sorry about that i'm going to continue i had paused the previous video and it stopped instead of letting me to continue so this is going to be part 2 the last part of this chemical reactions and bonds discussion so let me go back to where i left off so if you look at these two carbon atoms they're going to share a pair of electrons that's what that single line means and there's this is just a small portion of a big molecule but these two carbons are said to have the same electronegativity and that term means attraction for electrons so both carbons are going to share electrons equally because they have an equal electronegativity but not all atoms are like that when you see these two atoms here oxygen sharing electrons with a hydrogen that's a single bond right a single line a pair or two electrons being shared notice that the picture does not look the same these little light blue clouds here are representing electron motion and electrons being shared look at this little picture down here there's more blue around the oxygen o than around the h it's trying to signify that electrons are spending more time around the oxygen compared to the hydrogen atom up here the pair of shared electrons the two electrons moved around right we're not stationary but they were being shared and they spent equal amounts of time around each carbon that's not the case down here the shared pair of electrons those two electrons spend more time around the oxygen than they do around the hydrogen so in both cases a single pair of electrons two electrons are being shared but these two carbons are sharing the electrons equally the electrons are spending about the same amount of time beside of each carbon that's not the case down here and we say that this is due to different electronegativities oxygen has a different electronegativity compared to hydrogen oxygen is what i call an electron hog because it has more of an electronegativity an attraction for electrons compared to hydrogen so think of it as kind of like a tug of war they both have a hold of the electron rope but oxygen is pooling a little bit harder compared to hydrogen that's still shearing see it's connected oxygen if it was totally pulling the electrons away from hydrogen there wouldn't be this little bar at all right they wouldn't be sharing at all it would be a transfer of electrons that's an ionic bond we're talking still about shearing but just unequal sharing okay so there's two atoms that are important to biology that are especially attracted to electrons and therefore pool have a bigger electronegativity therefore pull electrons closer to themselves when they're sharing one of those is oxygen and another is nitrogen as you go to the right of the periodic table and adam's electronegativity increases but as far as we're concerned this course oxygen and nitrogen are the big electron hogs when they're sharing electrons are always going to have a stronger hold on those electrons and electrons are going to spend more time around oxygen and nitrogen because of that and that has a consequence okay and that's what i'm going to explain these little charges here okay but just to recap this upper picture nonpolar covalent that pair of electrons were shared equally among those two carbon atoms down here between oxygen and hydrogen that pair of electrons is shared unequally elect those two electrons are spending more time on average around oxygen than they are around hydrogen but they're still being shared so this upper picture is called a non-polar covalent bond equal sharing this second picture between oxygen and hydrogen is called a poor covalent bond unequal sharing now a little bit more on that polar covalent bond because of the unequal sharing the electrons are spending more time around the electron hog right oxygen same way with nitrogen if this was nh a shared a pair of electrons between a nitrogen and a hydrogen it would be the same picture than they are around the hydrogen so electrons are spending more time around oxygen than they are around hydrogen and that gives the oxygen atom in this case a slight charge see this little symbol here looks like kind of a little fancy q it's bigger down here that's a delta but think of it as partial oxygen has a partial negative charge because electrons which are negatively charged particles are spending more time around that oxygen because it has a higher electronegativity than hydrogen see here the shared pair of electrons are spending less time around the hydrogen so therefore hydrogen has a slightly positive charge that's because the negatively charged pair of electrons is spending more time to the left towards the oxygen so slight negative charge on oxygen slight positive charge on the hydrogen it's important to realize that they're not full charges they are not for example the full charge that sodium and chloride had on itself sodium had a full positive charge see no delta symbol chloride had a full negative charge no delta symbol that's because that was a complete transfer of an electron in this case we're still sharing electrons it's just electrons are spending slightly more time around one of the atoms so that's where you get this partial charge okay and this is actually going to set up some important properties of water that make it crucial for life these polar covalent bonds so take a look at this water molecule it's one oxygen and two hydrogens between each hydrogen there's a single bond with the oxygen so imagine there's a single line here and here where these arrows are so a pair of electrons is shared between this oxygen and this hydrogen and this hydrogen and that same oxygen now the reason why the arrows are pointing to oxygen is because oxygen is an electron hog oxygen is sharing electrons but it doesn't share evenly it pulls the negatively charged electrons closer to itself that's why there's partial negative charges on oxygen so think of it as slightly negative because the electrons that it's sharing is spending are spending more time around that oxygen the two hydrogens are slightly positive because the shared electrons are spending less time around them giving them a more positive charge okay but they're partial or slight charges they're not completely transferring the electrons they're sharing them so that's an important difference with ionic bonds you only have these partial charges when the covalent bond is of the polar variety because when you have equal sharing the electrons are spending the same amount of time on average between the two atoms so you don't have the negatively charged electrons spending any more time between one atom so you don't have any charges okay it's only with polar covalent molecules or compounds okay now before we go on to this last slides the hydrogen bonds before we go on to that i do want to give you some general rules of thumb you're going to notice in the study guide that i ask you based on a diagram that we were working on with the previous group of slides diagraming atoms and telling me whether they are going to be stable or unstable okay how do you know what type of bond an atom of form so will it form an ionic bond covalent etc there are some general rules of thumb so for example when you have atoms that have one electron two electrons three electrons or seven electrons in their outermost shell see here sodium has one the chlorine atom has seven so there are examples of this so again if atoms have one electron two electrons three electrons or seven electrons they have a tendency to participate in ionic bonds because for atoms that have one two or three valence electrons it's easier to lose those and have their second shell be stable with the eight electrons with atoms that have seven electrons in their outer shell like chloride it's much easier to gain one electron so those types of atoms tend to transfer electrons again atoms that have one two three or seven valence electrons those tend to transfer electrons to meet the octet rule and be stable all others atoms that have four five or six valence shell or outer shell electrons those tend to share electrons to be stable so general rules of thumb there that's how you would answer that study guide question okay now let's go over briefly we're going to get more into this third type of bond in the next section water but i'm going to briefly talk about this third type of bond this is the third type of biologically important bond so hydrogen bonds just like the name implies hydrogen is involved okay so these are bonds involving hydrogen but it's important to realize that hydrogen bonds are a consequence of polar covalent bonds forming so i'll show you what i mean by that let's take a look so let's take a look at this upper picture between these two water molecules so i'll explain the one on the right here coming up here's a water molecule oxygen and two hydrogens here's another water molecule oxygen and two hydrogens now they're not drawn they're not drawing out their full diagrams here but look at the lines a single line means a single pair of electrons shared so that's what oxygen in water does oxygen in a water molecule shares one pair of electrons with this hydrogen and another pair with this hydrogen same with this water molecule it's going to do the exact same thing so inside of a water molecule these are just bigger down here each one of these is a water molecule between the oxygen and the hydrogen that which i'm trying to point out here i wrote that in is a polar covalent bond as we were describing on the previous slide so oxygen is sharing electrons unequally with the hydrogen it's an electron hog pulling electrons closer to it okay unequal sharing that creates partial charges on the atoms involved in this case a partial negative on the oxygen because the electrons are spending more time with it and a partial positive charge on the hydrogen because electrons that are shared are spending less time with the hydrogen those partial charges can interact with other charged molecules so look water actually interacts with itself these water molecules are showing one two three four five here each one of those has two polar covalent bonds making it up right this oxygen sharing electrons unequally with this hydrogen this oxygen sharing electrons unequally with this oxygen or this oxygen sharing electrons unequally with this hydrogen excuse me so those are two polar covalent bonds setting up these partial charges now look at this hydrogen this hydrogen is going to interact with an oxygen that's part of a different water molecule a neighboring water molecule see these two these couple of dotted lines that's representing a hydrogen bond it's drawn in dots up here and that's a generally weak interaction it's a weak interaction between a partially positive hydrogen atom and a partially negative or fully negative other atom okay opposite charges attract so this partial positive hydrogen is going to interact with this partial negative oxygen as part of another water molecule so hydrogen bonds is when an already bonded hydrogen atom that has a slight positive charge is attracted to an already bonded negatively charged atom or an atom with a slight negative charge oxygen in this case so this is a hydrogen bond between these two water molecules see it's happening again this is a hydrogen bond hydrogen bond hydrogen bonds so individual water molecules are interacting with neighboring water molecules through hydrogen bonds but inside an individual water molecule there are polar covalent bonds so students have a tendency to mix those up within one water molecule that's the unequal sharing the polar covalent bonds a hydrogen bond is not unequal sharing it is a weak attraction of opposite slight charges right a partial or slight positive charged hydrogen interacting with a slight negative oxygen or nitrogen okay now even those these are relatively weak in other words these can be broken easily they reform really quickly okay and so they're really important to the structure of some molecules important molecules in us so we already mentioned the interaction interactions excuse me among separate water molecules but dna okay that large molecule is actually two-sided and the two side sides of dna interact because of hydrogen bonds proteins have a three-dimensional shape and that three-dimensional shape is due to hydrogen bonds within that protein in other words that's what's this that's what this picture on the top right is trying to depict this yellow blob think of that as a big protein and we're just looking at portions of that big protein here's a few atoms that's interacting on this point of the protein on the other side of the protein here's a few atoms interacting but look it's wrapping on itself because one part of it is forming a hydrogen bond with another part of it why here's hydrogen it is sharing electrons unequally as it does always with nitrogen and oxygen so it has a partial positive charge it is going to be attracted to a negative charge in this case a slight negative charge of an oxygen that's on the other end of that protein and so that's how proteins can wrap around themselves and form these big three-dimensional shapes and protein shape is actually very important to the function in the body and we're going to talk more about that in detail in the later part of this unit but that's what a hydrogen bond is it's weak individually but it can reform very quickly and many important molecules in us have lots of those hydrogen bonds okay so it's basically strength strengthen numbers but if we're doing an all-out ranking of strongest versus the weakest which one of these is the strongest by far covalent bonds sharing electrons is always a stronger bond compared to the others then ionic bonds when electrons are transferred and ions are excuse me ions that are oppositely charged are attracted to one another okay that's the second strongest and then hydrogen bonds they can break relatively easily but then they can reform but they are the weakest of the three so make sure you go back and practice those definitions there's lots of practice questions underneath the testing your understanding section of canvas okay both ones that i put together you'll see it in lecture exam one practice questions it'll be called chemical reactions and bonds that group of questions is relevant to this section and also there's questions in sachin as well the next set of slides we're going to talk about are going to build on this hydrogen bonding between water molecules and i'm going to discuss how that's important for water special properties and why it's so integral to life