Full transcript
0:03Hi. This is Mr. Andersen. Today I am going to talk about chemical bonds. Chemical
0:08bonds are attractions between either atoms or molecules. Now a little bit later we'll
0:14talk about intermolecular bonds, so those are things that are attaching molecules together
0:18but right now we're talking about straight up chemical bonds. In other words attractions
0:22between atoms. I've pictured four of them on this diagram right here. Our four different
0:28types are covalent bonds. So covalent bonds are going to be broken into two different
0:35types. These ones right here would be polar, so I'm talking about the water itself or the
0:42H2O. And then over here, this would be a non-polar covalent. Non-polar covalent, this is some
0:52diesel fuel that's been spilled. It's actually C16H34. And so in a covalent bond what you're
1:01doing is you're actually sharing electrons. You're not stealing electrons, but you're
1:05sharing them. And it goes from sharing them very equally in a non polar bond to sharing
1:10them very unequally in a polar bond. So these are the covalent bonds. Next one we'll talk
1:15about are going to be ionic bonds. Ionic bonds. In ionic bonds bonds you're actually transferring
1:21electrons between two different atoms and those become what are called ions. And those
1:28ions are what are attracting it together. And so in this case we've got NaCl, or just
1:33regular table salt. And that's an ionic bond. Now the other bond that's pictured here that
1:38I won't talk about today are going to be metallic bonds. Metallic bonds are found within metals.
1:44And they don't share their electrons. They kind of collectively share their electrons
1:48so it gives them cool stuff like hardness and conductivity. And so we're not going to
1:54talk about metallic bonds. But today I'm going to talk about covalent bonds, both non-polar
1:58and polar and then ionic bonds. And mostly what I want to talk about today how do you
2:02figure your what kind of bond it is. If you're just given the atoms, how do you know? Okay.
2:07So I want to digress a little bit and talk about the octet rule in kind of a round about
2:13way. When I was a kid, the most important toy you could have was Star Wars action figures.
2:19Now I'm dating myself a little bit. In other words if you went over to a kid's house and
2:24they had all these action figures it was going to be a great day because you knew they had
2:28them all. And so what were the big ones? At least on the good side, the big ones you had
2:32to have Chewbacca. You had to have a Hans Solo, Princess Leia. You had to have Luke
2:37Skywalker. You got to have your C3-PO, Obi-wan Kenobi. Maybe you had the Yoda. And then you
2:42had the R2-D2. Now if you had all eight of those you had a complete set. I remember flushing
2:49my R2-D2 down the toilet just inadvertently and it was like one of the most sad days in
2:54my whole life. And so if you had, let's say seven of the big eight action figures, you
3:01really wanted that last to complete your set. And so atoms are the same way. And they have
3:07what's called the octet rule. And so what does that mean? If you're oxygen, oxygen has
3:13six electrons. It would love to have eight so, it's got 1, 2, 3, 4, 5, 6. It would love
3:22to have eight. And so it can share those electrons with carbon and so it can have a complete
3:28set. In other words, the secret of life or secret of chemistry, at least half of chemistry,
3:35is that atoms are always searching out a complete outer level. In other words the want eight
3:40electrons in the outside level. Now likewise, carbons, since it's got four, it's got 1,
3:452, 3, 4, it can share those with the other oxygen. And so in carbon dioxide, they both
3:54have eight or all three of them have eight and so they're all happy. In other words,
3:57it's like have a complete set of action figures and you're good to go. Okay, so how do you
4:03know which type of a bond it is? Well this will vary a little bit depending on where
4:07you get your stats. But these numbers are pretty important to remember. If you have
4:11somewhere between an electronegativity difference of 0.5 and 0.0, it's a non-polar covalent.
4:18If you're electronegativity differences are between 1.7 and 0.5 then it's a polar covalent.
4:23And if it's above 1.7 then it's an ionic. Now first thing I need to talk about then
4:28is electronegativity. What is electronegativity? Electronegativity is a measure of how much
4:33you want electrons. And so the more electrons you want or the more you need those electrons,
4:41the higher you're electronegativity is. And so fluorine up here, fluorine has 7 valence
4:47electrons. That means if it can get one more valence electron it's going to have a complete
4:53set. In other words it almost has all of the Star Wars action figures except maybe Yoda.
4:57If it can get that last one, then it's going to be happy. And so the highest electronegativity
5:02of everything up here is going to be fluorine. It has an electronegativity of 3.98. And so
5:08as we move across the periodic table those numbers get larger. Also as we go up on the
5:13periodic table it increases as well. So who doesn't want any electrons? Well it's francium.
5:18Francium has an electronegativity of 0.7. That means it has one valence electron and
5:25it doesn't need to hold on to it that much. In other words it would be easy to give off
5:30that electron. It has a complete set right underneath it. And so it's going to be really,
5:34really happy. And so by looking at the differences between the atoms and their electronegativity
5:39differences, we can easily figure out what kind of chemical bond we have. So let's do
5:44some for example. Let's do water. And so you know that water is H2O. So all we do is look
5:51up the electronegativity of the two atoms. And so here's our hydrogen right here. Here's
5:56our oxygen. Well oxygen's electronegativity, I'm just reading it on this chart, so in my
6:01class you'd have to use your periodic table, it has an electronegativity of 3.44. I'm going
6:07to subtract that of hydrogen, which is 2.20. And so I get a difference of 1.24. That's
6:16the difference in their electronegativity between these two. So what kind of bond is
6:20that? Well I look on my chart. It's somewhere between 0.5 and 1.7. And so I know immediately
6:27that that's going to be a polar covalent bond in water. Let's go to another one. Here's
6:32diesel fuel. This is centane. Diesel fuel is going to be C16H34. So if I look at, it's
6:41hard to draw there, the bond between a carbon and a hydrogen, I just find them on the periodic
6:46table. So carbon has an electronegativity on 2.55. In other words it wants the electrons
6:51a little less then oxygen just did. Hydrogen has an electronegativity of 2.20. And so if
6:59I find the difference between those two I get 0.35. And so what type of bond is found
7:05in diesel fuel? Well it's less than 0.5 and so that's going to be a non-polar covalent.
7:11That also explains why when you have diesel fuel and you pour it into water they don't
7:15mix. Because one of them is non-polar and the other one is polar. And only likes will
7:21attract to likes. Okay. Let's go to the last one. That's salt. Salt, what is salt? Salt
7:28is NaCl. If we look up our electronegativity of salt, we're going to find sodium, oops,
7:35all the way over here so it's 0.93. I'm going to then find chlorine. Chlorine is 3.16. Three
7:40point one six. So I subtract 3.16 minus 0.93, as I try to do that in my head, that'd be
7:50like two point what . . . 2.23. So it doesn't matter if I got my math right. We know that
8:07that is greater than 1.7 so we know that' going to be an ionic bond. And so when you
8:13look at salt, these are actually chlorine ions that are attracted to sodium cations.
8:19In other words you've actually transferred that electron from the sodium to the chlorine
8:24and so those like charges are attracting it. If we were to do one more, this in ammonia.
8:31And so ammonia is NH3 and so you should be able to figure this one out. So first we look
8:39up hydrogen. Then we look up nitrogen. And so the difference is going to be 3.04 minus
8:482.20. And so which one is it? Is it non-polar? Is it polar covalent? Or is it ionic?