MIT OpenCourseWare http://ocw.mit.edu 3.091 Introduction to Solid State Chemistry, Fall 2004 Please use the following citation format: Donald Sadoway, 3.091 Introduction to Solid State Chemistry, Fall 2004. (Massachusetts Institute of Technology: MIT OpenCourseWare). http://ocw.mit.edu (accessed MM DD, YYYY). License: Creative Commons Attribution-Noncommercial-Share Alike. Note: Please use the actual date you accessed this material in your citation. For more information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms MIT OpenCourseWare http://ocw.mit.edu 3.091 Introduction to Solid State Chemistry, Fall 2004 Transcript – Lecture 15 First of all, welcome to the families. It's family weekend. So, welcome to the parents, sibs, and other members of extended family. I hope you have a lovely weekend on campus, and especially welcome to 3.091. I have nothing special planned today, just a run-of-the-mill lecture so you get a sense of what it's like to be sitting in one of these seats that you've paid so dearly for. I hope we can convince you that you've made a smart decision in putting your son or daughter on this campus. You will come to the quick conclusion it's not because of the facilities. Facilities here are nothing special. Classrooms, labs, and so on, it's the peer group. It's not the faculty. Faculty are OK, but I think it's the peer group. I think it's the students are going to be the friends and colleagues that makes it a special experience here. I'm teaching freshman chemistry, but it's taught from the Department of Materials Science and Engineering. And, I just want to make one little plug. If you walk up and down the infinite corridor, by and large it's ugly because it's decorated with administrative offices, with one exception. As you get to the east end, it looks like an engineering school because there are labs associated with the Department of Materials Science and Engineering. And, if you ever wondered what's going on in those labs, it's your chance to find out. So, this afternoon from 3:30-5:00, it will be possible to tour the undergraduate laboratory, which is on the south side, and the nanomechanics lab on the north side. So, if you're finding uncomfortable silence with your son or daughter, and things are kind of dragging on, walk down to the east end of the infinite corridor, and you can see some interesting things. They're going to have some demos, and whatnot. And, the other announcement is that on Tuesday, we will have quiz six based on homework six, but there is a few questions on homework six touching upon x-rays. And at the rate we're going, we will not have covered that material. So, ignore the questions on x-rays. But, the rest I think you should be familiar with. And, we'll test at the end of the period instead of the beginning. That way, you will have some time to interact with the recitation instructor. So, what I want to do today is something really important. And, it's fitting that the parents are here. Up until now, by and large, perhaps with the exception of some of the band gap material, what we've been covering, you could've seen bits and pieces of that in high school. And, if you've taken a lot of high school chemistry you might be familiar with it. Well, starting today, say goodbye the high school. This is solid-state chemistry now. That's why we have the crystal models appear. And, what we're going to do is we're going to begin with a little bit of review. And so, if you'll take a look at the handout that's going around, this will, of course, be posted on the web afterwards. But, just to follow along, I know the parents are here. It gives them some comfort factor to hold a piece of paper in their hands. So, that's your Linus blanket for this morning. So, I want to give the big picture. And the big picture is that the thesis of 3.091 is that electronic structure informs bonding, which then informs state of aggregation. And just to remind you, we started back at the beginning of September with the Bohr model of the atom, and saw that it was inadequate. Sommerfeld put in a few patches, came quantum numbers, Aufbau Principle, multi- electron atoms. Then we could explain the patterns in the periodic table. And then we posited octet stability as something that atoms strive for. And that gave rise to, first of all, just inert gases, explained them. But then we said, you know, things might engage in electron transfer. We could describe ionic compounds, then electron sharing, covalent compounds. Then we talked about metals, which is 75% of the periodic table, and en route saw the Van der Waals bonds as a fourth type of primary bonding. And then, we looked at secondary bonding, which is prevalent in covalent compounds, and we saw these three types, dipole-dipole, London dispersion, and hydrogen bonding. And, ultimately, that allowed us to decide whether something is a solid, or a liquid, or a gas at a particular temperature. And, we are particularly interested in 3.091, since this is solid-state chemistry, the circumstances under which something is a solid. So, up until now, I think you're comfortable with classifying materials on the basis of their bonding type, in other words, is it a covalent solid, or ionic solid, or a metal. But, I want to show you a little more interesting way of classifying because we are going to classify things according to atomic arrangement. And so, if you will flip over the sheet, we'll talk about atomic arrangement, and how it plays into classification of solids. By the way, solid, let's define solids. Solid is something that is dimensionally stable. It doesn't flow under its own weight. It has a volume of its own, whereas a fluid, either a liquid or gas, takes the volume of its container. The food goes to the walls of the container. The liquid in a gravity field sinks to the lowest point in the container. But, the container determines the shape. A solid retains its shape. So, now I want to classify solids by atomic arrangement. So, I don't care if they are metals or ionic compounds or covalent compounds. There are two classes: ordered and disordered. Isn't it nice when you can take things and classify them, just bifurcate? It's one or the other, just move through that decision tree. So, let's look at ordered solids. Ordered solids have a regular atomic arrangement, whereas disordered solids have a random arrangement. I put an asterisk next to random because it's not totally random. There is some local order, but there's no long-range order. So, the order is short range. And, what do we call such solids? The ordered solids are called crystalline solids. And, we have a plain, everyday word for such an ordered solid. It's called a crystal, and I've been using that term a lot. I talked about Davisson and Germer irradiating a nickel crystal. That's to distinguish the material for something that's disordered. If it's disordered, we say that the atomic arrangement is amorphous. And, there is a simple Anglo-Saxon word for an amorphous solid: glass. Glass is an amorphous solid. You might think glass has to do with whether something is transparent or not. And, I want to disabuse you of that. We've talked about transparency. And, what do we know about transparency? If we want to talk about transparency, if something is transparent, what do we do? We ask ourselves, is the band gap in that material greater than three electron volts? If the answer is yes, it's transparent. It is less than three electron volts, it's an absorber. So then, what does glass mean? Glass means no long-range order. Let me give you some examples. We could say, for example, we've got diamond. Diamond is crystalline. And, yet it's transparent. Why is it transparent? Band gap is about 5.4 electron volts. Obsidian, this is a glassy rock. It's got no long-range order. It's an amorphous material. And, it is opaque. Obsidian is opaque. So, here's an example of an opaque glass and a transparent crystal. So, as of now, I want to make sure that nobody in 3.091 from this day forward ever, ever says glass is transparent. I will show you before the end of the month metallic glass. How can it be a metal and glassy? Well, it must mean that there is metallic bonding operative. But, the atoms are not in a regular arrangement. And, that material is not transparent to visible light, because you know that metals have no band gap whatsoever. So: something important here to retain. So, the parents will now learn what the students have learned. When we go into a new unit, we begin with a history lesson. So, let's go in the way back machine. We'll go back to early efforts in trying to describe ordered solids. So, that's why crystallography is the study of ordered solids. Crystal comes from the Greek, krystallos, which is one of the terms that might refer to ice.