this lecture is about hydrogen bonds this is the third category of bonds that we talk about in this class and hydrogen bonds are critical to your understanding of water chemistry since living organisms are mostly water and a lot of living organisms live in water it's going to be a really important concept to understand we're also going to see hydrogen bonds and two other biologically significant molecules that we're going to talk about this semester so we're going to see hydrogen bonds in water which is what we'll be talking about in this lecture and in the coming lecture about properties of water you'll also see hydrogen bonds in DNA and also in proteins so you can see that these are really important bonds to understand they do differ significantly from calent bonds and ionic bonds and in order to really understand how and why hydrogen bonds form we have to take take a little step back and just quickly review Cove valent bonds specifically polar Cove valent bonds versus non-polar calent bonds remember that in polar calent bonds the electrons are not shared equally and the reason they're not shared equally is because one of those atoms has a much higher electr negativity relative to the other so let's look at the picture of that first let's look at who's electronegative and who isn't on the periodic table remember that that upper right corner of the periodic table where nitrogen oxygen Florine chlorine live is the most electronegative part of the periodic table so those elements have the highest pull on electrons the strongest pull on electrons so when they're sharing with something that's not the same level of electron negativity they are not going to share equally equal sharing of electrons is called a non-polar calent Bond hydrogen and hydrogen have the same exact electro negativity so they're going to have the same pole on those shared electrons and therefore share them equally chlorine on the other hand is an electron hog it's going to pull those electrons closer to it it's not sharing equally with hydrogen that has a lower electr negativity relative to chlorine chlorine is not completely stealing those away like it does with sodium in ionic bonding but they're still shared just not equally so let's write all that down as a reminder so polar calent bonds involves unequal sharing of electrons and this is due to a difference in electro negativity between the two atoms one of them has a much stronger pull on those shared electrons and therefore pulls those electrons closers not sharing them equally in that calent Bond non-polar calent Bonds on the other hand are equal sharing of electrons I'm just going to write equal sharing and this is due to similar or the same electro negativity so non-polar calent bonds equal sharing polar calent bonds unequal sharing what does this have to do with hydrogen bonding well let's look at water as our example remember that in a water molecule which is H2O meaning that it's two hydrogens bonded to one oxygen this is a coal Bond but more importantly it's a polar calent bond why is it a polar calent bond it's a polar calent bond because oxygen is very electronegative relative to hydrogen because it's very electr negative it has a really strong pull on those shared electrons so they're not shared equally in that Bond if we drew the electrons as two dots they would be orbiting closer to the oxygen and further away from the hydrogen so something like this remember that electrons have a negative charge so if something negative is spending more time time around you you're becoming a little more more negative and if something negative is orbiting further away from you you're becoming a little more positive and that's what happens in this water molecule because those electrons are spending more time with oxygen orbiting closer to oxygen oxygen is becoming a little bit negative and it takes on what we call a partial negative charge that is a lowercase Delta symbol I didn't do a very good job drawing that but that is the Greek symbol for Delta Delta negative is a partial negative charge it's not becoming an ion it's still sharing those electrons it didn't steal those electrons from hydrogen but those negative subatomic particles are spending more time orbiting around oxygen orbiting closer to oxygen Oxygen's becoming a little bit negative hydrogen's got that negativity orbiting further away sorry I'm blur in the background so my hands disappear at some point but because that negativity is orbiting further away from hydrogen hydrogen actually becomes a little bit positive so this is a partial positive charge so now we have a partial charge difference within this water molecule it was set up by electr negativity by the fact that this is a polar calent bond they are not sharing electrons equally those electrons are spending more time with oxygen orbiting closer to oxygen Oxygen's becoming a little bit negative and as a result hydrogen's becoming a little bit positive that is the key element to understanding how and why now hydrogen bonds are going to form so let's draw several water molecules because you would almost never have just one water hanging out by himself right it's you had a glass of water you've got millions if you had a lake it's going to be an infinite number of water molecules it's not really infinite but it'd be more than we could count so let's draw water now I'm going to draw that polar calent bond is just a line because otherwise it's going to take us too long to draw all of this some of them can flip this [Music] direction and now what's going to happen is all of these hydrogens are going to be partially positive and all of the oxygens are going to be partially negative so let me just quickly go through and write that because this is a critical part of the story I'll show you a better picture of that Delta symbol in a minute and then all of the oxygens are partially negative and now neighboring water molecules are weakly attracted to each other and this is what warms the hydrogen bond the hydrogen bond is going to be between water molecules it's not it's holding the water together the two hydrogens are bonded to that Oxygen by polar calent bonds but if we look at this water and this water and this one and this one they're all connected to each other by this partial charge difference attracting the opposite partial charge so these are hydrogen bonds hydrogen bonds are this weak attraction between the partial positive charge of hydrogen on one water and the partial NE negative charge of oxygen on another water molecule in water hydrogen bonds are what are called inter molecular bonds intermolecular means between molecules so so inter means between molecules they're not what's holding the water together this is not a hydrogen bond this is not a hydrogen bond those are polar coent bonds the hydrogen bonds are here they're what's connecting those neighboring water molecules together let me show you a better picture of that sorry got to scroll all the way up here so you can see those hydrogen bonds in blue connecting neighboring water molecules I told you that hydrogen bonds were also going to occur in water I'm sorry already just talked about water also going to occur in DNA and proteins so let me just quickly show you this we will talk about the structure of DNA and proteins in a lot more detail when we get to those macro molecules in lecture but you can see in this picture of DNA the two sides of the DNA are being held together by hydrogen bonds and this is secondary protein structure which is caused by hydrogen bonding between different amino acids that make up that protein so you will see hydrogen bonds again even after we finish water chemistry hydrogen bonds obviously always involve hydrogen and then that hydrogen is going to you have to be connected to something really electronegative which in chemistry is usually hydrogen bonding with oxygen hydrogen bonding with nitrogen these are the two we're mostly going to see in biological molecules hydrogen can also bond with Florine and that's going to be another example what is similar in all three of these examples is all of these are going to be polar coent bonds because in every single case all three of these oxygen nitrogen and chlorine are very electronegative so these are Electro negative atoms bonded to hydrogen that is not that Electro negative so in every single one of these cases these are going to be polar calent bonds unequal sharing of electrons and in every case those electr negative atoms are going to pull those electrons closer they're not going to share equally and so in every single case I want to make sure I keep my colors consistent the electron negative atom is going to become partially negative because it's pulling those negative subatomic particles closer to it and because hydrogen's like letting some of that negativity go it's orbiting further away from hydrogen hydrogen is going to become partially positive and now these regions of that molecule where these bonds occur where these polar coent bonds occur these regions are now going to be weakly attracted to other regions of a molecule or neighboring molecules and you can see if you look at DNA again you can see in that top hydrogen bond the oxygen on one side is attracted to the hydrogen on the other and then you can see the bond below that the hydrogen is attracted to the nitrogen in another region so that's what is going to make up the hydrogen bonds in DNA and it's also going to be what makes up the hydrogen bonds and proteins you can see the hydrogen bonding occurring between hydrogen and oxygen hydrogen and nitrogen throughout these proteins so those are hydrogen bonds again we're going to talk about how hydrogen bonding contributes to some of the unique properties of water as we move through those lectures on properties of water