shape of the molecules we will use some examples to illustrate this topic first CH4 methane we've seen this molecule before its Lou structure has carbon in the middle and four hydrogens make single bound around the carbon atom if we are going to take a look at the shape of this molecule we understand that the shape will be 3D while Louis structure is 2D on paper that's actually one limitation about Louis structure 2D it does not show the actual shape of the molecule so in order to determine the shape of the molecule we have to understand how the electrons are distribute carbon has four single bonds around it that means four groups of electrons we always call these groups of electrons as electron domain so there are four groups of electrons there are four electron domain on the central atom in order to make the molecule stable we want want to distribute this four electron domain as further away from each other as possible so in space if we want to distribute four groups of electrons evenly the best way to do it is do it in a tetraedron shapee so the tetraedron is just like a tripod okay the tripod for camera or anything else you have this three legs to let it sit on floor and you also have another stick pointing up that's the shape for method Tetra okay so we looked at the electron domain first to determine the shape and four electron domain means electrons are distributed in the tetraedron sh okay the bound angle in a tetraedron shape molecule like methan is 109.5 and this is the number that you will have to know the next example NH3 ammonia we've seen this example for Lou structures right three hydrogens attach to the nitrogen and there is one L pair on the nitrogen how many groups of electrons are around the central nitrogen we have three single bonds and a l pair so again four four groups of electrons four electron domains if we have four groups we want to distribute this four electron domain in space and the further away from each other as possible we have to distribute them in a tetraedron shape so the shape for the electron domain for ammonia is still in the tetrahedron shape but when we consider the shape of the molecule we do not consider that L pair molecules are made of atoms so we will only look at the atoms the shape of the ma molecule consists of that nitrogen and three hydrogen right so this shape we call it a trigonal pyramidal shape so you have to pay attention here because the of the presence of this L pair the electron domain is in tetrahedon shape but the molecular shape is trigonal pyramidal the bound angle here is 107 you don't have to memorize this number as long as you know the number will be slightly smaller than 109.5 that's enough so why it's smaller because L pair of electrons are relatively loose they do not have another atom on the other end to bind to them so they are going to occupy a little bit more space as a result the hydrogens will be squeezed down a little bit so the bound angle will be slightly smaller than 109.5 next example H2O we've seen that before the Lis structure has two hydrogens bonded to the oxygen and there are two lone pairs on the oxygen how many groups of electrons are on the oxygen four two single bonds two pairs of electrons okay there are four electron domains on the oxygen so the electrons will be distributed in the tetraedron shape same as before but if we consider the shape of the molecule we do not consider those two pairs of electrons only HOH right so HOH is in bent and we call this molecule shape as bent Bond angle 105 again you don't have to memorize this number as long as you know it's smaller than 109.5 okay for the same reason long pairs of electrons are loose they will occupy more space so the O Bond will be squeezed a little bit so the number is slightly smaller than the one in the real tetrahedron shape all right next example BF3 if we draw Louis structure boron in the middle three Florine atoms surround it we've seen B3 before and this is quite similar to that okay there should be L pairs of electrons on Florine but my point here is that I want to point out boron is electron deficient boron has only six electrons around it it does not have the oet okay how many electron domains are around the central Boron atom three because we have only three single bonds so we have only three instead of four electron domains all right if we want to distribute three three groups of electrons in space to make them as further away from each other as possible the best way to do it is to make them in the same plane and distribute them evenly okay this shape is called trigonal planer okay BF4 is in trigonal planer shape okay we don't have long pairs of electrons on Boron so the electron shape will be exactly the same as the molecular shape the bound angle in the trigal trigonal planer shape is 120 it's 1/3 of 360 of course all right next example is hcn the lowest structure of hcn carbon is a central atom it's single bonded to a hydrogen it's triple bonded to a nitrogen there's a long pair of electrons on the nitrogen now we are going to focus on the central carbon atom how many groups of electrons are around a carbon atom only two one is a single bot the second is a triple bond so only two groups of electrons when we have two electron domains we want to distribute them as further away from each other as possible how would we do that in a linear fashion right opposite to each other right so the Lis structure shown here is also the real structure of this molecule and it's called linear okay the shape is called linear the bond angle HCM because it's linear the bond angle will be 180° okay now please refer to the lecture preview that I posted on blackboard for a summary of the shape of molecules so you need to pay attention that we have electron domain shapes there are three of them tetrahedron shape trional planer shape and linear then we have different molecular shape based on the shape for electron domains okay so please review that table in the lecture preview for chapter one some problems to practice so I want you to draw the L structure of these three molecules the first one is Athan ch3 ch3 second one is called ath it has a cc double bond okay ch2 double bond ch2 the last one is called athine ch2 triple bond ch2 draw the Louis structures of these three molecules find out the shape around the carbon atoms okay and we will look at this molecules further more in our next class