15.2 Rutherford Model

All right, in the last lesson, we looked at Thompson's plum pudding model. We know that he was the first to determine the existence of negatively charged electrons being separate from the positive parts of the atom. However, he never really came up with a decent model other than the whole plum pudding type of thing. We also know that Millikan conducted the oil droplet experiment. The important discovery from this experiment was that he figured out that the charge on the oil droplet was in multiples of 1.6 x 10^(-19). He would balance the electric force with gravity using a glob of oil, and from that, he found that the charge on those droplets was always some multiple of 1.6 x 10^(-19). The smallest multiple was exactly that of the elementary charge. Other multiples would be two, three, four, ten times, or whatever bigger than that. So, he played a key role in figuring out the charge of these droplets. Out of all the discoveries in the field of electronics, one of the most important is the understanding of the elementary charge. It was discovered that the charge is quantized, meaning it comes in multiples of a specific amount, rather than being any random decimal. This discovery was a result of the advancements made by Millikan and Thomson, who determined the charge-to-mass ratio. With this knowledge, we can now determine that the charge of an electron is 1.6 x 10^-19. When we combine this information with the charge-to-mass ratio, we can calculate that the mass of an electron is 9.11 x 10^-31. It is important to note that this value is a calculated mass, not directly measured by any scientist. The next scientist to make significant contributions to the model of the atom was Ernest Rutherford. He conducted experiments with alpha particles, which are essentially the nuclei of helium atoms. We will discuss these experiments in more detail in future discussions. Ernest Rutherford conducted an alpha particle experiment with gold foil. He used a radioactive sample inside a lead box to prevent emission outside the box. However, he drilled a small hole in it so that the alpha particles would be emitted through that hole. He then shot the particles at various materials and found that he got good results when he shot them at a thin sheet of gold foil. You can imagine it like aluminum foil but made of gold. Gold can be made very thin into a foil, and we know it has a fairly dense concentration of protons. So, when he shot the alpha particles at the gold foil, he observed some strange results. Once they hit the gold foil, he could see the path the alpha particles took. Based on these observations, he concluded that other diagram, so there are basically three possible results. The first is when the alpha particle hits the gold foil and bounces straight back. The second is when it hits the gold foil and goes straight through. The third is when it hits the gold foil and is deflected at some angle. So there are different angles of deflection, either straight through or completely bouncing back. He was quite surprised by these results and was trying to figure out what was going on. Using Thompson's idea, he thought that if the alpha particle missed the electrons, it would go straight through. This means that the electrons were spaced far enough apart for the alpha particle to pass through their gaps without any interaction. On the other hand, he also realized that if the alpha particle did hit an electron, it would be similar to the Compton effect, where the incoming wave gets scattered at an angle and the electron goes off at another angle. So he used this idea to explain that when the alpha particle hits an electron, it gets deflected at some angle while the electron moves off in a different direction. The gold alpha particle is hitting the electron of the gold, then it's getting scattered. The third observation was the strangest: he found that some of the alpha particles hit the gold foil and bounced straight back. He thought that was pretty strange because that would be like having a 15-inch shell, like a cannon or big tank bullet, hitting a piece of toilet paper and bouncing straight back. So he thought that was pretty strange. What he then concluded was, if the alpha particle is hitting something massive, that would be a reason for it to bounce back. And if we know that the alpha particle is made up of two protons and two neutrons, and gold on our periodic table is quite a bit bigger (79), then it's hitting... Gold nuclei, which have 79 protons and 79 neutrons, obviously weigh more. So it makes sense that the smaller nucleus would have less momentum. As a result, it will bounce back perfectly straight when they collide. Based on this reasoning, Rutherford predicted that most of the mass is concentrated in the nucleus, with electrons orbiting around it. If the alpha particle goes straight through, it misses everything. If it hits the nucleus, it bounces straight back. And if it hits an electron, it deflects at an angle. This explains the three different results Rutherford obtained. Rutherford's model still accepted Thomson's ideas, but he proposed that the positive charge is concentrated in a nucleus at the center, similar to planets orbiting the Earth. And that's how he came up with his model. But as soon as he published it, there were some flaws. He thought they were just going orbiting around in circles, but if they're moving in circles, that means they have to be accelerating. And if they're accelerating, I mean if it's already charged, we know that from Maxwell, we said that will create EMR. And if you're creating EMR, that means it's gonna release some energy. And if everybody's energy, it's going to slow down. And those electrons would eventually crash. So if you had a positive nucleus and you have this electron in orbit, and if it's gonna slow down by losing energy, we know that it eventually flies in on itself. It gets slower and slower, and eventually would collapse. This doesn't happen. Our atoms are quite stable, so there's got to be a problem. So what's the reasoning? What's the right model then? That's to be determined. So we'll stop there, and we'll look at Bohr's model next.