 Ok, in this video I want to talk about a feature of an atom or different atoms called electronegativity. Electronegativity is a feature of an atom, and depending on how much electronegativity an atom has, it can cause unequal sharing of electrons between two atoms. So we spent a lot of time on the last couple of videos talking about how different atoms can share electrons. Here are two hydrogen atoms sharing a pair of electrons, and this is called a single covalent bond. This is one way of drawing a single covalent, this is another way. What I'm going to tell you is that different atoms can hog electrons even though they're sharing, and I'm using hog in the informal sense of the word. They might share electrons with another atom but they don't share equally. And the amount of unequal sharing that they do depends on how electronegative or how much electronegativity the atom has. So unequal sharing, this is kind of my cartoon of unequal sharing, this is how my kids share with each other, not terribly equally, but it does look like she got at least a little bit of the ice cream. So another way of saying what I've been saying is that some elements have a stronger pull on electrons than other elements. And I want you to remember that electrons have an electrical charge of minus one. So we can say that electrons have a negative electrical charge, that's going to be important in a little bit. I want to talk about covalent bonds again, but this time I want to talk about a special type of covalent bond called a polar covalent bond. Imagine that I have two atoms, hydrogen and bromine. And this solid line in between the two of them, this one right here, means that there is a single covalent bond. Single covalent bond holding the hydrogen and the bromine to each other. They're stuck together, they make one big molecule, which I'm going to draw like this little blob here. And this covalent means that they're sharing. But what I'm going to tell you is that, and you didn't necessarily, you couldn't have known this ahead of time, but what I'm going to tell you is that bromine hogs the electrons that are being shared compared to the hydrogen atom. It doesn't have the electrons all of the time, but it's not sharing equally with the hydrogens. Another way of saying that is that bromine is more electronegative than hydrogen. And because it's more electronegative, you can think of it as not sharing the electrons equally. They spend more of their time, even though this is one big blob molecule, the electrons spend more of their time on this side of the molecule. And I just said a minute ago that electrons are negatively charged. Now if the electrons were spread evenly around this molecule, then there wouldn't be any charge, any electrical charge on any part of the molecule. But that's not true. I just said that the electrons spend more of their time on the part that I'm highlighting here. Because they spend more of their time on this part, and because they're negatively charged, this part of the molecule, the right side of the molecule, the way I have it drawn here, has a little bit of a negative charge. It's not a full-blown negative charge the way that chloride, Cl1-, might have. You can think of this as a partial negative charge. So it's not a full-blown negative charge, but it's there. It's like an itty-bitty negative charge. But if the electrons are spending part of their time more than their fair share of their time on the right side of the molecule, making the right side of the molecule slightly negative, then the other side of the molecule has to be correspondingly slightly positive. So the left side, the way that I have this molecule drawn, has a partial positive charge. Because the electrons are just not spending their time over here, and so one side of the molecule is a little bit negative, one side is a little bit positive. The way that this is drawn by chemists to tell people that there's a partial negative charge is they write the Greek letter delta. This ugly-looking thing here is the Greek letter delta. It's a lowercase delta. So if you want to tell people that part of your molecule has a partial negative electrical charge, you write the Greek letter delta and you write a minus sign next to it. If you want to tell people that a part of your molecule has a partial positive charge, you write the Greek letter delta and you write a positive plus sign next to that in the upper right. So this is chemistry's way of saying, look, these atoms are not, they're sharing, but they're not sharing equally. The electrons seem to be spending more of their time on this side of the molecule and because of that, the other side is slightly positive, positively charged, and the right side, the way I have it drawn, is slightly negatively charged. So who cares? Fair enough. I guess that's a fair enough criticism. I'll show you an example where this is important in a few minutes, but I want to sort of introduce a new term. I said that this is a single covalent bond, but because there are these little electrical charges on either side of that covalent bond, what we say is that this molecule has electrical poles. The more fancy way of saying that is that this molecule has a dipole, but you don't need to know dipole. I just want you to realize that one side of the molecule is different from the other side as far as electrical charge is concerned. Sometimes people refer to these as electrical poles and because of that, this is not only a single covalent bond, it is a single polar covalent bond because people are saying, look, the left side of the bond is not the same as the right side of the bond. There's a difference in electrical charge on either side and it's a way of telling people that this is a special type of covalent bond. If I had two bromine atoms, imagine that I had two bromine atoms also with a single covalent bond. Now, a couple of minutes ago I told you that bromine is more electronegative than the hydrogen atom, but these are two identical bromine atoms. So even though bromine hogs relative to hydrogen, bromine hogging against bromine, it's basically going to be an equal fight as to who gets the electrons. Because of that, if I have two bromine atoms stuck to each other, they make one big molecule, but there is no little electrical charge on either side. They are sharing equally in this case. And if they're sharing equally, this is a single covalent bond, but it's called a single nonpolar covalent bond because, well, there are no little electrical poles on either side of the covalent bond. So people use this phrase, nonpolar, to distinguish these types of bonds where the sharing of electrons is equal from this type of bond up here where the sharing of the electrons is not equal. So I want you to know those terms. I want you to know what a polar covalent bond is and a nonpolar covalent bond. I also want you to be able to recognize when someone is telling you that an area of a molecule has a little bit of a positive charge or a little bit of a negative charge and how that relates to polar covalent bonds. The other thing I mentioned is I said that there's every element has a feature called electronegativity and some elements are more electronegative than other elements. What we would say is bromine is more electronegative than hydrogen, so bromine hogs electrons when it's sharing them with hydrogen. This is a periodic table showing the electronegativity of different atoms and the idea is the darker the color, the darker red, the more electronegative your atom is. Fluorine, it's not showing up very well here, but fluorine is the darkest box, so fluorine is the most electronegative atom. It's the biggest electron hog. And the general trend is if you travel in this direction, in this direction, in the periodic table, you become less electronegative. You become less of an electron hog. In the previous slide I was showing you bromine, which is pretty electronegative. It's a dark colored box compared to hydrogen, which is less dark. And so I said bromine was more electronegative so it hogged the electrons, and you can kind of see that visually with these colors comparing hydrogen to bromine. Again, I don't want you to memorize this chart. I don't want you to think about this too deeply. I just want you to realize that different elements have a different amount of electronegativity, which means they will hog electrons to a great extent or to a lesser extent depending on what they are and also depending on who they're sharing with. So keep that in mind. I'm going to talk about these two atoms on the next slide, so I want to highlight them as well. Oxygen is pretty electronegative. It's a bit of a big electron hog, especially compared to hydrogen. Hydrogen is not a hog. So if you have oxygen sharing electrons with hydrogen, the oxygen is going to hog the electrons at least a little bit, and so the oxygen will have a little bit of a negative charge. The hydrogen will have a little bit of a positive charge, and I want to talk about that on the next slide. I want to talk about water, and I just said that oxygen, here's oxygen in water, is more electronegative than hydrogen. Because of that what we can say is oxygen, or the part of the water molecule that's oxygen, has a partial negative charge, and the part of the molecule that's hydrogen, both parts, have a partial positive charge. So this is a polar covalent bond. So is this covalent bond over here. They're both polar covalent bonds in water. The moderately interesting part is this. Imagine that you have a bunch of water molecules maybe in a glass of water, and all of the water molecules are rolling around on top of each other. However if you have a second water molecule, here's my second water molecule, rolling by my first water molecule. Every once in a while the oxygen of the second water molecule, which has a little bit of a negative charge, will roll by the hydrogen of another water molecule which has a little bit of a positive charge. And because they have opposite little electrical charges, they are going to be a little bit attracted to each other. There's going to be at least a temporary pull between those atoms. It's not a strong pull, because this is a partial positive charge, this is a partial negative charge. So it's not as strong as an ionic attachment, but the pull is there. And so the way that I envision this who knows if it's really true, is that these molecules are rolling around next to each other. But every once in a while they point in just the right way that this oxygen is attracted to this hydrogen. And maybe they pause temporarily, because they're a little bit attracted. But there's enough energy in these molecules that they unpause and one water molecule keeps rolling around. But every once in a while they're pausing. This little pause is caused by this attraction, this partial positive charge, attracted to this partial negative charge. And people draw them, because they're real and they're measurable. They draw this attraction. However, they draw it this way. They draw it with a dotted line. The reason they draw it with a dotted line is to tell people that it's not as strong as these covalent bonds. Not even close to as strong, but it's still there. And this is a type of bond. It's a weak bond or relatively weak compared to the ones we've been talking about, but it's still a bond. And it's called a hydrogen bond. Sometimes beginning students get confused and they say, well, anything where hydrogen is attached to something else, they'd say, oh, that's a hydrogen bond. But it isn't. Not a hydrogen bond. Hydrogen bond has a special meaning. It's basically an attraction between a hydrogen atom, which has a partial positive electrical charge, and some other atom, which has a partial negative electrical charge. It doesn't have to be oxygen. There are other atoms that hydrogen can be partially attracted to, but oxygen is a common one, and it's especially talked about because water is so common. So this is the third type of bond that we are going to talk about in this unit. We talked about ionic bonds. That's when atoms with full-blown electrical charges get attracted to each other. We talked about covalent bonds. That's when they're sharing electrons. And hydrogen bonds is caused by polar covalent bonds. This is a polar covalent bond. These are polar covalent bonds. But they're not polar covalent bonds. They're just caused because polar covalent bonds make a little positive charge and a partial negative charge on different atoms. Because of that, they're going to be attracted to each other. So I want you to know this term as well. I want you to be able to recognize the dotted line. I want you to have a general understanding of how hydrogen bonds actually get formed. So that's it for this unit. Just a big summary. We've talked about ionic bonds. That's atoms sticking together because of opposite electrical charges. Covalent bonds. Atoms sticking together because they're sharing electrons. Polar covalent bonds. That's atoms sticking together, but they're not sharing the electrons equally. Nonpolar covalent bonds sticking together and sharing equally. And we just talked about hydrogen bonds, which is caused when you have polar covalent bonds. So that's it for this unit. Good luck. Bye-bye.