The Gold Foil Experiment
Ernest Rutherford along with his colleagues Hans and Ernest Marsden did a famous Gold Foil Experiment to determine that the atom is not like a tennis ball or a billiard ball but it does have a central dense region which we now know or now call as the nucleus and that the electrons actually revolve around this dense mass. So, just to compare, this is how J. J. Thomson thought the atom would look like; the electrons negatively charged, stuck in a positively charged dome. And, Rutherford and his co-workers through the Gold Foil Experiment showed that the plum pudding model was not exactly accurate.
Let’s get into this experiment now. They started out with a very thin gold foil, this is just like any other foil, say this one, Ok; like your tin foil or aluminum foil you would’ve seen that, right? Won’t it be just super if your mom packs your food using gold foil instead of you know, the aluminum foil which it’s normally packed in. Anyway let’s get back. They took this piece of gold foil, assume it’s gold even though it’s silver looking and shot tiny little particles at them and these particles were nothing but alpha particles, whom we just discussed about in detail.. Now, what they wanted to do was, hit this gold foil with the alpha particles and then they wanted to detect, what happened once the alpha particles hit the particles in this particular foil. To detect that, they had this material which would flash if an alpha particle hit it. So they take this material, bend it into a circle, put the gold foil right in the middle of this material like this. Now, what they do is, they shoot alpha particles from this side of the gold foil where there’s an opening over here, like Bam! Bam! Bam! And then most of the time they see flee flashes right here, right behind the foil. This indicates that the alpha particles are going right through the gold foil landing upon the other side. Sometimes behind the gold foil right here and in some cases much to their surprise, they see that the particles get hit right over here, over here, over here and sometimes even over here. This meant that the alpha particles were hitting the foil and deflecting off something or bouncing off something.
So, we can kind of summarize what they saw with a diagram like this. So, here is the circular detector screen and here is the alpha particle emitter, something like a gun which shoots out the alpha particles and then the gold foil is right here, in the centre; that’s what I showed you, ok. And most of the time when the alpha rays are coming through, the alpha particle would go right through and they would hit here and sometimes just sometimes they would deflect off, like you can actually see over here. In fact, some of the rays would actually bounce right back, like you can see over here or over here. So these…these lines are just indicative of the different parts there, the alpha particles actually took, on hitting the gold foil. You can clearly see that in this diagram.
Now, let’s look at the conclusions and very, very important conclusions that they came to. So if I take the gold foil like this, this would be the top view, right? How would you see this? Really, really thin. So, Rutherford reasoned that it should be say around thousand atoms you know, thick. Now, if I were to draw on this side what he reasoned, it would look like this and when the alpha particles were shot through, they saw that most of them came through the other side, like this. So, what did they reason? They reasoned that most of the atom would be empty space or something so so so light that it was so easy for the alpha particles to actually go right through them. It’s not very dense and very easy for the alpha particles to basically shoot through. The second thing they see is that once in a while the alpha particle actually hits off and goes on to the side and they reasoned that it should look something like this. Of course, you know that this is the nucleus now but at that time they did not know that yet. The nucleus wasn’t yet discovered so they felt that there should be something really, really hard so that the alpha particle should bounce off, like this. And, one more thing they reasoned is that maybe you know, just maybe since the alpha particle is getting deflected off you know, like this and since the alpha particle is positive the centre here might also be positive. Which is why, the particle is coming so close to the positive charge and then it’s getting repelled or deflected and moving in this direction. So, the conclusion was that there is something very hard and dense in the atom and once the alpha particle is hitting this very dense and hard material they’re either bouncing back or deviating from the path. Two very, very, very important conclusions. So, if I’m putting everything together here, when alpha particles are shot towards the gold foil most of the alpha particles pass through because they’re not hitting the dense material in the atom. And, if the alpha particle hits that dense material they get repelled by the charge or they get bounced back. And, this is exactly how the Gold Foil Experiment changed how scientists think about the nucleus.