In this lecture, we're going to study the material from concept development study 3 which actually has to do with the structure of an atom. in other words, we're going to push past the point where we know about atoms, at this point. And try to to really get into the nuts and bolts of what atoms looks like. Let's begin this process, actually then, by thinking a bit about what do we know about atoms. The answer, actually right now is not all that much. Let's see, what have we studied so far? We've determined the atomic masses of all of the different atoms in the periodic table. We've figured out how to do that. And, by using those relative masses in calculating numbers of moles, and comparing the numbers of moles of atoms in different kinds of compounds, we've been able to determine molecular formula as well. But that's about it, we actually don't know anything about sort of the inner workings of an atom, or what those pieces could gather. Now, could we for example use the atomic masses to have an understanding of why different kinds of molecules form. The answer to that is fairly quickly no, because if we look at atoms that have very similar atomic masses, they turned out to have very different kinds of molecular bonding. For example, oxygen likes to combine with two atoms to form H2O, but fluorine, which has an atomic mass only slightly more heavy than oxygen, only combines with one hydrogen. We compare the atomic mass of fluorine to the atomic mass of neon. Fluorine's 19, neon is 20. Fluorine is one of the most reactive elements in the periodic table, and neon won't form any molecules at all. What that tells us is that simply knowing what the atomic mass is, will allow us to determine what kinds of molecules will form. But will not give us any predictive method for figuring out what types of molecules will form, or any understanding of those molecules. So, what is it that does determine? What type of a molecule will form? For us to understand that, we're really going to have to have more information about what atoms essentially look like. What're their structures and so forth. One recent image of an atom is one here which is actually on IBM's website. This is an atomic force microscope image of a xenon atom sitting on a nickel surface, this blue region in here is actually a single xenon atom. But that's fairly useless for our purposes, it doesn't really tell us anything about the structure of the atom. Kind of need to be able to look to the inside of that, and to look to the inside of what we're really asking. What is it that makes up an atom? What are its pieces, what are those parts and how are those parts arranged and how do those parts arrangements differ as we go from one atom to the next? Well, how would we learn such a thing? If we really were to try to understand what the interior of this particular atom looked like, how might we go about that? It turned out that one way to do this historically was actually essentially just sort of throw things at the atom, that actually is to fire little individual particles. And the experiments which were actually done entailed something called alpha particles. Alpha particles are positively charged. And they are moderately massive, certainly much more massive by quite a lot, than electrons. Those are a couple of characteristics that we might know about them. One of the things we could do then, is to say, could we fire alpha particles at individual metal atoms. Well this was actually done in Rutherford's laboratory just slightly over 100 years ago. Now, the way in which they did it was to say, take a thin piece of gold foil. So, sometimes this is called the gold foil experiment. Quite thin, something like, a micron thin, which is about the width of a hair. And to fire alpha particles. At that gold foil was the expectation that when the alpha particle encountered the gold atoms in this foil, that it might scatter them in different directions. This is sometimes called the scattering or even alpha particle scattering experiment, where we ask, where do those alpha particles go? Maybe by following their paths, we can determine what was inside the atoms as they encountered. Set of the experiment might look a little bit more like this, as it's shown on your screen here. In this particular case you can see right in the middle of the area here, the thin gold foil. Here is a source for alpha particles being fired at that thin gold foil, and then to determine what direction those are scattered around, we surrounded detector out here. That lights up every time an alpha particle, encounters the screen in that particular direction. And then we ask, sort of, what is the distribution of those alpha particles that they fire out? What's actually observed? Here are the actual observations from the alpha particle scattering experiment. First, it turns out that most of the particles pass through that gold foil, not deflected at all, as if the gold foil were not even there. Let's go back and look at the image again here. And in fact, what happens here, as we can see, then most of the particles actually show up directly on the opposite side of the source, meaning that they passed through the gold foil as if there was no gold foil there. Secondly, some of these alpha particles, a small number of them, very small number of them, are in fact deflected and those would be the ones that are observed say, going off at angles around here or here, not passing directly through the gold foil without changing their paths, but only changing their paths slightly. And then finally, there are a few which actually come back in the direction of the detector here. Essentially, they are rebounded. It's a very small number which do that. But we've got three basic observations then, most of the particles pass straight on through, as if there was no gold foil there. A small number seemed to change their paths and an even smaller number seemed to turn around, and bounce back towards the source. If we stop to try to understand why that might be the case. The first of these is actually the one which is maybe the most striking, which is the idea that these particles pass through, undeflected as if there was nothing there. So our first sort of conclusion, coming out of the scattering of the alpha particles must be that most of the space of the atom is empty space. There's nothing there for the alpha particle to encounter so the alpha particles simply pass through as though there were nothing present. On the other hand, the fact that a few particles turn around and bounce back tells us that when the alpha particle does encounter something, it encounters something which is more massive than itself, sufficiently so that it turns back around and bounces back toward the space towards the source. So the mass of an atom seems to be concentrated into a tiny area, and we'll call that tiny area the nucleus of the atom. Meaning that it's sort of in the center of things, about which all the rest of the atom has grew. Well let's see, what else could we learn about the nucleus besides the fact that it's massive. And that it is timing, or remember that some of the alpha particles, and alpha particles are positively charged are actually slightly deflected as they move through the gold foil. What that tells us is that if positive charges are interacting with positive charges that would cause the alpha particles to deflect their path. So, we conclude that the neuclus has a positive charge. So that, for example, if there is a positivly charged atom here and a positively charged alpha particle's flying into it's vicinity, it might divert it's path slightly as a consiquence of the interaction of the positive charge. But, if it hits it straight on It will turn around and come back if that nucleus is more massive than the alpha particle. And lastly, of course, if the nucleus has a positive charge and if all of the atom is neutral, the electrons have to be in there somewhere with a negative charge that offsets the positive charge of the nucleus. And where are the electrons? The answer is the electrons are moving around and the space outside of the nucleus. So that if we now try to draw an image of what we think an atom looks like, it has a nucleus, which is positively charged, and then around the outside In this region out here there may be electrons flying around in ways that we haven't yet determined but which occupy the rather massive open space around. It turns out that the difference in the radius here, to the radius of this nucleus, is enormous. there's actually really no comparison whatsoever that there's about a factor of or an order of magnitude of 10 to the 4th difference between these two such that, for example, if I were thinking about the campus of Rice university which is of the order of a half of a mile in radius, and I were to place the nucleus at the center of the rice campus on the scale where the atom was the size of the rice campus, then the nucleus itself would only be about the size of a small marble. They're really very, very massive open space here in this nuclear structure's atom. So, that gives us this, sort of, fundamental picture that we have of of the atom. but it leaves us with some questions, here. For example, what is the difference between different kinds of atoms? It seems reasonable to imagine that those differences arise from the different number of positive charges, and the different number of negative charges, which are in each atom. How many are there? How many positive charges in that neuclus? Over here, I seem to have put, for no particular reason, 4 positive charges. We don't actually have any idea. What the number of positive charges might be in that atom? So we need some means to do that. Well, what do we know? Again, going back to the outset of this lecture, we said one of the things that we know about the atoms is we know all of their masses. So one possibility would be to rank order all of the elements in increasing mass order. And then sort of give them a rank order, a number, which we'll call the atomic number from smallest to largest. So for example, hydrogen is the lightest element, its atomic mass on a relative scale is 1, we'll call it, its atomic number 1. The second heaviest element is helium, we'll call it 2. The third heaviest is lithium, we'll call that 3. And all the way up the periodic table and keep in mind, we're just sort of giving names to the elements at this point, because we don't know those numbers have any significance. It's no different than saying one team is first in line, one team is second in line, one team is third in line, or in a classroom full student's one student is the tallest, the second student is the second tallest, the second tallest will be number two, the third tallest student will be number three. That doesn't mean that the height is 1, 2, or 3, only that that is the rank order. We're going to call that the atomic number. Now, what we're going to attempt to do is make some other measurement of a physical property having to do with the elements, which might tell us something about positive charges. This one comes out of left field. This one is actually not predictable as to what it is that was the appropriate measurement. However, this is the measurement which was done historically. What we actually measure are X-ray frequencies coming out of each atoms. Turns out if we take pretty much any material, and put it in the middle of a high intensity electric arc field, that X-rays start becoming emitted from the atoms inside that material. And those x-rays each have characteristic frequencies associated with them. We can capture those x-rays and measure them. And then what was actually done was to plot those x-ray frequencies as a function of the atomic number. Here's what the data actually look like if we do that. So this is x-ray emission and on the x axis here, is simply the atomic number. Which remember is not actually a physical property of the atom, it's just a rank order of the masses of the element with a couple of exceptions that we'll talk about in the next lecture. And here we've plotted against the x-ray frequency. Well notice we've got actually quite a beautiful curve when we did this. In fact, you can kind of draw a line through this data here and it looks either exponential or some kind of geometric growth, or perhaps parabolic. If it was parabolic, then this it something like Y equals X squared. And so, we ought to be able to say, take the square root of the data on the Y axis and see if we get a straight line. Here's that result. Now we've taken the square root of the frequency on the Y axis versus the atomic number. Now we get a spectacular result. In fact, a beautiful, beautiful straight line. These data form just about as straight a line as we ever see when we make measurements of physical properties. So if we stare at this for a little while, we think, this means not just that the frequency of the x-ray is related to the atomic number, but actually could be quantitatively predicted. For every single atomic number, there's an x-ray frequency on this line. And there are no X-ray frequencies that don't correspond to these simple atomic numbers along the way. What does that mean? It means a property of the atom is directly related to this rank order number, the atomic number, that we assigned it fairly arbitrarily. That means those assignments weren't arbitrary after all. It means that this atomic number down here, which remember before was nothing but a rank order of the masses, is itself a physical property of the atom. Now what physical property might that be? Well, importantly, these are all integers but the numbers we're looking at on this axis down here, notice are all integers. There not just some kind of number but they're specific number where we were actually. We use the integers for counting things. The unique aspect of the enditures is that they are used any time we have particles and then we can count those particles. As a consequence, any time we see enditures in science,we know that we're dealing with something particulate. What that tells us then Is that we have counted the number of particles of some type that is an associated property of every atom individually. And since we already know that there must be some number of positive charges. A reasonable conclusion is that these integers are in fact, the integers which are the number of positive charges associated with each atom. So our conclusion then, is that the atomic number, which remember is a mass ranking of the elements, with a few exceptions that we will talk about in the next lecture Is also equal to the number of protons which are in the nucleus of each atom. And is also equal to the number of negatively charged electrons which are in a neutral atom of each element. That number can vary if we have ions of those atoms, but what's invariant for each type of atom is the number of positive charges it has in its nucleus. As a consequence we now know a great deal about the structure of an atom. We know that there's a positively charged nucleus, we know what that positive charge is, we know that that nucleus is massive, and it occupies a very tiny region of the space of the atom. There's a vast, open space surrounding that nucleus in which the electrons move. And we know how many electrons there are in a neutral atom. They're the number exactly equal to the number of protons which are in the nucleus. Our next task is going to be to figure out how are those electrons arranged? How do they actually move, and how does that movement give rise to the properties of individual atoms. We'll pick that up in the next lecture.
