so what does this dotted line represent and I want you to notice that this is a dotted line and not a solid line like you see with the bond here between oxygen and hydrogen knowing what this dotted line represents is the first step into identifying intermolecular forces which is what this video is all about so let's dive in this dotted line represents the intermolecular force that is holding these two molecules together and that intermolecular force is called a hydrogen bond intermolecular forces are just forces that exist between different molecules that hold molecules together and these forces form due to attractions between two oppositely charged regions of the molecule when looking at intermolecular forces there are a number of forces we could talk about but in this video we're going to talk about the big three and that is hydrogen bonding dipole dipole forces and London dispersion forces and I've listed these forces here in order of decreasing strength with hydrogen bonding being the strongest and lard dispersion forces being the weakest this will be important in later videos when we discuss various physical and chemical properties related to different molecules so now let's look at each of these forces in more detail the first Force we will look at is hydrogen bonding hydrogen bonding occurs when you have a polar molecule that has a nitrogen oxygen or Florine bonded to a hydrogen here are some examples we see here on the left NH3 and on the right H2O we know that both of these molecules are polar and we see that they have a nitrogen bonded to a hyd hen in H3 and an oxygen bonded to a hydrogen in H2O so now we know both of these contain the intermolecular force hydrogen bonding so we know that two water molecules will be held together by the intermolecular force of hydrogen bonding because water is polar and it contains a hydrogen bonded to oxygen so now let's look at what exactly a hydrogen bond is the first thing to understand about hydrogen bonding is that it is not a bond the reason it is called hydrogen bonding is because it is a very strong force that can seem like a strong bond but it is not a bond this is because for a bond to form you have to have the sharing or transfer of electrons and as you're about to see the attraction here is due to different charges not the sharing or transfer of electrons so to understand what a hydrogen bond is and how it forms you have to remember that water is polar because the bond dipoles do not cancel out so in a water molecule we have the region around the oxygen atom that is partially negative and the region around the hydrogens are partially positive this means that if we have two water molecules get close to each other they will be attracted to each other and this attraction is the intermolicular force known is hydrogen bonding the next intermolicular force is dipole dipole forces remember that a dipole is when you have a molecule that is polar that produces two regions of opposite charge and when you have two dipoles get close to each other they will be attracted to each other just like two magnets and this will form the dipole dipole intermolecular force so here we have the molecule Co which is is a polar molecule we know it is polar due to the electro negativity difference between carbon and oxygen with the region around carbon being partially positive and the regen around oxygen being partially negative so if we have another Co molecule then the partially negative end of one molecule will be attracted to the partially positive end of the other molecule this attraction will form and this is called a dipole dipole intermolecular force and again it's important to emphasize that this is a force of attraction due to the regions of opposite charge and not a bond B which is formed from the sharing or transfer of electrons and the final force we'll look at is London dispersion forces London dispersion forces are present in all molecules and they are the only intermolecular force present in non-polar molecules to understand London dispersion forces you need to remember that bonds are made of electrons and electrons are moving so if we have a molecule of N2 we know this is non-polar because there's not an electr negativity difference between either nitrogen atom this means the electrons in these bonds right here are being shared equally between the two nitrogen atoms we also know that electrons are moving so if the electrons in the bond are moving there could be a point in time where the electrons are closer to this nitrogen here on the left this means that the region around this nitrogen will be partially negative since electrons have a negative charge this also means that the nitrogen here on the right would have a partially positive charge because we know the molecule itself does not have a charge and is neutral and if this nitrogen molecule were to come close to another nitrogen molecule then there would be an attraction that forms forms and this is what we would call a London dispersion force now if you think this looks familiar then you are right this should look just like a dipole dipole Force because that is essentially what we have we have an attraction form between two dipoles but what makes this different than a dipole dipole force is that this dipole is what we call a temporary dipole because it does not last the electrons are moving back and forth so while at a brief moment in time the electrons could be closer to one nitrogen over the other this does not last the dipole forms and the attraction forms but then it goes away this is why London dispersion forces are the weakest of the three intermolecular forces we have discussed in this video furthermore all bonds are made of electrons so this is also why London dispersion forces are present in all molecules because all molecules have bonds made up of electrons that are moving so that these temporary dipoles can form and then go away so to summarize here's a quick way to think of and remember the three main intermolecular forces es hydrogen bonding occurs when you have a polar molecule and you have a hydrogen bonded to nitrogen oxygen or Florine dipole dipole forces form when you have a polar molecule and it's not hydrogen bonding and then finally London dispersion forces are present in all molecules and they will be the only intermolecular force present in non-polar molecules understanding intermolecular forces is something that is very important in chemistry once we know the intermolicular forces we can begin to explain various physical chemical properties that are present in our molecules and if that is something you're interested in or if you need more help with Making Science simple check out these videos linked here [Music] [Applause] [Music] [Applause] [Music]