in this video I'll show you how to figure out the shape or the geometry of molecules we will use a theory called the vspr theory vspr stands for veence Shell electron pair repulsion all this means that electrons that are attached to the central atom are repulsed by each other remember electrons carry negative charges negative charges repel each other so as a result of that these electronic groups on the central atoms want to be as far away from each other as possible and so they will result in different Bond angles around the central atom so some of the terminologies that we'll use is ba which stands for bonded atoms and here we should include bonded atoms on Central atom So ba stands for the number of atoms attached to the central atom and then LP stands for long pairs again on the central atom not the outer atoms two examples ammonia which is NH3 nitrogen is Central it's attached to three hydrogens which are the outer atoms so if you count the number of atoms bonded to nitrogen 1 2 3 there's total of three atoms bonded to the central atom that's why I have ba equals to three so these are the three hydrogens and then we look at the central atom nitrogen it has one long pair and that's why LP is one so again LP stands for long pairs on the central atom here's one other example carbon dulite carbon is Central and it's attached to two sulfur ba for carbon sulfide is carbon D sulfide is two now there are two double bonds here's a double bond here we ignore the double bonds the question is not how many bonds are attached to carbon but how many atoms are attached to it so there's two sufers attached to carbon that's why ba is two bonded atoms on carbon is two and carbon does doesn't have any L pairs that's why the l pair is zero another example this molecule is called form Malahide carbon is Central it's bonded to oxygen it's also bonded to two hydrogens and that's why the ba for form Malahide is three there are no l pairs on carbon that's why I wrote down LP is zero there are L pairs on oxygen but oxygen is an outer atom so LP lirs only refer to the lirs on the central atom and then this case there zero let's do the next exercise this is a large organic molecule that has various bonded atoms and lwn pairs so I've labeled the first carbon is carbon a second carbon is B and then this was carbon C and oxygen D so let's concentrate on carbon a carbon a has four bonded atoms if you wondering where they are here they are here's a hydrogen attached to carbon a here's another hydrogen that's the second hydrogen thirdd hydrogen is right here and then we also have carbon B that's the fourth atom that gives us total of four atoms attached to carbon a and that's why for carbon a we have ba equals to 4 let's go back to carbon a there are no longone pairs on carbon a and that's why LP equals to zero moving on to carbon B carbon B has a hydrogen attached to it it's double bonded to another Carbon on the right and it's also single bonded to carbon a on the left that's total of three atoms so there's three rebonded atoms on carbon B and again there's no lone pairs on carbon B next comes carbon C here it is on the left it's single bonded to one carbon on the right it's triple bonded to another carbon that's two bonded atoms carbon C has no long pairs and then the last one is oxygen it's attached to Carbon on the left and a hydrogen on the right those are the two bonded atoms but also oxygen has two sets of Lone pairs so LP for oxygen d two so all of these bonded atoms and lone pairs are called electronic groups or sets so to figure out the molecular geometry of a compound first you need to figure out the bonded atoms and the lone pairs so depending on how many bonded atoms and lone pairs are attached to the central atom we end up with different types of molecular geometries so let's look at the first compound berum dihydrate berum is in the center that's the central atom because it's the single atom and it's attached to two hydrogens this looks like the lowest structure berum dihydrate now there's uh since there's only two bonded atoms these atoms want to be as far away from each other as possible according to the VPR Theory and so when that happens the bond angle between the hydrogen berum and the out the other outer hydrogen the bond angle here is 180 and the geometry that results because of that is called linear so anytime there's a central atom with two bonded atoms and no lone pairs on the central atom that forms linear geometry now if there's two bonded atoms and one lone pair such as in this molecule sulfur dioxide sulfur is in the center it's attached to two oxygens but there's also a lone pair on it as well so um Bond angle here we have three electronic groups we have two bonded atoms and we have one lone pair that's three uh groups these groups want to be as far away from each other as possible and the bond angle is close to 120 so here the bond angle is 120 here it's 120 and here is 120 now I have this less than sign why is that if there's a lum pair on the central atom this Lum pair repulses it experiences repulsion with these other bonding electrons and it actually pushes these two oxygens closer to each other which decreases the bond angle so the bond angle is a little bit less than 120 between the two oxygen that's why we have the less than sign so anytime there's a lum pair on the central atom the bond angle between the bonded atoms will be slightly diminished next scenario is when we have three bonded atoms and no long pairs on the central atom such as boron trihydride Boron is Central hydrogens are outer atoms and notice how I'm drawing these hydrogens one hydrogen points up the other one are slightly down this is because the bond angle throughout is 120 this is the perfect scenario where the electrons are far away from each other as possible and they experience the minimum amount of repulsion so here we don't have less than 120 this Bond angle equals to 120 if there's three bonded atoms and one L pair such as in the molecule ammonia the long pair is on the nitrogen it pushes the bonded atoms closer to each other and that makes the bond angle smaller here we have four electronic groups one group is the long pair the other three groups are the bonded atoms so if there's four electronic groups the ideal Bond angle to diminish the repulsion is 109.5 again because of the long pair the bond angle is a little bit less than 109.5 so this is called trigonal pyramidal when there's four bonded atoms and zero long pairs here's the four electronic groups all of them are atoms the bond angle is exactly 109.5 and this geometry is called tetrahedral and in the last case scenario if there's two bonded atoms such as in water two bonded atoms and two long pairs on the central atom again the bond angle is less than [Music] 109.5 but this is not linear geometry the bond angle is not 180 it's 109.5 or less and that's why the molecule is called bent so notice how there's two Bend geometries one has two long pairs and two bonded atoms and the other Bend geometry has two bonded atoms and only one long pair so the difference between the two is the bond angle in this case the bond angle is less than 120 in this case the bond angle is less than 19.5 so the number of long pairs one versus two will tell you which type of B geometry is present the other thing I want you to pay attention in this table is the shape drawing now shape drawing is the same as L structure so you start with the L structure the only difference between shape drawing and L structure is here we show the correct Bond angle so we draw the molecule with its correct Bond angles so the only possibilities in terms of bond angle will be either 180 or 120 or 109.5 you should never have shape drawing with 90° one angle or something other than one 180 120 or 109.5 whereas in loose structures Bond angles are irrelevant all we care about is how the central atom is connected to Outer atoms and where the L pairs reside here's some examples uh let's figure out the molecular geometry for these five molecules so the best way to do this is to uh draw a table in the First Column we draw the low structure so always start whenever you asked to figure out the molecular geometry of a comp compound start with the low structure the low structure will tell you how many bonded atoms and long pairs reside on the central atom so for water I draw oxygen in the center attached to two hydrogens um there's two bonded atoms and there's two long pairs on oxygen so when you look at the table that I just discussed this corresponds to bend geometry and notice how I'm drawing this I'm not drawing it as the linear yes the low structure looks like linear geometry but I need to uh bring the hydrogens down to show that this has B molecular geometry and that the bond angle is less than 109.5° next example for malah Heights I draw the low structure carbon is Central double bonded to oxygen single bonded to two hydrogens this is total of three bonded atoms and there's no long pairs on carbon so using the table above this corresponds to trigonal plan of geometry Bond angle throughout this 120 and that's why the shape drawing looks like this one group is up so I'm placing the hydro the oxygen the double bonded oxygen up and then the hydrogen slowly slightly pointing down off the horizontal plane third example carbon Tetra fluoride four bonded atoms and no l pairs on the carbon right carbon doesn't the central atom doesn't have any long pairs this corresponds to the tetrahedral geometry notice how I'm drawing a tetrahedral shape one group points up the other three have to point down so please don't draw any 90° Bond angle that are in the horiz izontal plane so throughout the bond angles are 109.5 next example phosphorus trihydride here's the lowest structure from the lowest structure we can see that there's three bonded atoms and one L pair on the phosphorus which corresponds to trigonal pyramidal geometry and again I draw the L pairs the long pair pointing up and the three hydrogens SL uh slightly pointing down the bond angle is less than 109.5 because of the long pair on the Central atom in the last example burum diiodide two bonded atoms zero L pair this is linear geometry 180 Bond angle and in this next page there's additional examples following the same rules that we just talked about and that's it for molecular geometry