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Rutherford’s Gold Foil Experiment

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INTERVIEWER: The Rutherford Gold Foil Experiment was one of the most important experiments in the development of atomic theory. It was important because it led us to a better understanding of the size and structure of the atom. Rutherford's experiment is very difficult to set up. So in this case, we're going to demonstrate it using virtual chem lab and doing a virtual experiment. So in this experiment, what we have here is we have an optics table. And in this case, we've put a americium 237 source, which produces alpha particles, a gold foil, and over here is a phosphor screen. You can see up here on this screen up here, the results of what's being seen on the phosphor screen. A phosphor screen works by having charged particles hit it, then it glows. Americium 237 is radioactive, and it emits these alpha particles. An alpha particle is a helium nucleus. What is the charge on a helium nucleus or on an alpha particle? In this case, a helium nucleus has just the nucleus and no electrons. And since it has two protons, the charge on the helium nucleus-- on the helium nucleus would be plus two. So in the Rutherford experiment, we're going to shoot these alpha particles through the gold foil, and then detect what happens on this phosphor screen right here. So as you can see, we have a little fat spot right here, and then we have these other smaller spots that disappear quickly. If we were to remove the gold foil, this is what we would get. We would get a small spot. So obviously, putting the gold foil in front of these alpha particles produces some change. Now what's interesting is what happens if we moved the phosphor screen over here to the side. So if we move it to the side, we can still see these little flashes from the alpha particles but no fat spot in the middle. And then if we move it over here to the side, at first glance, it looks like we see nothing. But if we wait long enough, we're going to see a little flash here. And those are hard to see so we're going to click this little virtual persist button and save the hits over time on this phosphor screen. So you can see that we're going to get about one hit of an alpha particle every second. That's not very much when you consider that the alpha particle source here is producing about a million alpha particles per second. So a million alpha particles per second are going through the gold foil, but about one out of those million is being deflected at this large 90 degree angle. So if we turn off our persist button and move the phosphor screen back here to the beginning, how are we to understand the results from this experiment? What does it tell us about the atom? And in this case, the experiment tells us this, that the atom is mostly empty space. Most of these alpha particles, a million per second go straight through the gold foil and you see just a spot in the middle, and then a few of them are deflected at wide angles. And then if you move the phosphor screen over here to the side, you see that even at a wide angle, they get deflected. So most of these alpha particles go right through the foil and nothing happens to them. A few of them hit something and deflected at a wide angle. And the interpretation is that, that is the nucleus. That's the core of the gold atom. So that's an important. Atoms are mostly empty space. But there's an even more important understanding or interpretation that we need from this experiment. And that is, why is this spot fat? If, again, if we move this over to the countertop here, we get a little spot. If we put it back here, why does that main spot still get fat if those alpha particles are not being deflected? And it has to do with the charge on the nucleus. So what do you know about the charge on the gold nucleus? What's the charge? So let's do a little thought experiment here. Let's assume that the charge on the nucleus is zero. So if this is my nucleus and this is my alpha particle, and remember it's plus two charge. And if it comes through but doesn't hit the nucleus but is close to it, what's going to happen? Well, if this is neutral and this is plus two, nothing's going to happen. It's going to go straight through. But if that was the case, we wouldn't see a change in that spot. We would see what our normal spot would be as if we didn't have this gold foil there in the middle. It would be a small spot like before. But if we put it in there, it's a big spot. So let's assume that the charge on the nucleus is positive. Well, if it's positive, and then we have a positive plus two charge alpha particle here on the side, what we would see is that, we would see a repulsion. Plus and plus are going to repel, and you're going to see that a small deflection of that alpha particle, and that is indeed what we see here. We see that these alpha particles are deflected just a little bit. So not only is the atom mostly empty space, but the nucleus is positively charged. And in the case of gold, the positive charge is plus 79. So when you get plus 2 and plus 79, that gets deflected at a wide angle. And so therefore, the nucleus is positive. But that brings up another important question. What happens to the electrons? If I've got a positive nucleus and I've got a negative electron, positive and negative, they're going to get sucked together. But from that experiment we know that it's always positive. So if I got negative and I got positive and they get sucked together, how does that electron stay there? Well, that's a key question. And the only answer to that is through quantum mechanics. And that's going to be saved for another video.