 Hi, I'm Zor. Welcome to Inizor Education. Today we will talk about electrons again, but that would be a final lecture in the topic which we call elementary particles. We did talk about electrons before and how they are spread around the nucleus. So we will talk about this a little bit more and we will talk about so-called double slit experiment. This lecture is part of the course called Physics 14 presented on unizor.com. I suggest you to watch this lecture and all other lectures from the website from the unizor.com. Well, because it's a course. So lectures are logically related to each other and I'm using something which I have already covered before in the subsequent lectures. Plus there are some problem-solving in certain cases. There are exams which you can take as many times as you want until you will get a perfect score. There is a prerequisite course called Math 14. It's on the same website. Math is mandatory discipline for physics. You cannot study physics without math. Although in certain cases, like for instance today, I will be talking mostly about kind of qualitative properties rather than quantitative properties of electrons. Okay, now electron is the third main particle which comprises the atom. We know about protons and neutrons which constitute the nucleus of the atom and electrons are somewhere around it. Okay, so we did talk about internal structure of protons and neutrons. We talked about quarks. So next question is, is electron also some kind of a combined particle which contains certain parts inside? Well, so far physicists did not find anything inside electron. So as of today, electron is considered to be an elementary particle. That's part of the so-called standard model which comprises all the particles. Okay, so electron is just elementary particles, nothing in it, at least to the best of our knowledge as of now. So what we can do is we can study its properties. So what are the properties? Well, first of all, electric charge. Electron is electrically charged particle. Now, when we talk about measurement of the electric charge, well in the macro world we use something like coulombs, which is a very large actually unit of measurement. In the particles world those which have electrical charge, these are protons and electrons primarily, we are measuring them in units equal to the charge of electron, because that's an elementary particle. There is nothing smaller. So the charge of the electron seems to be a natural unit of measurement of other charges. Again, we cannot really use it in the macro world, because we would have huge numbers and we don't want actually to deal with huge numbers. Inconvenient, but in the world of particles, that's actually the most convenient unit of measurement. So the absolute value of the electric charge of the electron is one. And considering electron is carrier of the negative charge versus proton, which is carrier of the positive charge. As we know, there are only two types of electric charges, positive and negative. We use the mathematical words positive and negative, but just because these charges operate very very much like positive and negative numbers. So the charge of the electron is minus one, charge of the proton is plus one. And as far as specifying the some kind of symbol actually for the electron, we use letter E and sometimes we use it with a minus as a superscript, just to signify that this is a negatively charged particle. Okay, so this is a charge measured again in the charge of the electron itself. So that's why it's minus one. Okay, next is what other properties do we know? Mass, right? So we have electric field and that's the charge. We have a gravitational field and that's the mass which actually plays the same role as a charge in the electrical field. Except there are two types of charges in the electrical field, positive or negative. In the gravitational field there is only positive, which is mass. Now mass of the electron is small. Actually, it's something around like one over two thousand of the mass of the proton. So the mass of the atom is concentrated in its nucleus. So that's small. So what else is important? Well, there is one more very important characteristic of electron, which we did not really talk about before. Now, if you put a magnet in a magnetic field, it will do something. It might rotate, it might move somewhere because there is north and south poles. And they attract each other, or if it's the same poles north to north or south to south, they repel each other, etc. Okay, fine. Now, let's forget about magnet. Let's talk about loop. Electrical loop with a current in it. If you remember from the part of this course called electromagnetism, that the loop with this electric current in it, the recto-electric current, acts actually like a magnet north and south. And we did talk about y, etc., etc. Now, if instead of this, we will have a charged object. Let's say charged ball and spin it around the axis. It will basically be the same as if electric current goes around the loop, so it will be like a magnet. And it will behave like a magnet. So in a magnetic field, so if you have some kind of a magnetic field, it will turn or whatever it do. So what's interesting is that when people were analyzing how electrons behave in a magnetic field, they basically found the same kind of behavior as spinning charged ball and they have decided actually to call this particular property of the electron spin. Now, obviously what's important is it's spinning this way or it's spinning that way. I mean depending on the magnetic field, etc. So there are two kinds of spins. I know you can call it up and down or positive and negative. It doesn't really matter. So that actually led people to analyzing where exactly electrons are positioned within the atom. Now, we already know that electrons are distributed among shells and every shell is divided into sub-shells. Now, within sub-shells, they are moving around certain, along certain orbits. They call actual orbitals. But we can view it as orbit basically within each sub-shell. So different sub-shells is different levels of energy. But within the sub-shell it's like basically on the same radius you can go this way or you can go this way on the same radius. But that would be kind of the same sub-shell, different different orbitals, but the same sub-shell if it's all the same distance from the from the center, right? Okay, so what's interesting is that based on certain properties which can be learned in quantum theory and we are not really doing this, so I'll just present the result. The famous physicist Wolfgang Pauli or Pauli has come up with a principle. It's called principle of exclusion. Pauli's principle of exclusion. Now, this principle says that if you have the same orbital, no more than two electrons can share this same orbital and if they do, if there are two of them, no more than two, but maybe one. So if there are two, they should be oriented differently as far as their spin is concerned. So they should be opposite. Again, the reason for this lies in the quantum theory and we are not talking about this. This is the principle. It's generally acceptable. It's accepted by physicists right now, so the sub-shells are divided into orbitals. And now, and now we can actually put everything we know about distribution of electrons around the nucleus into a very nice table. So the table would be like this. We will have a shell. We will have a sub-shell. We will have number of orbitals and number of electrons in each sub-shell. So shell number one, if you remember number of sub-shells within a shell corresponds to the shell number. So shell number one, number one, has one sub-shell. And again, if you remember, it was called the letter S. It's the first sub-shell. Now, number of orbitals, again, that actually follows from the quantum theory, is okay, if sub-shell has certain number M, then the number of orbitals is 2M minus 1. So that would be one. Sub-shell number one and this would be one. Two times one minus one. And it has no more than two electrons, right? So basically, as a result, the total of the first shell is two. Now, the second shell. Second shell has two sub-shells, if you remember, S and P. Number of orbitals. S is the first sub-shell, so it's one. P is the second sub-shell. Two times two, four minus one, three. Now, this has two. Each of these three has two. So all together this sub-shell has six electrons. No more sub-shells here. So the total is eight for the shell. That's the second shell, okay? Well, let's just repeat it for the third shell and that's it. Actually, in my notes for the lecture and every lecture has notes, by the way, like a textbook, I think I have four shells. So, but let's finish with number three. So the third sub-shell, it has three, the third shell, has three sub-shells. So it's S, P and D. First, second, third. Three and five, right? Number three times two, six minus one, five. Two of each orbital, so that makes it two, that makes it six, and that makes it ten, and the total is eighteen. So, eighteen is the number of electrons on the third shell. Now, the force will be searched, too, et cetera. So that corresponds, by the way, these numbers corresponds to whatever we were talking before, when we were talking about electronic structure of the atom. And basically what I have added is I added the concept of the orbital and principle of Pauli, which gives that every orbital can have no more than two electrons. So the maximum for this would be two, maximum of three orbitals would be six, for five would be ten, et cetera. So that's basically the structure of the electron cloud around the nucleus, as we understand it right now. Okay, and basically that's all I wanted to talk about properties of electrons and their distribution around the nucleus. Now, the second part, if you wish, of this lecture is dedicated to a specific experiment which basically puzzled physicists. So I'll talk about this. It's called double slit experiment. Well, maybe you heard about this, maybe not, but this is something which really puzzled physicists because they just don't know why it happens this way. They basically state that that's the way how it is. It's like in medicine. We, in many, many cases, we don't know the real underlying reason behind some kind of an illness. We know the symptoms and we treat symptoms without really a good understanding of what's on the molecular level happening inside the body. Well, same thing here. So I will present this as a symptom, as a behavior of an electron without real explanation of why it happens. Well, because I don't know. Now, the famous Nobel Prize actually, physicist Richard Feynman, has a very interesting lecture about the same thing. It takes about an hour and that was done a long time ago. And then recently I have found another lecture presented by someone called Al-Hli-Li, if I'm not mistaken. Al-Hli-Li, something like this. Now this presentation is much shorter, about less than 10 minutes, and the guy was very artistically presented the same thing with obviously aided by computer, with computer pictures, much, much nicer. I do suggest you to find on YouTube the lecture about two-slitz experiment done by this gentleman. He is in tuxedo with a bow tie. I mean, it's really very, very artistically done. I will not be as artistic, but I will try my best to explain what exactly this experiment actually is. I suspect that certain physicists actually were so much inspired by this experiment that actually that determined their profession as a physicist. So anyway, first of all, let's think about what happens if something like you have a box of sand with two holes, and you have some kind of a tray here. So you have a sand here. So a sand goes down through these holes, and it will form some kind of a two piles, right? Normal, everybody expects it, no big deal. Now, every particle actually, behavior is exactly like this. I mean, if you will take instead of sand some other particles, however small, you will basically have exactly the same result or big or whatever. Another example, which actually is the one which Richard Feynman was using, he was using a screen with two holes and some kind of bullets which are flying randomly. And then this is a screen, and they will also be in this particular kind of fashion across the across these openings, you will have the concentration. So you have two concentrations of bullets on this screen. Normal, fine. If we will do it differently, instead of particles, you will take light. And light, as we know, is electromagnetic waves, right? So there is a some kind of a source of light here. So there is a flat wave front. It goes to two slits, and you have a screen here. What will you have on the screen? Well, if slits are sufficiently close to each other and sufficiently small themselves, you will see interference picture. So you will see bright spot somewhere in the middle of the bright. And then less and less and less with so this is bright, this is dark, this is bright, this is dark. And why its interference picture? We did talk about interference in when we were talking about waves in the previous part of this course. Okay, that was explained, that was in that particular lecture. It all depends on how these two different rays from two different sources actually. And these are sources of new waves, according to the Huygens principle, if you remember. So how they come? If they come in phase, the distance is the same. So they come in phase, which means it's increasing the amplitude of each other. If they come out of phase, so the difference between these lengths and these lengths is exactly half a wavelength, then they will nullify each other. So one wave goes this way, another way goes this way. And whenever this is plus, this is minus. Whenever this is minus, this is plus. And the result will do this. If your waves are out of phase. If they're in phase, which means it's this way, the result will be double. That will be something like this, right? Amplitude will be greater. So, and that's kind of understandable as well. You have in phase, out of phase, in phase, out of phase, bright, dark, bright, dark, etc. Great. And that's also kind of understandable from the wave theory. So the particles behave like this. Two different lumps and waves behave like this. Interference picture. Great. So let's do the same experiment, but instead of light, we will shoot electrons. And that's exactly what double slit experiment is. Okay. So this is some kind of a source of electrons. It doesn't matter how we do it. And these are two slits. So this is kind of a screen and two slits. And this is some kind of a sensitive screen. For example, if you remember the old computers had CRT screens, cathode tuned, whatever, CRT cathode, I don't remember how it's called, CRC stands for cathode, whatever. So it's like old television, the same thing. The flow of electrons were bombarding. Well, it's some kind of phosphorus related material on the screen, on one side. And we see the image on another side. So this type of thing. So we will see what happens if this flow of electrons, what it actually does on the screen. Now, we kind of expect that electrons are just small balls, and they should behave like particles. So we expect that the brightness on the screen would be exactly like this. Two lumps, basically. The brighter right across it. And the further we are from these two points, the darker will be. Because there is some kind of dispersion, maybe, or whatever. Okay. What do we see instead? Again, I'm not as theatrical, but instead you will see exactly interference picture. So that would be the brightness of the screen, the level of brightness of the screen in this particular case. So electrons behave like a wave. Which kind of contradicts our common sense, right? Well, okay. Let's make this experiment slightly differently. Maybe electrons, when they go in bulk, many of them, they somehow feel each other. And that what makes this type of a distribution. Let's do it differently. Let's do it one electron at a time. So it will be hit from this electron gun, or whatever you call it. It will hit either this or that slot, or slit, rather slit. And see what happens. All right. Okay. So what we will have, we will have on the screen, a dot here, a dot here, a dot here, a dot here. So first it will be kind of a random. But gradually, as the time goes, and if we save the location of these dots on the screen, it will still be exactly the same kind of interference picture as we saw before. Now, since we are shooting electrons one at a time, there is no way they can actually interfere with each other. So how do they know that they should actually go in this way? Well, I don't know. Let's maybe, to clarify the picture, maybe we will make another interesting experiment. Right near this particular slit, we will put a special device which will basically blip or something whenever electron goes by, only on this one, not on this. So as we shoot electrons one at a time, we sometimes will hear this blip. And if we count how many times we shoot electrons and how many times we see the blip, or we hear the blip rather, you will see that's approximately 50%. So half of electrons goes through this slit and half goes to this. And our blip actually was registering. Okay. This particular electron went through this. And we do exactly the same as before, one electron at a time. Yes, what? The picture will be like this. As if they are particles. Go figure. And then what's interesting is, this gentleman I mentioned before, he said, okay, let's fool the electrons. Let's leave everything as is with this detection, the detection device, but we will switch the power off on this device. So maybe it's just plugged in a wall and we will just unplug it. So it's not really working. It's there, but it's not working. And guess what? Picture immediately changed to interference. Well, that's probably where I should end. I do suggest you to maybe watch the lecture on YouTube. It will be more, as I was saying, artistic, but the result will be the same. And maybe it will inspire you to become a physicist. That's it for today. Thank you very much and good luck.