the gold foil experiment was an experiment set up to prove the structure of the atom often it's called Rutherford's gold foil experiment although it was Hines gger and Ernest Marsden that actually performed this experiment under the supervision of Rutherford Rutherford was basically their boss so it was in 1909 that this team of scientists set to change our perception of the atom before we get to Rutherford's experiment let's just quickly revisit JJ Thompson's plum pudding monel this was the model at the time of Rutherford's experiment and it was the accepted model now Plum Pudding is a British dessert and I've never had any know what it looks like so I'm going to call this the chocolate chip cookie model and if you can think of a chocolate chip cookie we have all these chocolate chips and this is kind of like what the electrons would have looked like in Thompson's model so we have these electrons that are distributed throughout the rest of the atom and the rest of the atom all the cookie stuff here it would have been kind of a low density substance that was positively charged and so Rutherford's intent was to prove that this was correct or at least verify it was correct and here's what this experiment looked like Rutherford had a radioactive element inside of this lead box here and I've kind of cut out the lead box so we can see inside of it now this radioactive particle would have been decaying at a constant rate we won't get into radioactive decay in this video but basically what was happening with this part or this substance is that it was emitting alpha particles and these alpha particles look something like this and they are basically a helium nucleus all right so this would be a helium atom without the electrons and so these alpha particles would have been positively charged because there's two protons there and if you can imagine this radioactive element here just kind of emitting alpha particles just kind of in every direction here here all over the place just flying off this thing and what Rutherford and his team of scientists did is they had a little hole here right for these alpha particles to escape and these alpha particles are moving at incredible speed so it's almost like a gun and they'd have these alpha particles escaping and they fired these alpha particles at a sheet of gold foil and so they'd have this gold foil here right in front uh of the uh the the gun here the alpha gun firing at the gold foil and they had detectors set up all around to see where the particles ended up and if we look at the model of the atom and this is this is what we thought this is what they thought the model the atom looked like here just like Thompson did and so this is kind of zoomed in on this sheet of gold foil if we had our gold foil here we're just kind of zooming in onto a single atom they expected that all of the particles pretty well should just pass right through and they had a detector screen that these particles would show up on and actually this detector screen would wrap all the way around the uh foil here so it' be kind of a a circle there going all the way around and so they expected it all to kind of just hit the other side it's kind of like firing a cannon at a piece of tissue paper this kind of idea so if we had this Cannon and these cannon balls that were just being fired at this piece of tissue paper here you'd probably expect them to just blast right through that tissue paper and go on to the other side that wasn't the case though and what was ending up happening is that most of the alpha particles went right through that gold foil as if it wasn't even there but some of them would actually deflect they' kind of go off to the side and some of them this was really puzzling was that some of them would look like they even bounced off of that gold foil and came backwards so here's how Rutherford explained this the atam could not have looked like Thompson's model for the atom just couldn't have worked so what Rutherford thought is maybe we revise that model of the atom to include a very dense center right so we had this very dense Center here that was positively charged and the difference here would be that anytime an alpha particle remember alpha particles are also POS positively charged anytime they got close to that dense positive Center they deflect and if it wasn't as close it might deflect a little bit less okay but if it got close it would it would uh be repelled because positive is going to repel positive we may also have a situation and this is the evidence that Rutherford saw is that sometimes we had something coming almost right back and so that would have indicated almost a direct hit with the alpha particle to that dense center now the rest of the atom here would have been made up of something very very um low density and this is where the electrons would have been and it could have even been empty space and so Rutherford even thought that maybe most of the atoms actually empty space because most of the particles he saw just went right through as if there was nothing there and so the results here of Rutherford's experiment here's actually what his experiment would have looked like I have Thompson's model and Rutherford's model here this blue circle is the detector screen so we had to set up all the way around and in uh Thompson's model here what they were hoping to see or what they expected to see was all these particles mostly going or all of them going straight through really not impeded at all and what he did see was most of the particles going right through as he expected but some of the particles deflecting and some of the particles even coming right back and again that's the result result of that dense center of the atom and so he really came up with two big points to explain this new model and these were his two big points the atom has a dense positively charged Center which we know is the nucleus and the atom is mostly empty space and that's how Rutherford's gold foil experiment discovered the nucleus