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Physicists knew the interior of the atom contained protons, neutrons and electrons, but they didn’t understand exactly how they were organized. It took Ernest Rutherford to uncover our modern understanding.
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This episode is sponsored by: Casper, 8th Light
Rutherford’s Planetary Model of the Atom
Transcription services provided by: GMR Transcription
Female Speaker: This episode of Astronomy Cast is brought to you by Swinburne Astronomy Online, the world’s longest-running astronomy degree program. Visit astronomy.swin.edu.au for more information.
Fraser Cane: Astronomy Cast episode 378: Rutherford’s Atoms. Welcome to Astronomy Cast, our weekly facts-based journey through the cosmos. We help you understand not only what we know, but how we know what we know. My name is Fraser Cane; I’m the publisher of Universe Today. And with me is Dr. Pamela Gay, a Professor at Southern Illinois University Edwardsville and the Director of CosmoQuest. Hey Pamela, how are you doing?
Dr. Pamela Gay: I’m doing well; how are you doing, Fraser?
Fraser Cane: Good. So we think that people should do some science this summer.
Dr. Pamela Gay: Yes we do. Over at CosmoQuest, we are doing a Summer of Science to get people using their brains as well as going out and enjoying the sunshine. So we’re going to have a series of challenges for you, starting June 4. And we’re going to have a variety of prizes for participants, so go get involved.
Fraser Cane: That sounds awesome. I’ll get the kids to do it, too.
Dr. Pamela Gay: Sounds great.
Fraser Cane: So once again, we are please to welcome Casper Mattresses as a sponsor of Astronomy Cast. Pamela, still enjoying your Casper Mattress?
Dr. Pamela Gay: I am. It continues to be my happy day bed, Sunday napping mattress.
Fraser Cane: Well I’ve got a sort of interesting story, which is that Casper sent me a demo mattress, but I had to leave it in the United States. And so came back to Canada and realized that my mattress has been around for way too long. 12 years. And so apparently you’re supposed to change them every ten years anyway. So it was time to get a new mattress, and I loved the demo mattress so much that I actually bought myself another one. And I’m actually going to buy another one. So this is a legit thing. We actually really do love these Casper Mattresses.
So those of you who don’t know, Casper Mattresses, you order one online – they’re like, 500 for a twin, 950 for a king, which is a really good price. And they send it in a box, and it’s like some kind of magical black hole technology that, when you unwrap the whole thing, it kind of expands and –
Dr. Pamela Gay: It’s Mary Poppins in a box.
Fraser Cane: Right, exactly. It’s great. I’ve even seen unboxing videos for Casper Mattresses on the internet. You can go check that out. And it’s because they’re made of foam, but they’re sort of this special mix of latex foam and memory foam. And the crazy thing is, like, you can order, and then if you don’t like it, you can just send it back. They’ve got a return policy, delivery price is free, they’re made in the U.S., which – but they do sell them here in Canada, which is just terrific.
Dr. Pamela Gay: And unlike TempurPedic Mattresses, which is what our other mattress is, you don’t get hot in the summer is what I’m finding. We did the procrastinate on the AC thing, and the difference became clear.
Fraser Cane: Yeah, absolutely. So and Casper’s giving you, dear listeners, a deal, so if you go to casper.com/astro, you will get 50 dollars off your purchase if you wanna buy yourself a mattress. And again, I’ve gone through two of them now, I’ve tested out two of them now; I’m about to buy a third one, so you can join me in getting a good night’s sleep. So thank you very much to Casper, again, for continuing to support Astronomy Cast. We really appreciate it. And just go to casper.com/astro and pick up a mattress and get a special deal. So thanks, Casper.
Dr. Pamela Gay: Thank you.
Female Speaker: This episode of Astronomy Cast is brought to you by 8th Light, Inc. 8th Light is an agile software development company. They craft beautiful applications that are durable and reliable. 8th Light provides disciplined software leadership on demand and shares its expertise to make your project better. For more information, visit them online at www.8thlight.com. Just remember, that’s www dot, the digit 8, T-H, L-I-G-H-T, dot com. Drop them a note. 8th Light: software is their craft.
Fraser Cane: Physicists knew the interior of the atom contained protons, neutrons, and electrons, but they didn’t understand exactly how they were organized. It took Earnest Rutherford to uncover our modern understanding. So what timeframe are we talking about now?
Dr. Pamela Gay: So we’re still in the 1890s and early 1910s. Or up through the early 1910s. So Earnest Rutherford was actually a student of Thomson’s who we talked about last week, and this is someone who kind of personifies the idea of the nomadic scientist. He was born in New Zealand; parents were from England, but he was born in New Zealand.
Originally went to University on the South Island, then was able to continue his education on the North Island in Wellington. Got a scholarship to be one of the very few people allowed to study for an advanced degree at Cambridge without getting an undergraduate degree at Cambridge. Worked there under Thomson for a while, and then went to McGill University in Canada, and all the time chasing the ability to do more and more science.
Fraser Cane: And so Thomson of course, we talked about last week. He was the one who really started to puzzle out the nature of the electron. So when Rutherford kind of got into the act here, what was the state of scientific understanding of the interior of the atom?
Dr. Pamela Gay: Well, it really wasn’t yet. We were actually at a point where people still considered atoms as these immutable, basic building blocks of everything that couldn’t change. Alchemy was considered something that would never be possible. Now we know that alchemy, in terms of being able to make gold out of things, isn’t possible, but bombard something with enough neutrons, you generally can change its identity.
But at this point, the idea that radioactive decay actually changed the nature of an atom didn’t exist. The idea that one of the reasons the earth is warm is due to radioactivity didn’t exist. The idea that the sun was potentially powered through radioactive interactions, in this case, hydrogen building to helium and so forth and so on, the proton chain, none of those ideas existed. And then Rutherford came along.
Fraser Cane: And so what was the sort of idea then? They thought that the atom were just, like, little balls of whatever the thing was? Like, if you had gold, a block of gold was made of a bajillion little gold balls. And if you cut it down to the smallest piece you ended up with one?
Dr. Pamela Gay: Yes. And –
Fraser Cane: One little gold nugget.
Dr. Pamela Gay: And they were starting to get the idea of protons, but not really. It was actually Rutherford who, in the 1900s, would go on to name the proton, name the neutron. But they were starting to get the idea that there were certain fundamental particles. And this was where Rutherford really started to come in.
He did work on radioactive materials, realizing that through some – basically, if you left a block of radioactive material that was the right radioactive material in a container, it would end up coating the container with something that he called thoron. It’s an isotope of radium. So here he was starting to realize that nuclear reactions could produce a byproduct.
He started to classify what are the different particles that come off during basic radioactive, well, decay processes. They were still trying to figure out what was going on, but he was the first person to start to figure out that the alpha particle was helium, that the beta particle was tied to a reaction where you ended up with high-speed electrons coming off in decay processes. Neutrons were a thing, but it was actually one of his students that figured out what the neutron was in 1932. So they were starting to get at these ideas of these things existing, but they didn’t fully get there. He –
Fraser Cane: So you mentioned that he saw the substance that was the thoron –
Dr. Pamela Gay: Yes, thoron.
Fraser Cane: Thoron, right. So what kinds of experiments did he perform to try and uncover some of the nature of the atom?
Dr. Pamela Gay: He did a little bit of everything. He was one of these people that – he just kept trying thing to see what would the byproduct be? So for instance, he was one of the people that worked to try and figure out how do you stop alpha particles? How do you figure out what alpha particles are? And he initially gave the alpha particle, the beta particle, and the gamma ray their names. And then he figured out that the alpha particle was actually just helium. So he was the one that figured out that.
He then went on to start to figure out actually the reaction that we use in modern smoke detectors to figure out that there’s a fire in your house. He was the one who figured that out. If you take a bit of maranesium – which is what we use right now, he used a different radioactive material – you can create a chamber with a positive plate and a negative plate that have clearly charge built up on them. They’re connected with a battery, and if you put a small bit of radioactive material in them, that small bit of radioactive material will ionize the gasses inside the chamber. And that ionization process will cause charge to flow between the plates, and you can measure this current.
Well, what he discovered was, if he blew tobacco smoke – because, being a man of his time, he smoked – if you blow a bit of tobacco smoke into that ionization chamber, it will stop the current from flowing, because you’ve disrupted the ions. And it was realized, hey, this is actually a way to detect smoke. So the smoke detector’s actually thanks to Rutherford.
Fraser Cane: That’s awesome. But one of the most famous ones, of course, is the gold foil experiment, right?
Dr. Pamela Gay: Which actually isn’t what he got the Nobel Prize for.
Fraser Cane: Yeah, well that seems to go around. That sort of thing happened to Einstein. Here’s your Nobel for the Photoelectric Effect, as opposed to, here’s your Nobel for Relativity.
Dr. Pamela Gay: Right, so he got the Nobel Prize for his work on figuring out radioactive decay, for figuring out that atoms do transmute into other types of atoms through nuclear decay processes. He started to figure out the idea of the half-life, and then got the 1908 Chemistry Nobel Prize for that. And he was always a bit bitter that it was the Chemistry Nobel Prize instead of the Physics Nobel Prize.
But having completed this work that got him the Nobel Prize and did neat things like allow us to understand that the earth actually is really old, having done all of that work, he then went on to try and figure out what’s up with the atom. He was one of J.J. Thomson’s students. He understood Thomson’s plum pudding model of the atom. He actually went on to replace Thomson as the Cavendish Chair at Cambridge, leaving Canada to go back to England.
And once he was there, he started working with his graduate students, who were actually the ones who, as always, did the experiment that he gets all the credit for. And his graduate students set up a situation where there was an extremely thin piece of gold foil, and they fired a beam of electrons at the extremely thin gold foil, and looked to see how those particles would be deflected. And found that the deflection actually matched with the idea of gold foil being made up primarily of itty bitty, little, tiny spheres and a whole lot of empty space. And this finally got us to the idea that the bulk of the material in the atom is in the nucleus, which is surrounded by a cloud of electrons.
Fraser Cane: Right, and we talked about this a bit last time, right? That if the – you can imagine the atoms themselves, and if they were, like, a square – like a cube, for example, and you fired your beam of electrons, the electrons would just come bouncing back because it was just, like, a wall of tubes.
But if it was, like, a very large ball, then you would expect the beam of electrons to be bouncing off in various directions that would really compute with it hitting a sphere. But if it was a very small, but still kind of nugget at the very core of what was the atom, most of them would just pass right through, but some would deflect, and they would reflect as if they were ricocheting off this little, tiny sphere at the middle of what would be the atom. And this is what they found.
Dr. Pamela Gay: Right, so he was doing this work in 1909, the year after he got his Nobel Prize, and this was basically what went on to completely change how we see the atom. And it was kind of a done deal at that point. So what was neat is he systematically went through, and completely changed how we view the atom. We went from having no real understanding of its structure to Thomson thinking, well, maybe plum pudding idea, to Rutherford saying, no, it’s actually this structure that is concentrated in the center.
He then went on, working still with his graduate students, to start to pull apart the constituents of the atom. So in 1920, he postulated that the hydrogen nucleus – so take a regular, every day hydrogen that doesn’t have a neutron. That’s deuterium, it’s still a form of hydrogen, but take a most common form of hydrogen, the one that doesn’t have that neutron, strip away its electron, ionize it. What’s left, he dubbed the proton.
And so now we have this idea that atoms are built of something else. And this built on the work that he did in 1919 that showed that nitrogen, bombarded with an alpha particle, can lead to oxygen and something. That something was the proton. He started adding up the bits, starting to understand protons, how they add together to build up through the periodic table.
Fraser Cane: It’s sort of a – I guess – I’m trying to think of, like, what it must have taken for him to make that realization, right? That hydrogen, which is sort of this most abundant atom in the universe – although maybe they didn’t know that at that point. But that that is the building block. You can kind of imagine that it would be something else. That it’s all made of some other building block, even hydrogen, all the way up to whatever the heavier elements are. But in fact, that the lightest element is the building block that makes up the rest of the elements. I think that’s a pretty monumental discovery.
Dr. Pamela Gay: Well, and then to go on without any observational evidence, because we just weren’t quite there yet, to, in 1921, while working with Niels Bohr, he postulated the existence of neutrons. So here’s this person who, through his life, has just one after another figured out these key issues in the atom. Figuring out what the different types of radiation are, how nuclear decay processes work, figuring out what are the constituent materials. He jokingly referred to himself as the first alchemist in trying to figure out how the radioactive decays happen.
And what was also kind of amazing is it seemed like everyone he worked with went on to get a Nobel Prize. He simply managed to surround himself, or be lucky enough to be hired into situations that surrounded himself with the most brilliant people of his time, and he was just a farm kid from New Zealand. He wasn’t an aristocrat; he wasn’t noble born. So he was also one of the first scientists to essentially rise up out of nowhere to be one of our leaders of understanding.
Fraser Cane: Yeah, and work with Heinz Gieger, one of the people who worked on –
Dr. Pamela Gay: Was one of his graduate students.
Fraser Cane: Yeah, worked on radioactivity. Got some of his radioactive stuff from Marie Curie. Really part of the Justice League of physicists, right?
Dr. Pamela Gay: When he was at McGill, he worked with Frederick Soddy, who went on to get the Chemistry Nobel Prize. Just person after person that he worked with got the Nobel because they were literally redefining how we understand the microscopic, at the atomic level, side of our universe.
Fraser Cane: So okay, so I’m sort of – again, so he’s gotten to the point that he’s discovered the – or he has predicted the neutron, but it actually took a long time for anyone to actually find it, right?
Dr. Pamela Gay: Right, it wasn’t until 1932 that one of his graduate students actually was able to figure it out, James Chadwick. And we have an entire episode on that, back in the first or second year of Astronomy Cast. But it was actually really difficult to figure out, and it took being able to create a stream of alpha particles to bombard beryllium, causing the beryllium to give off the neutrons.
And for whatever reason – this is one of those scientific experiments that leaves me perpetually baffled. They figured out that if you put a block of paraffin wax into the stream of particles coming out of the beryllium that’s being bombarded with essentially helium, that the particles hitting the paraffin will cause protons to be given off.
And they were able to figure out, based on this really mind-blowing to me experiment, that the neutrons must have the same mass for the most part, as the protons, based on conservation of momentum. But since they didn’t detect any charge, couldn’t deflect things that thing was somehow a neutral particle.
Fraser Cane: Mm-hm. Just mind-bending. Oh right, Niels Bohr – he was working with Niels Bohr when he came up with this theory for the neutron. So again, just another of the superheroes of early physics. It’s really kind of amazing to me when I think about sort of where we are now. I mean if he was working in the 19 – in the teens and the 20s – really in the 20s is when he really postulated the modern concept of the atom. And the confirmation really wasn’t made until the early ‘30s, about the neutron. We haven’t even hit a hundred years of this. It feels like, man, it feels like we’ve known about the atom forever.
Dr. Pamela Gay: No, it’s all entirely new. And what gets me looking at this is he was someone that also went on to try and do good things in the world. So he was someone that when he went to Cambridge, by the terms of his scholarship, he couldn’t marry his sweetheart. So he left his fiancée behind in New Zealand. They got married and had a child when he moved to Canada, and throughout his life, just the importance of family comes up.
When he moved back to England during World War I, he switched over his work to try and figure out, not how to build bigger, badder weapons, but instead he worked on the sonar to try and understand how do you detect submarines so that there’s less death? So he worked on sonar for a time.
He was also part of the group that worked to try and bring as many scientists over to England as possible, to protect them during the war. So over and over, he’s working both on the let’s do amazing science, let’s raise up our students, let’s get our students Nobel Prizes while we’re at it, but he’s also just a good citizen. And you don’t hear stories like that very often in science.
Fraser Cane: Yeah, I hope our New Zealand fans are proud of their – of Mr. Rutherford.
Dr. Pamela Gay: They put him on their currency.
Fraser Cane: There you go. I think we’ve got Commander – no, we’ve got Marc Garneau on our currency. We’ve got an astronaut on our hundred – our fifty? Yeah. Okay, so I just wanna sort of close this up then, in that when Rutherford was done with the atom, what was his understanding of what it was then?
Dr. Pamela Gay: He actually got us pretty much all the way to our modern understanding of the atom. That you had a central core that was made up of protons that defined the identity of the atom. That if you changed the number of protons, you change the identity of the atom. This is where his joke about being an alchemist came up. He was the one who got us to the understanding of what the three kinds of radiation – alpha, beta, and gamma – that are the predominant forms of radiation, are. He was the person who understood that there probably is this additional neutral component that goes into all of this, the neutron.
He also started to make headway on what is the half-life, and how radioactive material plays a role in keeping the earth warm. How it’s the potential fuel for the sun that allows the sun to be old. And there’s the whole problem running up to that of is the sun powered by – they used coal to come up with calculations on how old the sun could possibly be. It was Eddington who eventually figured out the proton chain kinds of reactions that actually power the sun. But it was Rutherford who opened the door for nuclear reactions to be part of what allows things to last as long as they do.
Fraser Cane: So what do we then know now that maybe Rutherford didn’t know, specifically about the atom? Like when they thought that the electrons were still kind of orbiting around the atom, right? They didn’t understand the sort of modern concept of more of a cloud of probability, right?
Dr. Pamela Gay: Well, that were actually getting – that was all at about the same time. They were getting to the energy level ideas, they were getting to the discreet energies, they were getting to the Pauli Exclusion Principle. All of these things were coming up simultaneously throughout the world. And it was really this amazing age of – invention’s not quite the right word, but I guess intellectual revolution, in terms of finally having the pieces of the puzzle that were needed to see what the big picture was going to be.
What’s kind of sad is he actually died quite unexpectedly and fairly young in 1937, because he was too busy to get a hernia treated. So here you have this case of – he really kind of personifies all that’s right and all that’s wrong with science. He, like so many scientists, was the vagabond who moved from nation to nation trying to find the best place to get an education, the best job to take, put off getting married because that was what was required for the work. His health was not that great because he had a hernia, but he couldn’t be bothered to take time off to take care of himself because there was the science. And it’s just sort of like, this is all that’s right and all that’s wrong with science, even today.
Fraser Cane: So I mean – I guess one of the kind of other tragedies – the tragedy is that they didn’t have the tools necessary to start to kind of crack deeper into the atom, right? That we in our modern times know that there’s a whole other layer going on underneath the protons and the neutrons, and all of that. That you can actually break those things open and there are more parts underneath.
Dr. Pamela Gay: That’s true. People do talk about how he was the first to split the atom, in terms of getting an atom to decay into multiple bits that he could study, but the whole idea of the modern atom smasher that allows us to get at all the different additional particles that are out there, all of the unstable particles, all of the quirks that build up to create the proton. Bosons were something we still hadn’t really figured out. The idea that the nucleus is held together using – well, glue on bosons. All of these things – we weren’t there yet. But he got us to the point that we could see the need to figure those things out.
Fraser Cane: Yeah, it’s almost like finally getting a sense of what the landscape actually looked like. So really starting to see, like, okay, now I understand what this looks like. And then the next question is to again go, why? Why are they connected in the way they’re connecting? What are the forces that are bringing them together? What is keeping – like you say with the Pauli Exclusion Principle, with some of those other concepts. You’re starting to really get a sense of what’s going on in the overall environment. Why can’t electrons share a quantum state? Things like that.
And so they’re starting to really get a sense of what are the underlying rules and relations between these. Sort of the same kind of work that Einstein did, right? To say kind of like, why are things attracted by gravity? What’s really going on here?
Dr. Pamela Gay: Well, they’re actually, I’d say, different sides of the problem. You have Einstein, who was purely a theorist. His ideas came from looking around, thinking hard, working out the maths.
Rutherford, on the other hand, was someone who was considered perhaps the best experimentalist of his time, who systematically throughout his life, even as an undergraduate, was working on incredible experiments where was working at the time to figure out radio and how to detect electromagnetic radiation over great distances. He was working on x-rays. He was someone who poked and prodded at the world, trying to get it to give up its secrets. He wasn’t as interested in theorizing what must be out there, as he was at saying, okay, let’s figure this out by actually looking and taking it apart whenever we can.
Fraser Cane: Yeah. And even to this day, right, it’s still the experimenters and the theorists working hand in hand, who push the boundaries out. The experimenters who find the “that’s funny,” and the theorists who try to work at what’s going on. Or the theorists who try to provide some kind of path to search for the experimenters to actually go and go looking. Go look over here, go look over there. See if you see this.
Dr. Pamela Gay: And it is that constant back and forth dance, where the theorists have to match our current understanding of reality and then make predictions on what we’re going to find in the future. And the experimentalists, well, they’re the ones out there trying to dig into reality and either prove or disprove the theorists, or extend our understanding further, so that the theorists have to again play catch up. Rutherford was someone who extended things further, requiring that catch up to take place.
Fraser Cane: Yeah. Amazing. All right, well thanks Pamela. We’ll see you next week, for I think the final installment of our famous experiments series.
Dr. Pamela Gay: Yes.
Fraser Cane: All right. We’ll talk to you next week.
Dr. Pamela Gay: Talk to you later.
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Did you miss speak when you said Rutherford’s students shot electrons at the gold foil? It was alpha particles.
In the description of this podcast you say “Physicists knew the interior of the atom contained protons, neutrons and electrons …”
Rutherford’s gold foil experiments were conducted in the ~ 1910 time frame, whereas the neutron wasn’t discovered until 1932. You might want to modify the statement in your intro to more accurately reflect the current state of knowledge available to Rutherford and others in ~1910.