bond polarity and molecular polarity so let's take a look at the relationship between a molecule size and its boiling point with some data so here are three molecules name is methane for the first one it's weight um so so when i referring to size i'm really referring to the mass not necessarily the volume is taking out but how big it is in terms of its mass so it's it's mass or weight is 16 and it's boiling points around 100 kelvin so not celsius slightly different scale same gradations but different scale so methane weighs 16 boils at 109. ethane is a more massive molecule it weighs 30 and it boils at a higher point 145 propane the stuffing your barbecue is even more massive than both methane and ethane and boils at even a higher temperature point again this is in kelvin so you'd see the correlation here between the mass or weight if you're on earth and the boiling point as the mass increases the boiling point increases so generally speaking more massive molecules and in two ways of becoming a more mass molecule a the atoms inside of it could be bigger or there could be more of them so more atoms or bigger atoms or more bigger atoms would give the molecule more mass if it is more massive there are stronger intermolecular forces so intermeaning between the particles and so if you had a number of particles let's say these were i don't know the methane molecules we had above there they will have a certain attraction as all particles do towards each other and the larger those molecules are in terms of mass the stronger that attractive forces between the particles that is intermolecular force as opposed to intra which is what holds the individual molecule together so the bonding within methane is intra molecular the attraction of one method molecule to another methane molecule is inter molecular and so the more massive they are the stronger the intermolecular forces are the harder it is to separate them and therefore the higher the boiling point so the size of the molecule and again i'm referring to mass here not volume so the the mass of the molecule is not the only thing for sure the more massive it is the stronger the attraction between different particles the intermolecular force and therefore the higher the boiling point that is definitely affected by the mass but it's not the only thing that will affect the attractive force of those particles so if we look at these two particles here we look at some data on them one of them has a mass of 18 and its boiling point is 373 kelvin the other one is 44 so like twice as massive and look its boiling point is quite a bit less so we can see that in the previous example it was just looking at mass but here we can see that there's more going on to the picture it's not just mass that is going to be affecting the boiling point there's got to be something else as well if you recognize these names you may recognize some of these molecules these are alternative names for water water is a really small molecule relatively speaking and has a strangely high boiling point the other one is carbon dioxide not that big of a molecule but quite a bit bigger than water in terms of mass and it has a relative to water a lower boiling point so the question is why this is where molecular polarity comes in we've already talked about bond polarity that's looking at the difference in electronegativity pretty straightforward to do just find the difference between two numbers and that tells you what's going on in that bond whether it is a polar bond or not so bond polarity easy to do you can map it on the molecule done what we need to figure out now is molecular polarity so the actual once you've figured out what it's like in the molecule how is that molecule going to interact with other ones like it so you have to do this by determining the shape of the molecule as well as the bond stuff that we did earlier so we're going to be doing this first step where you figure out what's going on in the molecule we're going to consider it shape and then we'll be able to say okay how is this molecule going to act it's going to stick together really well or maybe not so we looked at carbon dioxide and water as our examples carbon dioxide we can draw the lewis structure for it and again you can have your carbons and your oxygens and you can end up once you've done your sharing and everything like that you end up with a structure that looks like that there's a double bond between the carbon and each one of the oxygens carbon's got four bonds oxygen's got two everyone's happy so it has polar bonds you go ahead calculate out what the electronegativity is of those bonds they are going to be less than 1.7 but greater than zero they are polar bonds the oxygen has greater electronegativity so it's going to hog the electrons leaving carbon with a positive partial charge not taking the electrons they're still lines there's still a molecule but it is hogging the electrons the oxygen is hogging them away from the carbon on both sides so for sure carbon dioxide has polar bonds but it is a non-polar molecule so really important to understand the difference between the bonds within the molecule whether they're polar or not that's easy to figure out then we look at the whole molecule and you decide based on its shape and what's going to be happening within the molecule is the molecule as an entirety it's the whole thing is it polar carbon dioxide is not a polar molecule though it does have polar bonds water on the other hand also has polar bonds so again you can draw your lewis structure for it we're going to have hydrogen and oxygen we're going to have these two so again if you draw a proper lewis structure you should end up with sort of one hydrogen pointing down and one off to the side that's why we when we're doing oxygen we go one two three four five six so we go in that pattern so when you go to bond your hydrogens you would put one on one side and i'll replace the pairs of electrons like so and so you end up with water looking like that not like that so this is the wrong way to draw water draw it like this get it correct more on that when you do vesper theory but for now again as long as you're placing those electrons one at a time before you pair them you'll end up with a decent structure for water not perfect but good enough for what we need to do with it so water has molecular polarity and again feel free to calculate the delta en of these bonds each one here is going to be larger than zero it's not the same atom but less than 1.7 so they are polar bonds so polar bonds check water's got them water itself is also a polar molecule so it has polar bonds and is a polar molecule carbon dioxide has polar bonds but is not a polar molecule so it is not just the bonds that determine the polarity it is also the shape and if you take a look at these two you can kind of figure out why we're going to be talking about this idea called dipoles and how they work to tell us what it is going to be looking like so these differences in electronegativity are going to be visualized using an arrow so we've done them before we draw drew these delta lowercase delta symbols so in this case hydrogen chlorine they have a electronegative difference greater than zero chlorines is greater so it is hogging the electrons away from the hydrogen it doesn't take them it's not over 1.7 it's under 1.7 so we still are drawing a line between them representing the shared pair it's just that chlorine's hogging some of the electrons in that shared pair so that is the delta symbols for it now what we're going to do is we're going to add on to that this dipole and essentially what it is it's an arrow that sort of emphasizes the unequal sharing the hogging of that's a terrible plus sign um the hogging of the electrons of the fact that chlorine is hogging the electrons and so we're pointing towards the one that has the partial negative charge to it and we're representing that with an arrow if you really want you could quantify it and you could say okay well i'm going to draw an error of this length based on what the actual delta e n is but we won't get it in that fancy so essentially it's an arrow that represents the difference in partial charges and it points towards the one that is hogging the electron the one that's more electronegative the one that has the partial negative charge now if we look at a molecule we can then say okay well if these are the dipoles they can help us to see whether or not the dipoles themselves cancel each other out because if they're there it doesn't mean the molecule is polar because they could work against each other and if they do they cancel each other out and the molecule ends up not being polar even though the bonds within it are polar so this is what we see with carbon dioxide we see our carbon and get that lewis structure up here a little more room here lone pairs there's our carbon dioxide and so we have a partial negative charge on our oxygens because they are more electronegative leaving a partial positive charge on our carbons and we can represent that with a dipole let's do a purple one purple dipole one for each of the bonded pairs and yes it is a double bond it doesn't change anything um so we have a dipole on either side of the carbons and you'll notice that they are pulling away from each other so think of this like a like a tug of war and if you have equal pull in opposite directions they cancel each other out and the molecule ends up being non-polar so carbon dioxide has polar bonds but e is nonpolar because the dipoles within it cancel each other out if they don't cancel each other out you end up with one end being different than the other and that is what defines a polar molecule has different poles to it one side is different than the other so we draw these dipoles to represent the unequal sharing we look at the overall picture and we decide is the molecule non-polar like carbon dioxide or is it polar like water because if we quickly brought up a picture of water here again let's put the charges on there hydrogens are going to be partially positive partially positive water is partially negative and so the dipoles are pointing towards oxygen you'll notice they kind of one points left the way i've driven it drawn it drawn it um one point in the left one's pointing up and so they do kind of point this interaction but they don't cancel each other out they're they're they're kind of adding together if you've done vectors in physics think of these as vectors you essentially end up with them not cancel each other out so you have if you imagined it this side here is positive and this side here is negative that's polarity water is a polar molecule so quick overview again same slide as before bond polarity is super easy just find the difference molecular polarity you've got to draw the molecule you've got to figure out the electronegativity difference of the bonds you've got to put the partial charges in you've got to put the dipoles in and then you've got to determine is the molecule polar based on all that so it is based on the shape and the polarity of the bonds so steps to determine whether a molecule is polar first off draw the molecule so again do your sharing get the bonding and everything like that then you're going to look up the delta en for each bond decide whether you're dealing with polar bonds or non-polar bonds make sure it is a molecule write the partial charges in your partial positive partial negative and add those dipoles in to help you visualize that difference then look at the shape and see do those dipoles cancel out if they cancel out it's a non-polar one sides not different than the other non-polar if they don't cancel out if they add together they don't cancel out the molecule will be polar one end is different than the other