How and when did we realize that atoms exist? How did we learn to understand their properties? How was Ernest Rutherford hogging all the science for himself? I discuss these questions and more in today’s Ask a Spaceman!
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EPISODE TRANSCRIPTION (AUTO-GENERATED)
During a recent astrotour, one of the people who went on the trip, Bill s, asked a very interesting question. He asked, how were atoms discovered? Atoms. Like, how how do we know that atoms exist? And it's such a wonderful question because the story of our understanding of the atom is not a movie.
It's not a single moment in time or a single experiment or a brief period of intense work with a few hard scrabble scientists struggling against the odds full of drama and intrigue and all hope it's lost until a single breakthrough moment where suddenly, briefly, nature makes sense and a new revelation is born into the world. No. It's a sitcom. It's a bunch of small, short stories, sometimes filmed in front of a live studio audience, and maybe there's a laugh track. Each story is kind of sort of connected to the others, but only loosely.
Like, in one episode, they might reference something that happened earlier or set up something to come in the future. Oh, will they admit their love for each other? Oh, I better keep watching. The tension is killing me. But each episode can be taken in isolation and enjoyed for what it is.
But at the end of the series, after a few years on TV, the characters have changed. They're in different jobs or different relationships, different paths. They've grown. Their understanding of the world has grown too. And you can't point to any one particular episode and say, that's it.
That's the moment it all changed. But when you contrast episode one with episode 100, yeah, things have changed. So what I have for you today is a few brief episodes that, taken individually, don't ever discover the Adam, and yes, I did the finger quote thing over discover, but taken together over the course of a century through multiple independent experiments, each one very well designed, very well crafted, answering a very particular narrow question, one little story. As you put all those together, you end up with an atom. You have a picture of an atom.
You have a coherent theory of nature that is able to explain many, many different kinds of observations because remember, Adam is a theory. Adam is the theory. Gravity is the theory. Evolution is the theory. Gravity, evolution, atom.
They're all theories. Atom. The concept of an atom is a theory developed to explain a bunch of observations and answer a bunch of experimental questions. So this TV series that I'm about to present to you, which I'm gonna call Eventually Adam for the sake of my own amusement, does have a few regular characters. The big lead star who's gonna come in, he's not gonna be the star at first, but he's gonna come in about halfway through is Ernest Rutherford, but there's some co stars.
There's JJ Thompson, Dmitri Mendeleev, John Dalton, Hans Geiger, Ernest Marsden. There's a special guest appearance by Albert Einstein himself. There's a lot of drama, a lot of laughs, and a lot of time wasted just setting up plot points that go nowhere. So you know a typical podcast episode of Ask a Spaceman, and also a typical sitcom episode. So without further ado, I present the new TV series hit Worldwide, Eventually End.
Episode one, JJ and the Ray Ray. By the late eighteen hundreds, everyone who knew anything knew that if you made a glass tube and sucked all the air out and stuck two electrodes in there and cranked up the electricity, you'd get glowy bits at one end. So you could turn electricity into weird glowing things. That's neat. The two electrodes that you stick inside your glass tube are called the cathode and the anode because reasons, and so the glowing light that would be inside this tube was called cathode rays.
And everyone was wondering for decades what the heck are cathode rays and what are they made of. Obviously, it had something to do with electricity because you needed electricity to make them, but we didn't really know what caused electricity in the first place. You know, if you have an electrical wire, we didn't know for a very long time what's flowing down the electrical wire. Is the word flowing even make sense in this context? We knew about electricity, but we didn't know what it was.
And so the cathode rays had something to do with electricity, but they also gave off light. I mean, rays is in there for a word, cathode rays. So was it something like radiation? Was it something like matter? Were the charges that were creating the electricity, were those being sent back and forth from the cathode and the anode?
Was that connected to the glowy bits? Was it different? Were they just carrying something along? Or were they their own thing? Like, it was just who knows?
So Joseph John Thompson, who apparently everyone called JJ, and I'm not sure if he preferred to be called JJ. He's like, folks, just just call me Joe. And they're like, no. We're gonna call you JJ because it sounds better. I don't know.
JJ Thompson, he tried working on this. And when he worked on it, he made the best dang vacuum tube that he possibly could, better than anybody else. And so he was able to make some really, really, really good cathode rays. Like JJ's cathode rays were top of the class, top shelf cathode rays in his little vacuum tubes. And he figured that cathode rays had something to do with electricity because you needed electricity going through your little circuit and into the into the vacuum tube in order to make cathode rays.
He did what any right thinking physicist would do and stuck the whole thing inside of a magnetic field. Because, hey, why not? It's on the list. You know, first you drop stuff, then you shove things around, and you stick inside of a magnetic field. It's on the list of things you do when you're trying to understand the universe.
So he put it inside he put his cathode ray apparatus inside of a magnetic field to see what happens. And what happens? The cathode rays bent. They moved. They curved.
That's interesting. So then he stuck his apparatus inside of an electric field, and what happened? The cathode rays bent. They moved their curve. He was able to bend the path of the cathode rays inside of an electric field and inside of a magnetic field.
The cathode rays, whatever they were, were responding to electric and magnetic fields the exact same way that charged particles do, the same way that matter does, and not at all like radiation does. I mean, you you take a blast of radiation, a beam of light, and put it through an electric field, it doesn't care. It just goes. But charged particles will care. They'll get their paths bent.
So he was able to conclude that cathode rays were made of matter. And if cathode rays are made of chunks of matter, which we'll call particles, then those particles must have, you know, charge, mass, all that stuff because chunks of matter have charge and mass. If you're made of stuff, you just have charge and mass. Okay? And the reaction of moving particles to an electric field depends on its charge and mass, and also its response to a magnetic field depends on its charge and mass, but in a slightly different way.
The math works out in a slightly different way. So what JJ did was that he twiddled the electric field. He put the thing inside of an electric field and ramped up the electric field to try to make this cathode ray bend down, and then he also put that inside of a magnetic field at the same time and cranked up the magnetic field to try to shove the cathode ray up. So you have an electric field pushing the charged particles, whatever is inside this cathode ray to make it a cathode ray. Whatever it is, it does respond to electric and magnetic fields.
And he was able to make it bend down because of the electric field and then bend up because of the magnetic field. And at some point, they would bounce, like, a little bit of electricity over here, a little bit of magnetic over here, twist the dozens. It's like you can imagine this cathode ray, like, bending up and down like, it didn't make that noise, but this is a podcast. And then it'd be, like, just even, like, perfectly balanced. And because the charged particles respond in different ways to electric fields and magnetic fields, he could do some math.
He knew how strong the electric field was and the magnetic field was because that's the way he designed his experiment. He knew how strong those fields were. He could take those ratios. He could take the ratio of the strength of the electric to magnetic field, do a little math over here, and he could figure out the ratio between the charge and the mass of the cathode rays. So he couldn't see these particles directly because they were very, very tiny, but he could figure out their charge and mass through their response to electric and magnetic fields.
And the big surprise twist at the end of the episode, the little cliffhanger, is that whatever the cathode rays were made out of, we had no idea they were made of something, but that something was about 2,000 times smaller than the hydrogen atom, which at the time was the smallest known thing in the universe. He called these little cathode ray particles, corpuscles because he was a little bit stuffy like that, But everyone else, including us, call them electrons. By the way, we still have cathode rays and cathode ray tubes. If you've ever seen an old TV that kind of monitors, it's called a CRT for a reason, a cathode ray tube. So that name never went out of style, but corpuscles sure did, and it just stuck with electrons.
But whatever these electrons were, they were 2,000 times smaller than the smallest known thing in the universe. What a cliffhanger. Episode two. Dimitri gets his periodic table. Speaking of smallest known thing, by the time that JJ was playing around with his cathode rays in the late eighteen hundreds, physicists and chemists were leaning towards the idea that all matter was made up of tiny little bits called atoms.
That's how JJ was able to make this remarkable discovery that cathode ray corpuscles, aka electrons, were surprisingly tiny. And this idea that maybe matter is made of tiny little bits called atoms is, of course, an old, old, old idea, just like all ideas. So whenever you I see this not just in the story of atoms, but in the story of basically any scientific insight. It starts with, like, ancient Greek philosophers debated the concept, and there was a school of thought that said atoms exist, and there was another school of thought that said atoms don't exist, or ancient Indian or Persian or Chinese, any literate society that was wealthy enough to afford a couple extra philosophers with you know, no useful or transferable skills whatsoever to hang around and think thoughts and write those thoughts down, and then their thoughts got preserved into the present day, which is a very, very difficult thing to do. I find it mildly disingenuous to say, like, oh, the story of the atom starts two thousand five hundred years ago.
Or, you know, or, like, the story of exoplanets starts four thousand years ago with the writings of this impossible to pronounce name. It's because as soon as you start having philosophers in your society, they just think all the thoughts. They have every possible idea, and they write them down. So there's a school of thought that thinks atoms exist. There's a school of thought that thinks atoms don't exist.
There's a school of thought that thinks the Earth is the center of the universe. There's a school of thought that thinks the Earth isn't the center of the universe. There's there's a school of thought for everything because they had every single ideas. So it's a little bit disingenuous to trace it back because all possible ideas have already been thought and written down. Sorry.
Side rant. So that was the B story in this episode. Now back to the A story. We knew for a long time that there was such a thing as elements, the fundamental building blocks of all things. Now, we disagreed on what the elements might be earth, air, wind, or fire or mercury and oxygen and carbon.
We had some mild disagreements about what those elements were, but we recognized elements that a complex thing like a tree or a bowl is made of simpler things in combination. And these elements, these simpler things could be combined and mixed together, and there was a limited set of elements in the universe. You can combine them, mix together, make more complex things, and that when you took a combined element and split it apart, you would get the fundamental elements, but the elements themselves were, well, elemental. You couldn't split an element into more fundamental things. That's kind of the definition of element.
There were different kinds of elements. Some were heavier than others. Some had certain properties like how well they played nice with other elements. And through the eighteen hundreds, it became apparent that there were some fundamental relationships between the elements, that certain certain properties of some elements seem to repeat as you went from lighter to heavier. So if you started with the lightest element hydrogen and went progressively higher, you would see some properties.
Okay. Like hydrogen is different than lithium. You know, lithium is different than carbon. Carbon, you know, and you would work your way up to high heavier and heavier elements, but you'd see like some properties would come back around. Like, oh, wait.
I remember these properties. They were back here in this element. And if you carefully arrange these elements in a table that highlighted this periodic behavior, you could, you know, make a periodic table of the elements, which is exactly what a lot of people, especially Dmitriy Mendeleev did. The chemists tell me there's a lot more going on, but I'll leave that to another show. But these elements, the fundamental building blocks of more complex stuff in our universe are just elements.
They're things. And the question was, could you take, say, a vial of mercury and chop it in half, and then now you have two vials of mercury, and then chop it in half again and have four vials, and then eight vials, and then sixteen, and thirty two, and 64, hundred 20, and 60 on and on and on and on. Keep dividing it up into smaller and smaller bits. Could you do it for all eternity? Could you have an infinitesimally small bit of mercury?
Or would there be the smallest possible chunk of mercury that you couldn't slice up anymore and still have mercury? That's a good question, isn't it? That's a really solid question. That's a great question. How small are the elements?
Before even Dimitri came along with this periodic table, there was a chemist by the name of John Dalton that made a really, really convincing case that, no, you couldn't chop things up forever. And he based his argument by looking at the proportions of various compounds. Let's say you have element a and element b. You can combine element a and element b to make a new thing, Happens all the time. And let's say when you pour them together, you you end up with a certain percentage of a and a certain percentage of b in your new thing that you just made, your new mixture.
But sometimes, you can combine element a and element b another way with a different percentage. So maybe there's, like, 10% element a and 90% element b in a compound, and but you can mix it a slightly different way, you know, with a slightly different temperature, you know, maybe you you jiggle your beaker, whatever, and you end up with 20% element a and 80% element b. What John Dalton noticed is that there's only a few allowed combinations of element a and element b. You can't just add them together any old which way. They only combine in certain percentages.
And by taking the ratios of the percentages, like, okay, in this compound, there's 10% element a, and in that compound, there's 20% element a. By taking those ratios, he found that these ratios, these proportions always reduce to very small numbers. Like, one of element a could combine with one of element b, or two of element a could combine with one of element b, or one of element a could could combine with two of element b, or two of a could be with three b, but you can never get like half of b or a quarter of a. It was always whole numbers, and it was always small whole numbers for all sorts of different elements for all sorts of different compounds. He argued that because these were always whole numbers and they were very small whole numbers, they were never like very, very big fractions, that the elements were made of atoms because you couldn't split an atom apart.
That these ratios were telling you that when you make a compound, you're taking one atom of element a and combining it with one atom of element b, or you're taking two atoms of element a with one atom of element b. But you can't have half an atom, a quarter an an a seven m. You have to have atoms. There is a smallest possible bit of any element. That was a very convincing case, but, of course, you can't see the atoms themselves.
You can only guess at their existence from the way these proportions operate, and this was in the early eighteen hundreds when Dalton was operating. And by the late eighteen hundreds, this picture was, you know, generally considered right, but there were a lot of detractors because, you know, we hadn't actually seen an atom. And because through this picture, through the Dalton picture, through the Dimitri Mendeleev picture, the lightest element is hydrogen, therefore had the very smallest atom. The atom of hydrogen was the smallest thing in the universe, the smallest possible thing in the universe. That is until JJ came along with the whole cathode ray business and upset the apple cart.
Episode three, Einstein, Episode three, Einstein, the Brown Noser. So in the early eighteen hundreds, there was a dude named Robert Brown. He was looking at pollen grains sitting in water because he was interested in pollen grains sitting in water, I guess. But he noticed something else. The pollen grain moved.
It wiggled. It jiggled. It danced. It it snuck around the water. And at first, he thought maybe the pollen grain was alive since alive things tend to move, but then he looked at definitely not alive things like crushed up rocks, and they were moving too.
What was going on? He had no ideas, but he wrote about it, and he wrote about it really, really well, much better than how he wrote about pollen grains. So we call this kind of movement Brownian motion. You can see this kind of motion too. If you look at dust in an air when sunlight is streaming through a window and it hits it just right, you can see that dust in the in the air, and you realize you really need to clean, but that's a problem for another day.
If you look at the dust, some of that movement is smooth from air currents just washing around. But some of that is like hergy, jerky, zig zag, wiggly motion that the air currents can't explain. That's Brownian motion. Robert Brown was studying this about the same time that John Dalton was coming up with this atomic theory, but they didn't talk much. And so this Brownian motion problem was unsolved for about a hundred years.
Let that sink in. It was a well known problem, a well known question, what causes Brownian motion? For a hundred years, all through the eighteen hundreds, this problem was unsolved. In the meantime, this idea of atoms was starting to gain ground, that the elements were made of atoms like we saw in the last episode, and the physicists who were developing the laws of thermodynamics were thinking about things like temperature and pressure and entropy of air or water or fuel in a piston, like this whole steam engine thing, and they were making some very convincing arguments to make their, you know, their math come out right. They were trying to explain, like, what is temperature?
Why is there a relationship between temperature and pressure and volume? Why? They were able to come up with a very, very good answer that the relationships between temperature and pressure and volume and entropy and all that are due to countless microscopic movements. That when you look at something like air, there's a bunch of tiny air particles that are wiggling all around. Some of them hit you, and that's your sensation of air pressure, and that how that's how heat is transferred from one thing to another through these countless microscopic motions, and they had some really awesome math to back it up.
So maybe maybe there was a connection between these countless microscopic motions, like if air or water is jiggling around at a sub microscopic level. You know, Brownian motion is kinda jiggly too, so maybe there's something there. It was Einstein himself in a paper in nineteen o five, the same year he dropped the relativity bomb on everybody. He cracked the riddle, and his answer was atoms. It's all atoms.
Imagine you're in a party, and it's a crowded room. There's lots of music and you start in the middle of the room. You are a Paul and Greg. You're surrounded by people. Some of the people wanna talk to you.
Some of the people wanna ignore you. The some of the people bounce off of you accidentally. They just, oh, you brushed your shoulders. Sorry about that, dude. Yeah.
Oh, no. No problem. Some people come up to you and chat and stop you from moving. Some people want you to say, hey. Hey.
Hey. Come over here. I want I want you to meet someone. I want you to think of, like, moving through a party. I don't go to a lot of parties, so I'm just kind of hypothesizing here that this is exactly what parties like.
I see I see parties on TV, which is how I'm basing this analogy. They're busy places and you're constantly distracted, and your movement is erratic. Your movement is zigzag and hergy jerky and wiggly ziggly. You can't move in a straight direction because you keep running into people. And for various reasons, the people randomly give you a new direction.
Maybe you'll go forward a little bit. Matt, maybe now you're gonna bounce over here. Now you're gonna come back here. Now you're gonna go up there. You're gonna go over here.
You are the pollen grain and the people are the water molecules or the water particles, whatever they are. It's through countless random interactions between the water particles and the pollen grain that give rise to this strange Brownian motion. Einstein didn't stop at just a party analogy and something that sounded nice. He went and did the math because that's what Einstein does. And his big trick was to provide a connection between something you can see and measure, like, say, how quickly a pollen grain wiggles across the fluid, with something you can't, like the number and mass of the atoms, the particles in the water.
After he wrote this paper a couple years later, experimentalists got around to doing it and voila, you have an atomic theory on solid ground because now you can connect the chemistry understanding of Dalton and Mendeleev, of the elements, with the physics understanding of these sub microscopic motions, and it all made sense. It all hooked together. Einstein happened to win a Nobel Prize for that. And now for a word from our sponsors. Eventually, Adam is brought to you by Patreon, purveyor of fine science communication products, especially patreon.com/pmsutter, including but not limited to Ask a Spaceman, Space Radio, and experimental science communication projects to reach new audiences.
For that smooth, silky finish, pick Patreon. That's patreon.com/pmsutter. We now return you to Eventually, Emma. Episode number four, Ernest Learns the Alphabet. So in the late 1800s and the early 1900s, we knew that there was this thing called radioactivity.
Since a common thing, this is basically all of nineteenth century science of we know a bunch of things, but we we have a hard time understanding them. But there was this thing called radioactivity discovered and characterized by the Curies, which is another fantastic episode, feel free to ask, and that there were certain elements like uranium that if you stick next to a photographic plate, you would, like, kinda get a picture of it, which is weird because you wouldn't use any light. Usually, you think of you need light to take a picture, but a a rock of uranium could take a picture of itself. And I also suppose if you took that chunk of uranium and stuck it next to bodies, you get cancer, but that's also another story. So once again, what the heck is this radioactivity?
Is it radiation like light? Is it particles like matter? What is going on? Enter Ernest Rutherford who was a student of JJ Thompson. By the way, JJ Thompson also got a Nobel Prize for his work with cathode rays, and Ernest got really interested in the problem.
So he did what any good student does, and he copied the methods of its teachers, and he stuck those radioactive materials into all sorts of electric and magnetic contraptions to see how things behaved. And he found that radioactive emission, radioactivity, whatever is causing these photographic plates to expose, you know, to do the things they do, they came in three flavors. There were three kinds of radioactivity. One kind could be stopped really, really easily like some air or a thin piece of metal, and that kind of radioactivity wouldn't go very far, and it didn't respond to electromagnetic fields. Another took a little while to stop but did respond to electromagnetic fields.
And the last could go on basically forever and didn't care about anything or anybody at all. So he named them alpha rays, beta rays, and gamma rays. It was pretty obvious that the gamma rays were radiation because they were behaving like radiation. Didn't respond to electromagnetic fields. We're just going straight directions.
We had just recently discovered X rays. That's that's a whole other show. Feel free to ask. And so these gamma rays, we figured were radiation. And the beta rays, because they curved under electricity and magnetism, Rutherford figured that these things were the same thing as the cathode ray.
So beta rays and cathode rays turns out to both be electrons. Interesting. So when a thing radiates, when it's giving off radiation, it gives off light, it gives off electrons, but then it also gives off these alpha rays. And alpha rays were really tough. He suspected that they were particles because they could be stopped really easily like particles tend to be, but it wasn't until he developed a super duper strong magnetic field that he could finally get there past the bend.
And after the playing the same math games as with the cathode rays and the beta rays, he was able to figure out that the alpha rays were really just chunks of helium. Helium? Helium. That's interesting. Why?
Why do radioactive chunks of rock leak helium? Well, you can kinda see leaking light, you know, this kind of radiation, the gamma rays, and and the electrons, which we realize was also a part of electricity in the cathode rays, but helium? Why? Why does uranium spit out helium? Well, we'll just have to solve that problem another day, I guess.
Episode number five, Geiger counting. A few years after classifying the three kinds of radiation, Rutherford was working on a hunch. The idea of atom was starting to take shape, especially with Einstein's work in nineteen o five. But Rutherford's old boss, JJ, remember him, had found that some particles were smaller than atoms, subatomic particles, if you will. Those cathode rays, those electrons, were smaller than the smallest known atom, the hydrogen atom.
So did that mean that atoms weren't really atoms, that you could chop them up, that they were made of, you know, smaller bits? We knew we had electrons, which were very tiny and negatively charged, but atoms themselves, if you just look at a hydrogen, it's very large and neutrally charged. So how do you put these pictures together? How do you build a coherent picture of nature where you have you know you have subatomic bits that are small and negatively charged, and then you have atoms that are relatively large and neutral. JJ developed a model that I am 100% serious.
I'm not joking. This is the name of it. And this is not a name we give today. This is the name they use back in the day. It was called the plum pudding model.
Rutherford wanted to test this because he was a scientist, and that's what scientists do. There's a model that tries to explain reality. Well, we got it and put it to the test. And by nineteen o eight, he was a little more of a big deal. He got a Nobel Prize for his work on the radiation stuff, so he didn't have to do it himself.
He made his students do it instead. And he had two students at the time, Hans Geiger of Geiger counter fame and Ernest Marsden of I guess he didn't make a little clicky box, so nobody remembers him fame. And they set up a contraption that took these alpha rays, these alpha particles, which we now realize were helium atoms or something related to helium atoms, shot it at gold foil. He used alpha radiation because he knew it was heavy and it could go fast, so it made a really good probe, and he knew it was tiny. So it was like it was a good thing to use to understand other things.
He used gold because he could bend it down and fold it down into very, very, very thin sheets. So he knew he was looking at, you know, maybe a hundred atoms, 200 atoms at a time in a very, very thin sheet. It was a very good setup. And they used the fluorescent screen at the end because what they were doing was taking these alpha particles, these helium atoms, nuclei, whatever they were, shooting them at the gold foil, and then seeing what happens when they came out the other end. And they would strike the fluorescent screen and they glow and then Hans and Ernest, Marsden, not his boss, Rutherford, would sit in the back and count.
They say, okay. There's a glowy bit there. Okay. Mark it down. Glowy bit there.
Mark it down. They would sit in a dark room hour after hour, day after day doing this. I believe most classic physics experiments are just a means of torturing your grad students, and this is a perfect example. In the plum pudding model and JJ's model, it made a prediction. This is a good scientific theory where it's trying to explain current observations and make a prediction for future observations.
In the plum pudding model, the alpha rays, the alpha particles, would pretty much go straight through the gold with only a few slight changes here and there. Because in that plum pudding model, an atom is just a kind of diffuse ball of positive charge with little electrons swimming around like plums inside of a plum pudding or we could we would call it maybe the the raisin loaf model where there's the loaf of bread is the atom, and the whole bready part is just this, like, diffuse positive chargey thing. And then there's little nuggets of negative charge, the electrons, which are the raisin. So we can call it today the raisin bread model. And in this model, if you shoot the alpha particles at it, well, you know, there's not a lot of interaction.
There's not a lot of interaction. The alpha particles would just, like, kinda swim through that gunk and then come out the other side. And they you might get little changes here and there, like, if there's a random chance interaction, you know, you know, a slight little charge, it might hit an electron here. So you get tiny little deviations, but mostly it would just plow on through. But instead, almost all the alpha particles went through completely straight, and some got really bent.
And some, about one in 20,000, bounce back the way they came. And, yes, that's one in 20,000. That means you have to count 20,000 of them before you get the one that bounces back the way you came. And now you know why Geiger went on to automate this kind of particle counting thing. It's like an obstacle course in there.
These alpha particles were slamming into the gold foil, but it was mostly empty space. If you look at a gold foil, it looks totally solid, but no. It's mostly empty space. It would just sail on through. But every once in a while, there'd be like a giant pylon.
And if you imagine you're an alpha particle like racing through this totally empty field, and most of the time, you can just make it through the empty field and no one notice it. You don't notice anything. You get any deflection, whatever. But every random chance, you slam into one of these giant pylons, and you get bounced back the way you came. The only conclusion from this that Rutherford could come up with was that an atom is not a plum pudding or raisin bread at all.
It has a small, heavy, positively charged core surrounded by a swarm of tiny, light electrons. It's a very different picture. You discovered the nucleus. The nucleus of the atom is small and heavy, capable of stopping an alpha particle in its tracks. So it has to be heavy, has to be positively charged, and the electrons must be outside of it because this nucleus can't take up a lot of space because most of the alpha particles make it on through just fine.
It's only very rarely that they hit that nucleus. And then Rutherford realized that these alpha particles that he's been playing with the whole time are actually helium nuclei. So that was the crack. That was that's what cracked radiation, that the uranium was breaking apart. And when it broke apart, that released some energy in the form of radiation gamma rays.
It spit out some electrons, and the nucleus itself was breaking apart. And when it did, a chunk of the nucleus came out in the form of a helium nucleus and that was his alpha radiation. Episode number six, Just Charge It. Just when you thought it was over, Rutherford comes back for a hat trick, a trifecta of science. He was able to develop a mathematical model for how much path deflection he should see, assuming his new picture of the atom of the important bits are concentrated in the middle.
Like, if you have now that new model, you know that JJ's model is wrong, but you're not exactly sure what the picture is, so let's paint a picture and let's make some predictions. And so he was able to develop he ran through the math and do some calculations that he could further test and explore. And by playing with different elements, not just gold, but other elements, he's able to measure the charge in that core. So, okay, you do gold. You get a certain amount of deflection, a certain number bounce back, etcetera.
Okay. Now you do silver. You get a certain amount of deflection, a certain amount of bounce back. Now you do I don't know. Name another element.
It doesn't matter. I don't know what he did. I didn't put it in my notes. So you can assume he did this with a few elements. And what he was able to measure by doing all these repeated experiments was the charge, the amount of positive charge in the nucleus, in the core of these atoms.
And it was always some multiple of the charge of hydrogen. So if hydrogen has a charge of one, then he would do another element and it would have some multiple of that and some all of it and never like a fraction, it was never three and a half or five and a quarter. It was always five, eight, 15, 20 three. It was always whole numbers, and it was always proportional to its atomic weight. And the atomic weight was a number that the chemists had been using for a century to simply place an element on the periodic table.
Like hydrogen is element number one, helium is element number two, Lithium is element number three. The atomic weight of hydrogen is one. The atomic weight of helium is two. The atomic weight of lithium is three. It was just a number, like an index.
But with Rutherford's experiment, he was able to put the final piece in place. An element's position on the periodic table, which is used to describe its properties and behavior, isn't just a label. It's a thing. It's a physical quantity. It's the amount of positive charge the atom has.
It's the number of protons. If you have one proton, your atomic weight is one. Your element is one hydrogen. If you have two protons, your atomic weight is two. You're element number two, helium.
And the properties of hydrogen are different than the properties of helium because they have different numbers of protons. But sometimes the atoms are heavier than just the number of protons. Well, then there must be some heavy particles in the nucleus that are neutral, no charge at all. Neutrons. And so there it is.
After over a hundred years, a picture of the atom emerges, a core made of protons and neutrons that is small and dense, surrounded by the electrons. An atom. Eventually. This miniseries from the first episode to the last, like I said, took over a hundred years to play out. And it was serious, dedicated work.
Not a lot of cliched accidents. I mean, JJ was surprised by his results. Rutherford was surprised by his results, but they were in a position to be surprised because they were testing something. They were designing experiments to answer very specific questions. They were grinding through it.
They were getting it done. All the way from Dalton's and his predecessors' realization that matter might have a smallest bit to basically the modern picture of the atom. A hundred years of serious dedicated effort. And of course, that was just one mini series. There's the spin off series that introduces quantum mechanics to explain all the behaviors of the atom, but that's another show.
You'll have to tune in another week. Thank you so much for listening, and remember my book is for sale, Your Place in the Universe Understanding Our Big Messy Existence. If you love space and astronomy, if you wonder how we fit in and how science shows us how we fit in and how we've grappled with it over the past few hundred years, then this book is for you. You can go to pmsutter.com to buy it. It has links to Amazon, Barnes and Noble.
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I find it mildly disingenuous to say, like, oh, the story of the atom starts two thousand five hundred years ago. Or, you know, or like the story of exoplanets starts four thousand years ago with the writings of this impossible to pronounce name. You know, it's because as long as as soon as you start having philosophers in your society, they just think all the thoughts. They have every possible idea.