 Hi, I'm Zor. Welcome to a new Zor education. I would like to start a new chapter in this physics course. It's also about atoms and it's about nuclear reactions. Well, when people talk about nuclear reactions, the first thing which comes to their mind is the bomb. Yes, it's a huge nuclear reaction with amount of energy released. Absolutely enormous. And the question is where it comes from. So today's lecture is kind of a foundational information about this energy which comes when certain nuclear reactions are happening. The name of this lecture is Mass Defect. And actually Mass Defect is the source of the energy. But let's talk about this step by step. So, first of all, let's talk about how we measure the mass when we are talking about particles. Obviously, kilograms and other normal, I would say, units of measurements are kind of difficult to apply to these tiny particles. So, what physicists were thinking about is at the time, at least, they were thinking that something like a proton or neutron are the most elementary particles. At that time, they didn't really know about quarks, for example. So, protons and neutrons, they do have very, very similar mass. And the physicists decided that something like a mass of one proton or one neutron would be a nice unit, since it's an elementary particle. Then, let's say the atom contains a certain number of protons, a certain number of neutrons and a certain number of electrons. So, the unit which is associated with a proton or a neutron being a unit, a one, would be a nice unit of measurement on that particular scale. So, what they have decided to do is they took an atom of carbon, and not just any carbon, but concrete carbon, which is 12, 6, 6 protons, 6 neutrons, the atomic number is 12, and it has obviously 6 electrons as well. So, they took the mass of this, which they can measure, and divided by 12. So, that would be a nice unit, since it's 12 protons and neutrons, we divide by 12, so the unit would be about one mass of proton or neutron. So, that was kind of the unit which was established to measure the mass of elementary particles. Now, using this unit, they have much, much later actually, calculated precisely mass of protons and neutrons and electrons actually, so the elementary particles which are always in any atom. So, and that particular mass was, oh, by the way, this particular unit was called atomic mass unit, or AMU. Sometimes, it's just simply U units. When they're talking about something, they can put number either AMU letters or just U letters, which assumed this is atomic mass unit. Sometimes, it's called DA for Dalton, the chemist and physicist who actually invested a lot in research of atoms and their chemical reactions. So, this atomic mass unit is used on that scale, and when physicists had much better equipment to measure, they have measured that the mass of proton is, and I have to 0072766 atomic mass unit. So, that's mass of the proton. Mass of neutron is 10086649 atomic mass unit, and electron has a mass 00005486 atomic mass unit. Well, as you see, electron is much, much smaller. It's about less than one-two-thousandth of mass of the proton. Okay, so these are exact measurements of the masses of proton, neutron and electron. Well, but obviously with that great equipment, physicists could measure mass of the atoms. So, they have decided to start with a simple atom, a very small number of protons and neutrons, which is deuterium. Deuterium is having one proton, one electron obviously, and one neutron, which makes this atomic number two, one proton plus one neutron. So, the mass of this was also measured, and it was 2.014102 atomic mass unit. All right, great. But then they have decided to basically compare. What's the components of this? Components of this one proton plus one neutron plus one electron. So, let's just sum these numbers and we will get 2.0164901. I count it, that's true number. Okay, so it looks like the atom of deuterium is 2. whatever atomic mass unit, but the sum of its component is more than that. Well, that's kind of a strange, but that's defect. That's something which is called mass defect. All right, maybe it's a strange kind of a circumstances related only to this atom of deuterium. Let's talk some other example. Well, I took another example. We will have to talk about uranium 23892. So, it has 92 protons, 146 neutrons, and 92 electrons. 92 plus 146 is 238, that's atomic number. All right, so again, there are some measurements, 238.98499. That's what uranium atom of uranium is having as a mass, atomic mass unit. All right, now let's calculate this. 92. this plus 146 plus this plus 92. this. And we will have, actually I'm wrong. This number is for this sum. That's 238, 239.98499. And the atomic mass of atom of uranium is 238.02892. So again, we have this sum of components 239 is greater than the atomic mass of the atom itself. So, why? Where the mass goes to? Matter doesn't visit you. Matter can only be converted from one state into another. And something must actually be preserved. Something like conservation, conservation of energy, conservation of mass. We have many different conservation. So, what's going on here? Why do we have such a discrepancy? Why do we have mass defect? So, the sum of components is greater than the whole which these components compose. Well, and the answer is in the most famous formula of physics. Yes, arguably, obviously, maybe there are some other good formulas. But some people really think that this is the most famous formula in physics. This is a formula suggested by Albert Einstein in 1905 as part of his special theory of relativity. And it basically states that any matter which has mass, mass is m, contains within itself certain amount of energy which is equal to this. Now, the c is speed of light. Well, it's huge. Now, it's square. So, it's even more huge if it's possible. So, the amount of energy within something which is even small like one gram is huge. Now, why? Well, very simple reason. Now, you remember that we need energy to keep nucleus together, for example, of the atom. Because protons are repelling each other. And we need something which we were talking about, inter-nucleus forces which are actually holding the nucleus together, the strong forces. So, this is the energy needed to keep the atom together. So, if you will take some of the components of the atom, all these protons and neutrons, why are they a mass greater than the mass of the atom itself? Because part of the mass of the components is converted into the energy which holds the nucleus together. And electrons and their orbits, etc., holds atom together. So, this is the reason why we have basically an integral structure of the matter. The matter is containing molecules, molecules contain atoms, etc. To hold it together in certain order, to hold our world from disintegration, we need some energy. And the energy is basically taken from each component. So, since every piece of matter holds certain amount of energy in itself, this energy is used when we are combining pieces of the matter together. So, the energy is used to hold them together. So, this is the reason for our world to still be as it is, basically, without the defect of mass. The components would just disintegrate practically, because there will be no forces which hold them together. So, to maintain the order in the universe, we need energy. And whenever, from the chaos, all the protons and neutrons are separated and some kind of a big bang, whatever. So, whenever we have such a chaotic conglomerate of protons and neutrons and other particles, etc., if we want to have something meaningful, some planets, people, buildings, etc., we need an energy which holds them together in this organized fashion. Without this energy, our world would disintegrate and the energy comes from this mass defect. So, that was basically my main kind of a point which is in the foundation of the nuclear reaction. And obviously, my next statement, which will be the next lecture, is that whenever we break apart matter, we basically destroy this integrity. And now we will have certain components which have greater mass. So, why do they have greater mass? Well, because certain amount of energy is converted into the mass. And certain amount of energy just flows away, obviously. And that's the reason for something which is called fission. Whenever the nucleus of heavy element like uranium, for instance, breaks apart, it releases certain amount of energy which held this matter. And that would be a subject of the next lecture. So, thank you very much and good luck.