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Current time:0:00Total duration:11:24

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this is a quote by a physicist as a comment on one of his experimental results he said about his experiment he said it was as if you fired a 15-inch shell at a piece of tissue paper paper and it came back and hit you so let's talk about his experiment and what he was doing Rutherford at the time had been doing a lot of research on radioactivity he was friends with Marie Curie and her husband Pierre and he had done a lot of studies looking at the different kinds of reactivity and more specifically he was curious at this time about Apple asari alpha particles which are he didn't actually at the time she didn't know what they were but we now know that they are helium 2 plus nuclei so that means we have two protons in the nucleus since it's helium and it also has two neutrons and it doesn't have any electrons and thus it has a two plus charge so what brother fer did he took a piece of radium and he put it inside a lead box and the lead box had a small hole in it on one side so that the radioactive elf particles could come out of that hole in the direction that he wanted and then he shot the alpha particles at a piece of gold foil a very thin piece of gold foil and he was curious to see if he could learn a little bit about the structure of the atom using this experiment so if we look back at our quote we would say that our alpha particles here the alpha particles are the bullets that are coming out of our alpha particle gun and gold foil is our tissue paper and we have these pretty fast and massive alpha particles that we're shooting at it but why was Rutherford expecting expecting that to happen here it's not necessarily straightforward at least to me why you would think these alpha particles would just go straight through the bold foil so what brother freed at the time was doing was he was testing the plum pudding model so this is pretty early in history where we we being scientists way back then knew that JJ Thompson another physicist had just discovered electrons so we knew the atom the atom had something had these particles in it that were small that were really small we knew that there were less than 1% the mass of a hydrogen atom so way smaller than an atom and we knew they were negatively charged so I'm going to call them electrons because we know their electrons now and so JJ Thompson knew that electrons existent based on his experimental results and he proposed based on his results that an atom looks something like a plum pudding and if you don't know what a plum pudding is because maybe you're not British or you just don't like dirt you could also imagine it looking something like a choc chocolate chip cookie so we have these little negatively charged particles that are stuck inside the atom but most of the atom is made up of a positively charged soup and this was mainly because the atom overall has to be neutral scientists knew that atoms were neutral so there had to be something there to cancel out the negative charge of the electron so because Rutherford was starting with this in his mind for what the gold atoms look like he could actually do mathematical predictions on what the alpha particles to do and what he predicted was that they would just go straight through you can use physics equations to look at the electric field that's generated by this positively charged soup and it turns out that the field because the charges spread out all over the atom the field is very weak and so what he thought would happen was that all the particles would just go straight through and then occasionally one of them might be bent a little bit since we do have a positively charged soupy atom um depending on where the the alpha particles goes through he thought you might see a little bit of deflection but mostly it should go straight through and I guess we started with a spoiler because we know that well he didn't quite get what he expected so what exactly did Rutherford see well he shot his alpha particles at his tissue paper and he saw most of the particles go straight through just as he expected in fact he saw almost all the particles go straight through he saw a couple of them be deflected a little bit so they got deflected off their path maybe about 1 degree so barely enough to be able to see it and you know if he had not been a curious chemist we would maybe still think right now that this is what an atom looks like but luckily Rutherford was a very thorough chemist and he also thought it might be interesting to detect whether particles came not just here he didn't just put a detector screen here he put a detector screen that went all the way around which so all the way around almost all the way around giving enough space for the alpha particles to go in and he he was being really careful here because he really didn't expect to see anything right around here or here here or really anywhere except for here but it turned out that for every one in one in 20,000 alpha particles or some crazy tiny number like that for every one in 20,000 alpha particles he saw the particles hit the gold foil and bounced back and that's crazy right that's exactly what you don't expect when you hit a piece of tissue paper with a bullet so the first thing he did I think was not go hmm this is really crazy we just want to know what price here I'm pretty sure the first thing he did was this is weird and then he probably checked his experimental results and he tried to repeat it and he checked everything to make sure nothing was going wrong and it turned out that yes something was actually happening this one and 20,000 alpha particles was real so what did this mean this meant that we needed a new atomic model we had to explain somehow that a tiny fraction of the alpha particles was getting bounced back so how did he do this he knew there was something in the atom that was tiny massive and positively-charged and he knew that had to be tiny because not very many alpha particles interacted with it because most of them went straight through he knew that had to be massive and positively charged because well the electrons are really small and most of the particles went straight through so whatever these particles are interacting with had to be very small but really heavy which is how they bounced right back so he made a new model of the atom that incorporated these requirements and what he said was that there must be something in there with these properties which we now call the nucleus and it's really tiny in fact he was able to calculate exactly exactly he was able to calculate approximately how big it was based on how many alpha particles hit it and he said it was approximately 110 thousand ten thousandth of the volume of the atom and then what else do we have we know we have this nucleus which is positively charged and tiny and massive and then we also have our electrons and then what's the rest of the atom doing based on all of this that means most of the atom is actually empty space since the electrons are really small and the nucleus only takes up one ten-thousandth of the radius the rest of that space is all just nothing which is kind of crazy so based on this particular model that Rutherford makes that may next he was able to explain his results he was able to explain that most of the alpha particles just went straight through and then every now and then an alpha particle would come really close to the nucleus and then that would get deflected a little bit and even more rare an alpha particle may hit a nucleus straight on and then I would get bounced off because the nucleus is super heavy and because it's positively charged so it would repel the positively charged alpha particle Rutherford called this particular model or we call it now I'm not sure which actually he called it the nuclear model this actually looks pretty similar to the modern picture of the atom that most people think of there's a lot of questions that are still not answered here like what exactly the electrons are doing or where are they where are the electrons but because Rutherford proposes new model other scientists were able to design new experiments to test it and not very long afterward we had a pretty good picture of what was going on on the level of the atom