Free Chemistry NEET Video Lectures for Structure of Atom

00:00:01 [Music] [Music] Hello, my name is Sabhisachi Mishra. I'm an assistant professor in the department of chemistry in IIT Karapur. In next few classes we'll discuss about uh the fundamental particles that constitute everything around us. Whether uh you see anything around you whether living beings or non-living things, a piece of wood, a piece of metal, a stone or the dress that you are wearing, the food that you ate or even even our own body. Everything is made up of by set of fundamental particles.

00:01:00 These fundamental particles are the building blocks using which the entire universe has been created. In this lecture series, we would see the properties of these fundamental particles. We'll learn about the stories behind their discoveries and their their properties, their uh experimental validation and their significance in in uh modern science. In last 200years, scientists have made significant progress in obtaining a very clear understanding of the structure and property of matter. Even long ago, something about 2,5

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00:01:47 years ago, roughly around 400BC, there were Greek philosophers who were thinking deeply about the structure and properties of the particles that make matter. These philosophers had great visions. They they thought deeply about the subject. when they saw some let's say a stone a metal uh a piece of metal they thought that I can break it into half and each of those that broken piece I can take and I can break it further into half and I can continue to break them into smaller and smaller particles

00:02:29 until a situation would come where I cannot break this particle anymore then I attain a stage of uncutability that I cannot cut this matter any further or I cannot divide this matter into half. This uncutability or indivisibility of the matter is what they uh called as atomio. This is a Greek word which means that something which is not cutable for you cannot cut cut it further. This term atomio gave rise to the term that we know today which is atmal particle and it was believed that it is something that cannot be cut further or

00:03:24 cannot be divided uh into smaller pieces. These philosophers had wonderful thoughts. They were great ideas but they were not backed up with scientific evidence. So there were no experiments which were conducted to validate their hypothesis. It was 188 that the first scientific theory was proposed regarding the structure of the atom or the structure of the matter. That was by Dalton in his famous Dalton's atomic theory which was proposed in the year 18

008. John Dalton was a British science teacher. He hypothesized that

00:04:11 the matter is composed of tiny particles. Tiny indivisible particles. tiny indivis indivisible particles and he called them atom. He imagined these atoms to be hard hard spheres. Now he saw that there are different kinds of elements. There are iron, there is there is platinum, there is gold. So he said okay I see different elements and these elements are composed of different atoms and these atoms that constitute an element have identical properties. Let us see. Let's see. Uh let's imagine

00:05:12 that we have element A uh which is which is composed of atoms A and we may have element B which is composed of another set of atoms. I am showing them as as spheres with dots. They are these are B atoms. The properties of the atoms that constitute element A are all identical. The properties of B atom are B atoms are again identical but the properties of atoms A and atoms B are very different. So you can imagine there are rather elements uh so let's call them C. So we have now defined three different

00:05:53 elements with three different sets of atoms and each atom atom A atom B atom C they have distinct uh physical and chemical properties. But then John Dalton also saw that we have compounds the chemical compounds. How do the compounds form? The he suggested that the compounds form by the compounds are composed of several atoms with different composition or different ratio. So different atoms of different elements combine with different ratio to form a compound. So let's say I took one of A type atoms. I took uh two B type atom

00:06:42 and then I take one C type atom. So I made a molecule or a compound which is called A B to C because I have one A atom, two B type atom and two B type atoms and one C type atom. This is how the compounds form. This is what John Dalton professed. Then he also saw that there are chemical reactions during which the atoms change from one compound to another. So he described the chemical reactions. Chemical reactions is the place where the atoms are exchanged. exchanged the atoms are exchanged let us see you

00:07:30 have let's suppose I have a molecule which is ABC and then I have another molecule which is BC and they are reacting what I do get is let's say I get a B2 plus C2 if you look carefully you would see that the number of atoms of A or B or C type in the left hand side are exactly equivalent to the number of atoms of A B and C type in the right hand side during this chem chemical reaction neither I created any new atom nor I destroyed any atom what I essentially did was to exchange an atom from one molecule to another molecule so

00:08:05 this is what chemical reaction was all about in Dalton's uh hypothesis it was an interesting theory explained uh several uh experimental uh results but it also had a few drawbacks for example it could not say why the atoms of element A and atoms of element B have different properties. So different atoms different atoms have different properties. Dton's atomic theory suggested that all the atoms of element A have same identical property. All the atoms of B have same properties but atom B and atom

00:08:55 A they have different properties but why should they have that that was not explained by Dton's atomic theory then here for example I wrote down this molecule called A2C suppose this AB2C molecule exists what Dton's atomic theory did not explain is that why a certain composition or a certain ratio ratio of these elements form a particular molecule. Why the other compositions are not stable? Why do molecules have certain certain composition and why not some other ratio? These questions were not explained by

00:09:45 John Dalton's atomic theory. Again there were some new experiments which uh took place after Dalton's atomic theory was was proposed from which it was known that atoms absorb or emit electromagnetic radiations but Dalton's atomic theory could not explain why the atoms should absorb or emit electromagnetic radiations. Dalton's atomic theory was was an interesting hypothesis which described some chemical uh some experimental results. However, it had many uh limitations. In mid 1850s there were a series of new

00:10:42 experiments which were carried out that showed that atomic Dton's atomic theory has several flaws. We'll discuss about those experiments. Uh some of those experiments they are called cathode road cathode ray discharge tube experiments which were carried out in during mid 1850s. Michael Faraday was the one among the pioneers of of this kind of experiments. This cathode ray discharge tube experiment. We'll we'll discuss about the experimental setup. So it is essentially a large glass tube that you

00:11:23 can see. Uh it is connected to a a vacuum pump uh using which you can evacuate the gas which is present with within this um discharge tube. Then it has got two metal plates. Let us draw this way. These two metal plates were uh connected by a potential difference. So a voltage was applied. So we we use this as positive positive terminal. This as negative terminal. So my this is this is negative terminal. This is positive terminal. The negative terminal is called cathode. The positive ter terminal is called

00:12:29 anode. One thing that you would see is that I have kept a hole in the anode plate. the plate which to which the positive terminal of uh voltage is connected to. There are two important things here. First we created a vacuum. So that means very low pressure exists low pressure and the second thing is that we are going to apply very high voltage. When you do that, we see in this cathode ray discharge tube experiments that several rays they originate from this cathode terminal and they travel towards

00:13:17 the anode. So you would see several rays and these rays are called cathode rays. You cannot see them see see these cathode rays in naked eye to to be able to uh visualize them. What they did was create a hole in this anode plate so that these rays can pass through this hole and come to the back side of this anode plate and will hit on this glass surface of this discharge tube. Before that this glass tube the side of this glass tube was coated with some phosphorescent material. Uh typically one uses zinc sulfide

00:14:16 coating to have this uh phosphorescent coating. The property of phosphorescent material is that when charged particle come and hit them, you would see a bright flash, a spark. You would see here a bright light. You would see that actually would be uh a way of visualization that this rays which we are calling cathode rays, they have come and hit this surface, this glass surface. We'll discuss some properties of this cathode range. First was it was observed that they always travel in straight

00:14:57 line. They always travel in straight line as it was uh seen they originate from cathode and they travel towards anode. So that is why they are called cathode rays because they they originate from the cathode um terminal and these rays also have kinetic energy. How can we know that they have kinetic energy? How can I know how can we know that? If we can fix a propeller, a small propeller here, then you can see that these rays will hit on this propeller and then this uh the fans will start rotating. So you would actually convert

00:15:47 the kinetic energy of these rays which are called cathode rays into mechanical energy. This experimental setup was taken by another scientist JJ Thumpion and he modified his experimental setup slightly. Instead of a uh cylindrical tube that you saw in the previous uh picture, uh JJ Thumpson, a British scientist, his cathode red tube had a different shape. It looks like an a bottle with an elongated neck. We'll see why he has uh adopted this kind of shape for this for his experiment. Again like a typical

00:16:29 cathode ray uh discharge tube. We have one cathode terminal. So a metal plate cathode. We have one anode here with a hole uh at the center. And the two terminals are connected to very large potential difference. So this is negative. This is positive. This is my anode terminal. First it was this glass chamber was completely evacuated and then a high voltage was applied across these two terminals at the cathode and anode plates. When you do that of course we know that that rays will start from cathode and they will travel towards

00:17:30 anodes. So and we would because of the presence of this hole the rays will come and hit this screen. This screen is again uh coated with phosphorescent material so that when it hits the screen we get a spark. Let's call this point A. All right. So this is what we had obtained from regular cathode ray discharge tube. What JJ Thumpson did was he tried to investigate the properties of this cathode rays. So what he did is he applied another electric field along in this manner. So he applied he subjected this cathode

00:18:14 rays to a to another electric field. He used a particular polarity. Let let us assume that he we defined it this one. This is the positive terminal. This is the negative terminal. When this electric field is applied, what he saw was that these cathode rays which were going in straight line, they started getting deflected and their deflection was in a particular direction. So they started from here and they felt the presence of electric field and then they started getting deflected and by getting

00:18:50 deflected they hit the screen at a different point B instead of point A. So these cathode ray beams were actually bending in the presence of electric field. We know one thing is that like charges repel each other and opposite charges attract each other. So what we see here is that these cathode rays are getting attracted towards positive terminal and they were getting repelled from the negative terminal. This suggested that these cathode rays are negatively charged. He then switched off this electric field

00:20:01 and what he did was to introduce a magnetic field. Let us assume that he used uh a mag magnet in this manner and when he used this magnetic field he saw that this cathode rays so before he introduced the magnetic field he switched off the electric field. So that means the ray cathode rays instead of hitting the screen at point B they came and hit the screen at point A. Now again by introduction of a different field this time it is magnetic field the cathode rays again get deflected but this time they are defle getting

00:20:39 deflected to another opposite direction. Let us call that where the point where the cathode rays hit the screen let's call that point C. So now we using these two setup one was electric field another was magnetic field one was electric field second was magnetic field by using these two different fields JJ Thson showed that these cathode rays are getting deflected in opposite direction. Now we have got two different uh fields whose strength we know because we are doing the experiment on that. Now

00:21:22 if we think why these cathode ray beams are getting deflected we we think that okay of course they are charged particles in addition to their charge they also will have some mass because if a particle has different mass you would require a different amount of field to deflect it by a certain magnitude. So the deflection the degree of deflection depends on two properties of the particle. One is charge and the other one is mass. What Jay Thomson is did is that he adjusted the electric field and magnetic

00:22:03 field. Now he switched on both electric field and magnetic field simultaneously and he adjusted the strength of the electric field and the magnetic field in such a way that the cathode ray beam instead of getting deflected towards point B or getting deflected towards point C was brought again back to the straight line and it can be achieved by carefully tuning the strength of the electric field and the magnetic field. So we have two field strength that we can tune and we have two different properties on which this deflection

00:22:35 depend on the basis of several careful experiments. Uh it was obtained that these particles the cathode rays they have a fixed value of E by M which was determined to be 1.758 into 10 ^ 11 kum per kilogram. Right. This is very interesting. So he he saw uh he could get an estimation about the charge per mass ratio. He repeated this experiment by changing this metal from one metal to another metal by using different metals for cathode and anode. And he saw that no matter which metal he is using as cathode and anode the E by M

00:23:26 ratio remains the same for all metals. This is very interesting because to begin with this cathode discharge tube was completely vacuum. there was there was nothing inside uh this tube except for the cathode and anode terminal. Now by passing high voltage some particles which have certain mass certain charge certain value of charge per mass ratio s these particles they are uh created and they they these particles have same E by M value irrespective of the nature of the material. So that means there are

00:24:12 two things. One is that now John Dalton's hypothesis said that atom was the smallest building block the tiniest particle that you can see. But here Jay Thumpson said that uh no there is another particle which is negatively charged and this negatively charged particle is present in all metals different elements. So there exist another particle which is fundamental in nature and is present in all the metals that he tried out. So he suggested so there is something other than atom which is which is a

00:24:54 fundamental particle. Now what he obtained from this experiment was the charge to mass ratio but he neither had an estimation about the charge of this particle nor did he have an estimation about the mass of this particle. And this was resolved by another set of experiment. This time it was carried by uh Robert Milik. This experiment is is very interesting. It is called Milikans's oil drop experiment. The experimental uh setup some looks something like this. You have a large uh chamber uh which is fitted with two metal

00:25:31 surfaces. You have one metal surface at the bottom uh one towards the uh top and the top metal plate has a hole. It's a tiny hole at the center of this of this plate to this chamber. Uh Milican connected is some a thing that is called uh an atomizer. This is an atomizer. Atomizer is nothing uh but a mechanism with which you can create small particles. It is also used for example in the perfume that you use. You press the perfume and then you see small particles coming out of the perfume and

00:26:20 there is very very tiny particles. So this is this has similar mechanism and this atomizer was connected to some oil. So when you press this atomizer you actually create lot of oil drops here in this part of the chamber. And these oil drops because of gravity they will come down. And since there is only one small hole these oil drops will come out of uh this this region of the chamber only through this hole and they will travel downwards because of gravity. because of gravity which is pulling the

00:26:57 oil drops down they will come down. What he did is was to introduce a small telescope here. What would you what would he do with this telescope? He can look at the motion of these oil droplets when they travel along this direction through this telescope. By doing this he can monitor how long these oil drops are taking to travel in a distance. From there he can have an estimation of the mass of these oil droplets that his atom atomizer is creating. So we have an initial estimation about the mass of

00:27:36 these oil droplets. All right. Uh this was not the end of this uh arrangement. He connected these two metal plates to a potential difference. uh let us say that we have the there is a battery here. So we have a pos with put the positive terminal this side negative terminal this side. Of course when I apply a potential difference if these oil droplets are chargeless this is not going to affect them. They will simply go by uh gravity. It's not only gravity which is actually playing a role. There is also a role of viscosity

00:28:14 because we are creating these small droplets and when they uh go down in the air there is a viscous force. But for our discussion we'll ignore that. We'll consider that only because of gravity these oil droplets are coming down. Now what he additionally did is that he introduced X-ray in this manner. When he introduced X-ray, X-rays are radiations with extremely high energy. They have so much of energy that the they can ionize the gas present within this chamber. So when you apply the X-rays, the X-rays

00:28:52 hit the gas molecules and they ionize them. And when they ionize them, these gas molecules release negatively charged uh particles. And these negatively charged particles stick to our oil droplet. So now I had this oil droplet, but this is now sticking with some negatively charged particles. Right? Now after introduction of this X-ray these oil dropleters are no longer chargeless. Now they have a certain charge and I am applying an electric field depending on the polarity of the electric field that I have

00:29:27 chosen. Now you can see because these are negatively charged oil droplets they would be attracted towards the positive plate. So they will go upward. So now there is another field that another force because of electric field which will pull this charged droplet upwards and gravity is anyway pulling it downwards. Now we have two different uh forces. The gravitational forces can be written down as mg and the electric forces uh forces due to electric field can be given as Q. Q E E is the strength of the electric field that I'm applying

00:30:07 and Q is the charge of this oil droplet. Now if I use a very very strong electric field which is much stronger than gravitational uh pull then of course you would imagine that these oil droplets will suddenly go up and will get will stick to the uh upper plate. If I use a very weak electric field, the gravitational pull will will win and this oil droplet will come downwards instead of going upwards. But if I can carefully manage or carefully tune the strength of the electric field, I can attain an equality where QE the upward

00:30:46 pulling force because of electric field will be now equal to the gravitational pull which is mg. G is a universal constant. I know that mass of the oil droplet I have an estimation already. E the strength of the electric field that I know because I am doing the experiment. The only unknown that I am left with is the charge of the charge on these oil oil droplets which are called Q. So he tried out different values of electric field and he saw that he was getting some interesting values for the charge. So he was getting uh sometimes

00:31:22 he was getting 1.6 6 into 10^ -9 kum. Sometimes he was getting 3.2 into 10^ 9 - 19 colum. Sometimes he was getting 4.8 into 10 ^ -9 kum and so on so forth. If you look carefully at these numbers you see that all these numbers are integer multiple of this value. So what he was getting was as charge as the integer multiple of 1.6 into 10 ^ -19 kum. From his experiments he concluded that the base elementary value of this uh charged particle is 1.6 6 into 10^ -9 and if two of these particles are sticking to the oil droplet he's getting

00:32:17 3.2 and if he's get three uh of these particles are sticking to this oil droplet he's getting 4.8 into 10 ^ -19 kum and so on so forth but the basic value the fundamental value comes out to be 1.6 6 into 10 ^ -9 kum. So he suggested so this must be the charge which is which which we call it Q and we call that the charge of the electron must be 1.6 6 into 10 ^ -19 kum since already we know that these are negatively charged we can use a negative sign here from previous experiment that is the experiment of JJ Thumpson we had

00:33:

00 an estimation of E by M as 1.758 into 10 ^ 11 kum per kilogram now we have an estimation of E we have an estimation of E by M from these two equation I can get this the value of n which is 9.01 into 10 ^ -31 kilog which is now the mass of this fundamental particle which has a negative charge of this value 1.6 6 into 10 ^ -9 and this particle we called electron. If you look at this the mass of this electron it turned out that it is several thousand times smaller than the smallest atom that was known that is the

00:33:50 hydrogen atom. So Dton's hypothesis was at was that the atom is the smallest indivisible particle. There is nothing inside the atom. atom is the basic building block. But because of the experiment of JJ Thumpson and Robert Milik, it turns out that all all elements have these electrons which have a particular value of mass which is much smaller than the mass of an atom. So therefore there exist a particle which is smaller than than than an atom and these particles have negative charges. Now this was wonderful.

00:34:31 However, there is one problem. The problem is that these fundamental particles which we call them electron are negatively charged but atom as a whole is charge neutral. So there was a need to propose a new model for atom and this is what was done by JJ Thaman in his famous plum pudding model of atom. He said that okay uh the atoms undoubtedly have electrons which are negatively charged particles. So to counter these negatively charged particles there must be a positive charge in the atom. So imagined he

00:35:12 imagined that atom uh is a atom is spherical. He said that okay the positively charged positive charge must be distributed uniformly over the atom. So the positive charge is uniformly distributed. So there is a positive charge density over the atom. This is what they believed. And then electrons which are the negatively charged particles electrons are embedded into the atom or the positive charge to the positive charge density. He imagined the atom to be something like this as is shown here. The atom is

00:36:12 spherical in shape. The green color shows the positive charge distribution and the blue colored dots that you see they are electrons which are embedded uh randomly uh in in this positive charge distribution uh of the atom. He uh likened it to the famous u British desert plum in pudding plum pudding model. This is what is called plum pudding model which can also be uh imagined in terms of watermelon. You see the watermelon the flesh the red flesh of the the fruit part of the watermelon uh can be

00:36:50 imagined to be the positive charge distribution and the seeds the black seeds that you see are essentially the electrons which are embedded into this fruity part of watermelon. You can call it plum pudding model or watermelon model. This was a revised version of the model of atom uh which was proposed by JJ Thumpion which is a one-step adv uh advancement of Dalton's atomic model because Dalton did not disting did not imagine the presence of electrons. He just said that atoms are hard spheres.

00:37:27 But then there were a series of experiments uh which were carried out that suggested that JJ Thomson Thomson's plum pudding model is is not that accurate. It has got its flaws. We'll look at those experiments. Now one of one such experiment was carried out by Ans Ratherford which is called which is famously known as Rutherford's gold foil experiment that was carried out in the year uh 1911. The experimental setup is uh shown somewhere here something like this where you have a large circular screen.

00:38:08 You have a large uh circular screen which is coated with phosphorescent material. Again you can use zinc sulfide. So this the inside is is is coated with phosphorescent material. He has used a gold foil here. Gold foil is a it's a it's a very thin foil that is kept in this way. It's just like a piece of paper. He has a source of alpha particles. You can imagine it like a gun which emits alpha particles. Alpha particle is simply doubly charged uh helium. So these are positively charged particles

00:39:07 with with known masses. So what he does is that there is a gun in this source uh that shoots alpha particles and these alpha particles they they are passed through a lead plate and then these alpha particles they come they hit the gold foil like this and if they pass through the gold foil they will come and hit the screen at this point. These as I said these alpha particles are positively charged particles and this is coated with a fluorescent phosphorescent material. Uh the characteristic of this material is

00:39:47 that when it is hit with a charged particle you see a spark you see a bright light here. So this experiment was carried out uh in dark room. All right. So what he did is that he started shooting uh alpha particles after alpha particles thousands and thousands of alpha particles and he tried to see where these alpha particles are hitting in this screen. What did he observe? He observed first the most of these alpha particles most alpha particles they go undelected. Undelected means these rays simply

00:40:34 travel in straight line as if there was no barrier. That was one of one uh primary observation that most of these alpha particles are just going through the gold foil as if nothing nothing is there in in on their way. But he also saw a small number small number of alpha particles they undergo small angle deflection. What is that? He's when these alpha particles come they hit here this on the gold gold foil and instead of going straight they go like this and they hit the screen at different point or they

00:41:19 can also it can also go and hit uh somewhere but this happens only in some small number of cases very rarely but it's a significant observation rarely there is a phenomenon of bouncing back the ray the alpha particles simply bounces back. So that means it goes and hits the gold foil and it comes back or it under goes a large angle deviation deflection and it hits the screen somewhere in this region. It either bounces back or very large angle. These are the three primary uh observations that he made. First, most

00:42:05 alpha particles go through this gold foil undelected as if this foil doesn't exist, as if there is no barrier on their path. A small number of alpha particles they undergo small angle de deflection. Uh that means some something is obstructing their path and only rarely they calculated about one in 20,

000 one in roughly 20,

000 uh alpha particles. they actually either bounce back or undergo very large angle uh deflection. Now this this was an interesting uh observation but it required uh some amount of

00:42:47 thinking as to what might be happening. Now you imagine that this is your gold foil which is very thinly uh thin foil. So there is only a uh the thickness is u extremely small in the order of uh a few nanometers. You can imagine uh in the following way imagine that this is your gold foil that you saw that was standing within the c circular loop and I'm showing you the uh cross-section from from this direction. You'll see that well the gold atoms since the foil is very thin I imagine only a few layers of atoms are present.

00:43:26 I do not know the structure of the atoms but I know that this gold foil is composed of some atoms. Let's first imagine if Dalton's atomic theory were true, what would have happened? If so, I'm shooting this uh alpha particles. I'm shooting these alpha particles. If Dton's atomic theory were correct, we would have seen that these uh hard spheres would have deflected these particles. So most of most of the times the alpha particles would have come back but that was not the case. What was

00:44:09 observed is that most of the times these uh alpha particles they passed through that. So they were going through like this. Since most of the times they were going through his first conclusion was that atom has a lot of empty space. That was the first conclusion because uh the these alpha particles felt nothing. They just simply uh passed across the gold foils as if no nothing was there. So it must the atoms must have lot of empty space. This is what he uh concluded. Fair conclusion. The second

00:44:54 thing is that this what what he saw is that when some cases when they were the alpha particle was coming it saw something we do not know what but it saw something because of which it got deflected instead of going straight it went it went went uh this way it under went some deflection. Why would it why would that happen? That could happen if we remember these are positively charged particles they have certain mass these uh alpha particles these positively charged with certain mass particles they are going

00:45:31 and they are getting deflected it might have happened that there are some positive charges in this in this atom which is actually repelling the alpha particles that are coming and these positively charged particles have certain mass because of which there is a collision and then there is a deflection. So from this it turned out that well the atoms atoms have some positively charged particle with some mass. Right? That is why you see some deflection. But what was the third observation? That was only rarely one in

00:46:22 20,

000 times that you saw that this was the ray that was coming was getting completely uh was was bouncing back. When would it when would that happen? The complete bounce back will happen only when if if the ray the sorry the alpha particle that we're traveling hits a massive charged particle positively charged particle if it hits directly then the complete retract retracing of the alpha particle pathway can be observed. So from the second observation it was it can be concluded that atoms have positively charged particle with

00:47:06 with mass and these positively charged particles with with mass they are concentrated at one place only when the alpha particles hit this particular place where the charge and mass are concentrated You see this bouncing back by carrying out this experiment several times. He concluded that by then it was known that that a typical atom had the size of 10 ^ - 10 m. So this positively charged core with the mass must have the size of 10 ^ -5 m. And this core that we call which has which is where all the masses are concentrated and the

00:48:05 charges are concentrated it was called as nucleus. So from ratherford's ratherford's gold foil experiment it was uh learned that the atoms have a core which is known as nucleus that where the charge and the mass are concentrated. So now we can uh propose rather force atomic model. So he imagined that that atom has a central place the nucleus which has which contains the mass and the charge positively charged and the electrons they go around the nucleus in in fixed orbits. Uh we can imagine uh we

00:48:48 can we can reimagine this experiment by considering that what is what would be the outcome what would be the uh observation if JJ Thumpson's plum pudding model were true. Plump pudding model would have said that the positive charge is uniformly distributed. If the positive charge were uniformly distributed, you would have seen that the most of these alpha particles would have got deflected. But that was not the case. Since only a few particles got deflected and very rarely you saw a complete deflection, it was said that

00:49:20 the plum pudding model was wrong. And here comes a new model for atom where which says uh positive charge of the atom is centered at a core place that is called nucleus and the electron which carries the negative charge they go around the uh the nucleus. This is the story of the discovery of nucleus. In the next class we'll discuss about the internal structure of the nucleus. We'll see that the nucleus is composed of neut neutrons and protons and we'll also learn about the stories behind their

00:49:55 discoveries. Thank you.