammonia we had one lone pair of electrons repelling these bonding electrons up here for water we have two lone pairs of electrons repelling these bonding electrons and so that's going to change the bond angle it's going to shorten even more than in the previous example so the bond angle decreases alright so this bond angle in here decreases to approximately 105 degrees rounded up little bit so thinking about the molecular geometry or the shape of the water molecule so we actually call this bent or angular so this is a bent geometry because you ignore the lone pairs of electrons and that would just give you this oxygen here and then this angle alright so you can also call this angular so we have this bent this bent molecular geometry like that or angular and and once again for molecular geometry ignore your lone pairs of electrons so these are these are examples of three molecules and the central atom and all three of these molecules is sp3 hybridized and so this allows us to this is one way to figure out your overall molecular geometry and to think about bond angles and to think about how those hybrid orbitals affect the structure of these molecules