intramolecular forces and intermolecular forces so let's talk about the difference between those two words and then some of the properties that arise due to these intermolecular forces which of course are come about because of the intramolecular forces so to clarify intra meaning within and so if you have a molecule it's going to be held together by shared pairs of electrons so that would be the intramolecular forces and you can look at the partial charges and the polarity of the bonds and the dipoles that's all intra that's within the molecule if you're drawing a molecule you show all that information then we have our intermolecular forces and so you can't show an inter molecular force because with a single molecule because this is between different particles and just to get this in here this is with in so in tros within inter is between and so if you want to show an intermolecular force you've got to draw at least two molecules and then you can say okay well how are they going to interact with each other and so if you're talking about a pure substance you've got two of the same molecules and you can talk about the interactions between them and again that intermolecular force will depend on what the molecule is like which is what the intra molecular force shows you so once you get the intramolecular forces figured out you can say okay well what then will be the intermolecular forces and then what are the results of those so again figure out the intra molecular forces what are the particles like and then you can figure out what they're doing the intermolecular forces and all the properties that arise do that so again intramolecular forces um we can calculate the electronegativity difference and that's that's sort of the main thing that we're doing here to figure out okay with these two atoms when they um stabilize they they what's going to happen with their electrons here and they could form ionic bonds and you have ions you could have sharing of electrons so you have covalent bonds molecules and there's also metallic bonds when you have just metals coming together they don't transfer their electrons right because if they're the same atom their delta n would have to be zero but they also don't act like molecules they form this crystal it's like a giant crystal where they all share their electrons within there and that's called a metallic bond all three of those are intramolecular so we have this force within the compound itself that sort of determines what type of compound you have once we know we're dealing with molecules then we can do some interesting work on the intermolecular forces so if you took one of those molecules and another one of those molecules and put them together how are they going to act and if we stick to pure substances it's a little bit easier definitely they're going to have what's called london dispersion forces or sometimes just dispersion forces or sometimes just london forces all of that's referring to the same thing so anytime you have a molecule it's going to be attracted to other molecules regardless the molecules are made of atoms the atoms have protons and electrons and so if you get them near each other the protons of one will be attracted to the electrons of the other and that is called london dispersion forces so that's always going to be the case the more atoms you have or actually the more protons and electrons you have so if you have bigger atoms it's even more so the more protons electrons you have within a molecule the more attracted it will be to other molecules then we have these dipole-dipole forces so this is all that work we did on polarity so if you do a structural diagram of a molecule you figure out how the electrons are being shared who's hogging whose electrons the delta en the electronegativity difference the dipoles all that and you figure out whether you have a polar molecule or not if you have a polar molecule those dipoles in one molecule will be attracted to the dipoles in another molecule and like two magnets get attracted to each other but it's not magnetic attraction it's electrostatic and so polar molecules are going to be attracted to the oppositely charged pole on another molecule and again that's quite a strong force as well as we saw with water it can be so strong that it actually gets its own label so a particular type of dipole-dipole force that is so strong it actually gets its own sub section are hydrogen bonds and this is when you're dealing with hydrogen bonding to something super electronegative like oxygen fluorine or that should actually say nitrogen not fluorine for sure fluorine but also nitrogen so hydrogen bonded oxygen hydrogen bonded to fluorine and hydrogen bonded to nitrogen will all create dipoles of course because you have unequal sharing but really strong dipole so strong that they form this thing called hydrogen bonding as an intermolecular force so remember you can't have this unless you have more than one molecule so you'd never show an intermolecular force with only one molecule because that doesn't make sense that would be an intramolecular force as you're talking about if you want to do an intermolecular force like a hydrogen bond you've got to have two different molecules to show that so nonpolar molecules the only thing holding them together and attracting them towards each other i should say a group of them together are these london forces or dispersion forces or land and dispersion forces so they are fairly weakly attracted they are attracted because they do have protons and electrons and so they are attracted to each other's protons and electrons but it is relatively weak it doesn't take much energy to get them moving to the point where they can fly apart from each other turn into a gas polar molecules though have these dipoles which give them a partial positive partial negative n and so they are more strongly attracted to each other and so if you had a comparable size molecule that had a dipole versus one that didn't the ones that have the dipole are going to be more attracted to each other they're going to be held more tightly together and so you can see that a non-polar molecule of a given size versus a polar molecule of vitamin size they're going to be held differently and so we end up with these non-polar molecules being very easy to turn into a gas whereas these polar molecules might be more difficult to turn into gas you give them more energy to do so and remember this is we're looking at two variables here the polarity of them is one variable but also the size is also going to factor in here so you've got two variables if you have a really big nonpolar molecule it will stick together well because it's so big and again if you have a small polar molecule it won't stick together as well so it's both the polarity as well as the size that will interact together to tell you what sort of properties it's going to have so let's look at water as an example of a particularly polar molecule it is quite small but it also has strong polarity to it it has the ability to form these hydrogen bonds because you have hydrogen and oxygen attached to each other and remember that is not the hydrogen bond right so if you look at a molecule of water the bond between the oxygen hydrogen is not the hydrogen bond that's the common misconception right because the oxygen hydrogen are attached to each other that gives them the ability to create a hydrogen bond and the hydrogen bond is actually the fact that this hydrogen has such a partial positive charge because it's bonded to this oxygen that that partial positive charge is going to be really attractive and it could be attracted to a partially negatively charged oxygen the attraction between those two different molecules which again is a intermolecular force that's the hydrogen bond and you only get that when you have molecules that have hydrogens bonded to oxygens or fluorines or nitrogens so if you had hydrogen bonded to fluorine let's say a simple lewis diagram here um you're going to have a really negative fluorine and a really i should say partially negative fluorine that really partially positive hydrogen so again there's no hydrogen bonding you can't show a hydrogen bond with a single molecule but what you could do is you could say okay well if i had a group of these guys there's another one here again it's going to have a really partially negative fluorine a really partially positive hydrogen and then the force between those and so you have to draw it differently you can't draw as a line because we already use that to show a covalent bond but there is going to be an attractive force between the two different molecules which again is inter molecular um and again if it's it's if this is an intermolecular dipole force let's say the dipoles in here as well so we have this positive dipole i'll try ended positive end of the dipole at the hydrogen and here we have a negative end at the dipole at the fluorine so you have this very parsy negative fluorine that's part of this molecule and you've got this very partially positive hydrogen that's part of this molecule that's all intra molecular it sets up the condition for a intermolecular hydrogen bond so because of this if you have these hydrogen bonds you have very very strong dipole-dipole attraction and you get some interesting properties out of it like what we see with water one of the neat things with water if you've ever seen insects walking on water they're able to do this because of the strong dipole-dipole force that is in water which is so strong it gets its own name hydrogen bonding and this keeps the water molecules really attracted to each other especially at the surface where they're not surrounded by other water molecules because there's nothing above them so they're really attracted to the ones beside them and below them so the surface tension ends up being quite strong on water and the things like insects take advantage of that they're able to step on top of it or if you've ever seen a droplet of water sitting on a surface that is doesn't like water is hydrophobic um it will sort of bead up on that surface like a waxed car a couple of beads of water so they run off and don't dry on the car and that again is because the water molecules within that bead of water are super attracted to the ones beside them let's not draw water linearly so you have this water molecule and you have this water molecule and again it is the force between them that is the hydrogen bonding so we see that high surface tension high solubility well is another thing we have ionic and polar substances really dissolve well in water we call it the universal solvent because it dissolves other substances so well not everything but a lot of things quite well as long as they have a charge so ionic things that you're trying to dissolve ionic solutes have charges like sodium chloride and so water which also has charges is able to get in there and and be attracted to those ions the ions attract to it and so they're able to break up the ionic crystal and it dissolves quite well not all ion substances but a lot do and then polar substances as well will dissolve in water because the polar substances themselves have polarity the water has polarity so they're attracted to each other and they dissolve high heat capacity as well again if you have a water molecule that is really attracted to another water molecule because there's one there's another there and we have a hydrogen bond between them as well as all the other water molecules around i am just drawing one here um because of the partial positive and the that's supposed to be a negative dipole here and again we have this dipole on this one with a partial negative quite large partial quite large partial positive hydrogen so you get this hydrogen bonding between them and it's surrounded it's happening all around it and so if you want to start moving them you're going to try to mess with this hydrogen bond here right you're just pulling it on one side and moving to the other side so if you're trying to get them moving faster get them vibrating faster which is what heating up is it's going to take a lot of energy to do that and once they're doing it's going to hold on to that energy quite well so they call this heat capacity these molecules of water are really attracted to each other so it takes a while a lot of energy to get them moving and once they're moving they keep moving they hold that heat quite well as well we see some of these properties um coming together when we start creating these mixtures so we've heard that oil and water don't mix and remember it can't be because water is not attracted to oil right because all particles are attracted to all molecules attract other molecules right it is that the water itself is so attracted to other water molecules they've got the hydrogen bonding here that's happening between the molecules as well so they're really really attracted to each other that they don't get particularly attracted or essentially they sort of exclude the oil they all group up together because they're super attracted to each other and then the oil molecules just sort of end up being left out if they're less dense they end up on top if they're more dense among water but they definitely end up being separated from the water because water is so attractive itself or other molecules like it it excludes oil from that group and it ends up being on its own this is why we say oil and water do not mix it is not that they're not attracted to each other it's just water is more attracted to other water molecules excluding the oil