Today, we're going to begin our study of chemistry right at the foundation with the atomic molecular theory. The material we're going to cover here is in the first Concept Development Study titled, the Atomic Molecular Theory. And you should read that and familiarize yourself with it in preparation for listening to this lecture, as well as the second one. So at the foundation, chemistry is really the study of atoms and molecules. What are the properties of atoms, how do they combine to form molecules? What are the properties of molecules based upon the atoms that make them? And how do atoms rearrange in the course of the chemical reaction to form new molecules out of the original molecules? And what are the properties of those new molecules that we observe from the chemical reaction that's taking place? And how does this all relate to the energy associated with chemical reactions? So we really need to understand existence of atoms molecules, and that is in essence then, the atomic molecular theory, which is what we will develop in these first two lectures. So just as a foreshadowing, where are we going with this, what is it that we want to know? If we look at the world around us, think about all of the objects, all of the spaces around us and so forth. What we discovered, what we will discover, is that all of those materials are all made out of tiny individual particles that we can't see that are called atoms. Atoms themselves are the fundamental particles that make up the pure substances that we call elements. And as you can see, as defined here, the elements are those pure substances that cannot be further simplified. There are over 50 million known substances, pure substances compounds as we will call them in our world and only a little over a 100 of those only 100 materials are elements themselves the simplest of these. And even out of those little over a 100, far few of those are really commonly occurring in most of the compounds. So, how is it possible that such a small number of pure substances, the elements, are able to combine to form such an amazingly large number of, of other kinds of substances? And the answer is that atoms combine in simple integer ratios to form molecules as seen here. And that molecules are themselves now the fundamental components of the larger set of pure substances that we call compounds. And compounds are pure substances, which can actually be broken down into elements. These are really the fundamental principles of chemistry. Everything we know about chemistry is based upon these simple precepts and then sort of flushing those out with more details. So if we're going to understand chemistry, we're going to have to believe in these things. And at the outset, what we might want to then ask is, how do we know that atoms exist? After all, if you look at the substances around you, you don't actually see the atoms. The atoms are far, far too small for us to detect with our normal senses. So we need some other means to do that. Well, in a contemporary world, it turns out that there is a way, that in essence, we can see atoms. We can use an instrument called a scanning tunneling microscope. In this particular case, the image that I'm showing you was taken at IBM's Almaden Labs about 25 years ago now. And in this image, what we are actually seeing with each one of these little bumps here is an individual iron atom and it's sitting on the surface of a piece of copper. There's another one here. In fact, every one of those little red things there is an iron atom. Now, how this particular image was produced, how a scaling tunnelling microscope works. How they use that gadget to precisely arrange the atoms to form this rather interesting looking character set that actually means atom is a topic for a much, much later lecture. All this really implies that we actually have a means by which we can sort of see atoms. But there's really two problems with taking this as the conclusion. One is, we could never have built a scanning tunneling microscope if we didn't already know that atoms exist. In fact, it's based upon the fundamental principals of the atomic theory as it was developed over many, many years. But even beyond that, imagine that we did build a scanning tunneling microscope when we took this image. We would have no real means to know that what we should do in interpreting that little bump there. We shouldn't interpret that necessarily as an atom, we wouldn't know actually what that little blob was that we were taking a look at there. It's only because we already believe in existence of atoms that we know that those little individual points on there are in fact iron atoms. So if that's the case, how would we observe atoms? In fact a little over 200 years ago, when we found that molecular theory was developed. What observations were made that persuade us in the existence of atoms? What actually can we observe? We'll pause for a moment. Look at the objects around you and think about what their properties are. What are the things that you see? And what observations can we make about those things that we see? I put a few of them down here. Every object around you has some sort of volume, it takes up some space. It has mass, it weighs something. It has some density that gives us some sense about whether we think it's a light object or a heavy object, depending upon its mass per volume. It's got a shape that may vary in the course of some kind of chemical or physical process. It has color and those are, are interesting properties as well. And then, there are other more chemical things, like say solubility or conductivity that might be interesting properties to study as well. It turns out that the property that we're actually going to study here is density. I'm sorry. [laugh] It's not density at all, it's mass. The property we're actually going to study here is mass. The reason why we're going to choose mass is actually for reasons that will be clear as we study the next couple of slides here. What can we study in mass? To do that, let's actually take one particular chemical compound here. Here's our compound. It's going to be copper carbonate. And as the name implies, copper carbonate is a material that's made up of the elements copper, carbon, and oxygen. If mass is an interesting thing to study, then maybe what we should do is weigh out particular sample of the copper carbonate. Say 100 grams as is suggested here of the compound and decompose that 100 grams of the compound into its constituent elements, namely the copper in the carbon in the oxygen. And then, we can take the masses of those elements after we decompose them. If we run that experiment, actually, what we discover is that there is 51.5 grams of carbon produced when we decompose the copper carbonate. There is also 9.7 grams of carbon and there's 38.8 grams of oxygen. Now, that's just a experimental result. There's no way that we could have known that, that was true. We run the experiment and we see what we get. But there is an interesting observation there. Interesting observation one is, if we notice carefully, summing together the masses of the constituent elements and adding them all up is actually exactly 100 grams. And that's kind of reassuring, because we started off with 100 grams and we wound up with 100 grams. So apparently, nothing got lost along the way. That is what suggests that mass is actually an interesting quantity to study. Let's run this experiment again. This time let's take 200 grams of the compound and let's see if in fact every time we do this experiment, we wind up with a similar kind of result, okay? We'll run the experiment, and it turns out, if we decompose 100 grams, I'm sorry, 200 grams of the compound, we get 103 grams of the copper. And we get 19.4 grams of the carbon. And we get 77.6 grams of the oxygen. Those are interesting numbers, for a variety of reasons. First of which is, if I add them all together, I get exactly 200 grams again. Suggesting that, in fact, what we start off with is what we wind up with. In some sense, then, what we're discovering is that mass is actually a conserved quantity. That the total mass is the same at the beginning as it is at the end. And in fact, if we study many, many chemical reactions of this type, we will actually observe something called the law of conservation of mass. That every time we take some chemical process, and we weigh all the materials that begin in the process, and we weigh all the materials that we wind up at the end of the process, that the sum is the same. What this allows it to do, is actually consider the mass to be essentially sort of an accounting system, by which nothing gets lost. During a chemical reaction, we can sort of find out where everything went by taking masses. So that's an interesting thing to do. There's also something else interesting about these data. Probably, if you're good with numbers, you've noticed this already. But let's go back and take a look, and notice that here, out of 100 grams of the compound, 51.5 grams were copper and out of 200 grams of the carbon, compound, 103 grams were copper. This number is exactly twice of this number, just as this number is twice of this number. In other words, when I double the mass of the compound, I doubled the mass of the copper, copper involved. And likewise, you can see that I doubled the amount of carbon and I doubled the amount of oxygen as well. It appears then, that in fact, the elements are fixed proportions to each other by mass. In fact, just looking at these data, I can tell that copper carbonate is 51.5% copper regardless of the sample size that I took. And, it is 9.7% carbon, regardless of the sample size, and it is 38.8% oxygen, again, regardless of the sample size that we have exactly the same mass ratio in each one of these things. We should test and see whether or not that shows up sort of consistently. So let's run a couple of other chemical reactions. This has only been one, particular one. Here's one. Let's take a compound which is a combination of carbon and chlorine, we'll call it carbon chloride for now. Won't give you the molecular formula, because I don't know what the molecular formula is, because I don't know that molecules exist yet. I don't even know that atoms exist yet. But if I take 100 grams of this carbon chlorine compound and break it up into the carbon and the chlorine, what we observe is that, that 100 grams is 92.2 grams of chlorine and 7.8 grams of carbon. The sum of those two is clearly 100 grams, so in fact we have conserved the mass just like we thought that we should. But likewise, if we were to take say oh, 50 grams of the carbon chloride. And measure what the amounts of mass are that, of, of the carbon and the chlorine which are involved there. It turns out we get 3.9 grams of carbon and we get 46.1 grams of the chlorine. Well, that's interesting. What that tells me is that the mass ratios of the chlorine and the carbon are exactly the same regardless of the sample size. Here's a different way we could actually look at this problem. It's a lead sulfide question. So we get the same kinds of things here, right? We're going to take a 100 grams of the compound combining containing lead and sulfur and break that compound into the constituent lead and sulfur. And here, you can see, that the compound comes out to be 86.6% lead and 13.4% sulfur. What if we tried to change those? What if we said, you know, look I'm not happy with that? I would like to see if I couldn't wind up with some different ratios. So I'm just going to find a 100, I'm going to find 1 gram of sulfur and I'm going to mix that together with 10 grams of lead. Maybe I could actually combine them in that proportion. And then, I would predict that I would get 11 grams of lead sulfur, because I now believe in the law of conservation of mass. So I'll react them together and try to get 11 grams of lead sulfide. It turns out if I do react them together, I get 7.52 grams of lead sulfide. Again, I'm going to write out the name lead sulfide, rather than give the molecular formula. Because we don't yet know that atoms exist, so we can't know what the molecular formula is. Well, that's interesting. There's a couple interesting things about it. One is I didn't get 11 grams of lead sulfide, the other is I didn't get 11 grams of anything. And I ought to of conserved mass during the course of this chemical reaction. Well, it turns out we did. There's just 3.48 grams of lead left over. What that means is apparently, I can't just combine lead and sulfur in any amounts that I want to. I can only combine them in a particular ratio, but let's try again. Let's add some extra sulfur this time. Maybe if I add sulfur, I can make more. Notice, I only use 6.52 grams of the lead, so I'm only going to use 6.52 grams of the lead again, but this time I'll use more sulfur and see if I can't get more lead sulfide out of that amount of lead. And the answer is no. I get 7.52 grams of lead sulfide exactly again, and let's see then if that's the case. Is there some left over? Well it turns out, yes, there is. There's 1 gram of left over sulfur. In fact, then I can only really react 1 gram of sulfur with 6.52 grams of lead to wind up with 7.52 g of lead sulfide and not have anything left over afterwards. What that tells me actually is that there's a fixed mass proportion that has to be combined, and that fixed mass proportion is pretty clear from what we have back over here on the screen. Apparently, lead sulfide, the lead sulfide is apparently 86.6% lead and it is 13.4% sulfur and I cannot vary those proportions. What we've actually discovered is a new law of nature here. We've discovered something that is called the law of definite proportions. You can see what it says here and it's exactly what we just observed. In a compound, the component elements are in a definite ratio by mass. We've seen that with the lead and sulfur. We saw it with the carbon and the co, and the chlorine. And we saw it with the copper, and the carbon, and the oxygen. That regardless of the sample size we take, we can only get particular proportions. And even if we try to force together different proportions, we don't actually wind up forcing the, the elements to combine in some different way. This is out of our control. It is a fundamental law of nature that these materials will react the way that they do in the mass proportions that they do. Now, does that mean that atoms exist and the reason that they're restricted is because we can only combine atoms in specific ways? It turns out that the answer to that question is, yes, but have we proven that that's true? Is it the case that the law of definite proportions has proven to us that the reason, for example, that I can only combine this amount of lead with this amount of sulfur is because these are lead atoms and sulfur atoms and they are combining in a particular integer ratio? Well, maybe not. In fact, the chemists 200 years ago were not persuaded by the law of definite proportions, because we could answer the following questions. Could we combine things in definite proportions that are not particles to arrive at particular combinations by analogy, for example? I can take red and I can take yellow and I can mix them together and I can get orange. And if I want to get a particular shade of orange, I have to mix together a particular ratio of red and yellow. But that doesn't in any way imply that somehow [unknown] the colors red or yellow, or particularly, doesn't imply that at all. In fact, I could combine the red and yellow in different ways and get different colors of orange, but for any specific color orange, I can only get a particular combination, even though, there don't seem to be particles. Could I mix together particles that don't go together in definite ratios? The answer to that question is also yes. Let's consider, instead of red and yellow as abstract colors, let's make them M&M's. I can combine red M&M's and yellow M&M's in any kind of mixture that I want to in any proportion that I want to. And so, just the fact that I know that they're particles doesn't lead to definite proportions. The conclusion then is that we have not yet proven that atoms and molecules exist. We need to take some more data, but we are well along the right track here. And what we will now do is examine other mass ratios. And we'll compare those mass ratios in the next lecture to be able to establish the existence of atoms and molecules.