To tell you about one of those important experiments in physics, that's for other foods alpha particles scattering experiment. It's an important experiment because it changed our model of the atom. That's the model we use to explain how matter works. The story really starts all the way back in ancient Greece with the kind of word atom. And it comes from the Greek word atomos, which means indivisible. Democritus, a Greek philosopher, about 400 BC, talked about if you kept cutting up a piece of, let's say, cheese, you'd eventually get to a piece of cheese that you couldn't cut any longer. And that would be one atom of cheese. This idea was kind of forgotten in the dark ages and they thought that things were made up of earth, water, wind and fire. And that kind of makes sense logically and worse is why you should be well logical conclusion. Because if you burn some word, fire comes out. So it makes sense to say that the words made of fire. Dalton in the early 1800s suggested that in that fact, everything was made of individual indivisible atoms. He modeled them like billion balls. They were solid and unbreakable. The atoms that made iron were different to the atoms that made copper, different to the atoms that made oxygen and so on. And he worked this out by doing lots of chemistry. There were certain fixed ratios of elements within compounds. So Dalton's model was prevalent for about 100 years because it explained the evidence that they had at a time. It explained those ratios that he found during chemical reactions of the masses. About the end of the 19th century, JJ Thompson was investigating these things, CAFO READ. He worked out that actually the CAFO READ was a beam of tiny particles, one 1800th the mass of a hydrogen atom. And he knew they were negatively charged because they were coming from the negative plate, the negative electrode of a power supply of a cell. He theorized that these must be tiny negative particles which could be taken off detached from an atom. Thompson was the first person to discover that actually you could take things out of atoms. So here's my electron gun. This is the CAFODE. It's a bit of metal that's going to get really hot and it's going to release the electrons. They're going to be accelerated and attracted to this positive plate here. And they're going to be accelerated through this gap. And that's going to be my CAFODE READ. And my beam of electrons. Looks just actually like I'm shining a beam of light. But by bringing a magnet close to this, the beam is deflected by that magnetic field. He was able to deduce the mass and charge of the particles. So Thompson needed a model to explain what the atom was really like. He came up with what we call the plum pudding model. Well, we don't really eat plum puddings today, but I guess it's something a bit like a scope. There's quite a lot of cakey stuff and that there are these detachable pieces, these raisins or plums. He said the atom was a bit like that. The cake was the positive charge and the raisins or the plums were the negatively charged particles that he discovered in his CAFODE READ. That was a successful model because it explained the evidence that they had at the time. In science, when we have new evidence that cannot be explained by the old model, then we have to change that model or replace that model. So I think it's that conducting experiment which provided that new evidence and changed this model was earnest brotherhood. To understand how am I mentors this was for brotherhood and for science, you have to put yourself in his shoes. He's working for Thompson. Thompson is his professor. Everything he knows has come from Thompson. And to say to Thompson, no, I have different evidence. My experiment shows that your model, the famous J.J. Thompson, your model is actually incorrect and we need to refine that model. That took a lot of courage and that was a huge step forward in physics. Brotherhood's experiment took place in an evacuated chamber and he essentially fired alpha particles which are massive, energetic, positively charged particles at a very thin piece of gold leaf. He was expecting to see all of the alpha particles going straight through the gold leaf because their energy would mean that they would just kind of bash that cakey stuff out of the way. There was no reason for them to be stopped. But that didn't happen. Most of the alpha particles did indeed go straight on through. There was a significant number that weren't detected on the opposite side from the alpha emitter. Like any good scientist, Brotherhood repeated his experiment, check the results, check the methodology, refine the methodology. And his last change was to move the detector around at an angle to the gold leaf. And he found that the alpha particles were being scattered. They were being deflected by the gold leaf. Some of the alpha particles have been deflected through small angles which means they changed direction slightly as they went through the gold leaf. But that still didn't account for all of them. Some of them were found on the same side from which they'd come. This was astounding to Brotherhood and he said this was as astounding as if you'd fired an artillery shell at a piece of tissue paper and it'd come back and hit you in the face. So with this new evidence he was forced to change the model. These large ball bearings represent the nucleus in Brotherhood's nuclear model of the atom. So he made three observations. First one, most of the alpha particles went straight on through the atom. And from that he could conclude that most of the atom was actually empty space. Secondly, some of the alpha particles were deflected and he could do that by moving a detector around and counting the amount that was scattered through small angles. Now because that track implies there must be a force away from the nucleus of the atom we can say this nucleus must be positively charged because our alpha particles are positively charged as well. And finally the final observation was that some very few of them particles were deflected back the way that they came. This means that that nucleus must have a lot of mass because we knew these alpha particles had a lot of energy therefore it needed something massive and positive to be able to deflect it back the way it came. A lighter thing, a lower mass thing would have just been knocked out of the way. It's from the analysis of these so analyzing how many and how much and how close they could come to the nucleus that we get this measure of the size of the nucleus being times 10 to the minus 15 meters across the diameter of the nucleus being 10 to the minus 15 meters across and the overall diameter of the atom in this is no idea to scale or to add on the electron cloud there would be around 10 to the minus 10 meters. What brotherhood didn't have evidence for was where the electrons were in the atom. He kind of guessed that they might be kind of orbiting around the exterior of the atom but because they would have been attracted to that positive nucleus they surely would just spiral in and kind of crash. So it took Neil's board to kind of think about that and work out that there has to be a certain amount of energy in that electrons orbit keeping it in its orbit. I'm going to call those units, we call them energy levels. Brotherhood whose experiment showed us this amazing thing that most of the atom is just nothing at all and that almost all of the mass is concentrated in a tiny tiny tiny tiny pump and that part is positively charged and that there are some orbiting electrons around the outside of the atom that have very little mass and are negatively charged. Neil's board puts the ice in on the cake and gives us the idea of the energy level and allows us to do all of what comes after just quantum physics.