Class 11 Chemistry: Exploring Atomic Models

Hey everyone welcome to homeschool and welcome back to class 11 chemistry series. We started with the second chapter atomic structure that is structure of atom. First chapter is already completed video link is provided in the description you can go and watch it out and coming to the second chapter. See in my previous videos I have discussed about the discovery of subatomic particles, discovery of electron, proton, neutron and discovery of charge to mass ratio of electron right and we have seen few questions on that particular aspect and coming to today's topic we are starting with atomic models. See once the subatomic particles are discovered Then it is very much necessary to understand how these particles are arranged. So where are protons present? Where are electrons present? And how is our neutrons located inside the atom? You know this idea is very very important to understand. So these atomic models will help us in giving that idea. I mean to say they will help us in making us understand about the arrangement or the presence or the location of protons neutrons and electrons inside the atom okay so different scientists have come up with different ideas actually for example we will study here you know thomson's model of an atom later we will go for rutherford's model of an atom after that bohr's model of an atom right so like that various atomic models we will study So, the chapter is all about the studying atomic models. Say the first atomic model is Thomson's atomic model. So this atomic model you must have studied in your schooling also, isn't it? Thomson's model of an atom. See here, you know what? We study various atomic models from that we get lot of knowledge and we get lot of ideas like how electrons protons can be present. But finally all these are not accepted models. Thomson's model, Rutherford's model and later you will study Bohr's model. You know these three models are not accepted models. Okay and finally we are going to study quantum's model of an atom and that is widely accepted model. So till date we are accepting quantum's model of an atom. Okay. Say to understand about quantum's model of an atom it's very much essential to understand about these basic models first. Okay. So to understand that real accepted quantum's model this idea is very very important. That's the reason we are studying all these atomic models. okay so initially i'm starting with thompson's model of an atom say this is a very simple model of an atom and thompson says that thompson says that you know what uh the protons are uniformly distributed okay in the atom and first and foremost he says that atom is spherical okay very important point he says atom is spherical Okay, so and you know the radius of an atom will always be around 10 to the power of minus 10 meters. So this is what the size of an atom. Okay, size of an atom is how much? Around 10 to the power of minus 10 meters. So this is actually the radius. This is a value of radius of an atom. Okay, fine. Say he said it is spherical no? So its radius will be of this much and radius itself we call it as size isn't it? Atomic size means what? It is the distance between the center and the outermost shell we say and that distance is exactly equal to the radius. So here radius itself is nothing but size of atom. So he said it is spherical and it has a radius of around 10 to the power of minus 10 meter. And then what he says is in this spherical shape, in this spherical shape, protons, protons are uniformly distributed. Protons are uniformly distributed. Okay, so they are spread all over the spherical shape. So protons are what? Positive charges, right? So protons are nothing but our positive, positive charges. So these positive charges are spread all across the atom. So they are spread all over the space of an atom. Okay, so this is what we mean by protons are uniformly distributed. That means they have spread all across that space, that spherical space, right? And fine. And where are the electrons present? He says that electrons are embedded in this spherical shape here and there. Okay, so he says electrons, electrons are embedded. End. They are embedded in the sense they are present here and there. Right. So just like a watermelon. You know what? Thomson's model of an atom is also called as watermelon model. Watermelon model. Or it is also called as plum pudding model. So why do we call it as watermelon or plum pudding model? If you cut watermelon, how does it look? you will have full red color, right? And black seeds are distributed here and there. So just like that, what is that red color compared to red mass is nothing but positive charges. So positive charges are distributed like that all over the atom, just like the red part of watermelon. And what are those black color seeds? Black color seeds are nothing but your electrons. So here and there, the electrons are embedded, okay? So he compared the black seeds as electrons and he compared that red mass which is there in a watermelon as the positive spheres that means they are our protons so protons are nicely distributed all over the area and here and there electrons are spread that that that's what we call embedded okay so this is how he gave justification uh for the arrangement of protons and uh you know And you know by Thomson's time neutrons were not discovered. Okay so Thomson's model, Rutherford's model, Bohr's model will never talk about the neutrons because neutrons were much recently discovered. They were discovered in 20th, near to the 20th century. Okay so they didn't give any idea about neutrons. So this is what all about the Thomson's model of an atom. and one more thing he told there is there is electrostatic electrostatic force of attraction force of attraction between protons and electrons okay so between protons b means protons and e means electrons okay so it is that electrostatic force holding protons and electrons to stay together isn't it so that's what is main idea about thompson's model of an atom and observe one thing here the mass is distributed uniformly uh in the atom okay say the mass of an atom is due to the mass of each and every part right so that's what we mean by mass is uniformly distributed right so one more point try to note it down mass mass of an atom is uniformly distributed. Okay. So if you want to find out the mass of total atom, you know, it depends on each and every part of atom. It's not that the entire mass is concentrated at one place. No. So that's what we say. The mass of an atom is uniformly distributed. Right. So these were the important points that you have to remember on Thomson's model of an atom. But anyway this is not the accepted model. Say you know if protons and electrons are present the atom cannot be stable for longer time. And he did not actually electrons are movable particles. Say they won't stay in one place actually they keep moving. uh you know throughout the place you know he didn't give justification for the uh movement of electrons right so that's the reason it had lot of limitations but still uh he could say that atom is spherical both positive negative charges are there and they are as a whole the atom is neutral because the number of positive charges is always equal to the negative charges uh you know he gave idea about the neutrality of an atom He says the atom is neutral. Yes, that is acceptable. But the way he said, you know, the protons are distributed all over and here and there electrons are embedded. You know, that's not a good idea, right? So that is the greatest limitation of Thomson's model of an atom. And now let us go for the Rutherford's model of an atom, which is very, very, very important. See guys, coming to Rutherford's model. He did an experiment called alpha ray scattering experiment. Alpha ray scattering experiment. Okay, so this is really a very important experiment which you have been studying right from your schooling, right? So once again, we will try to revise this experiment. And try to remember, I told you in the previous classes also, alpha rays are nothing but helium nuclei. It's nothing but helium. nuclei. So what do you mean by helium nuclei? It is this. So helium will have two electrons and two protons. No, if you remove those two electrons, you will have only two positive charges, which we will represent as He plus 2. So this helium nuclei, which is totally a positive in charge, right? So that helium nuclei particles are called as alpha rays. So one point is very clear. These helium nuclei. that is these alpha particles are positively charged particles okay fine so the experiment is something like this and he took a source of alpha particles say the one a particular element or a particular chemical reaction which can produce alpha particles is what we call the source of alpha particles okay so there are so many you know elements which can emit alpha particles right so such a source was taken initially so this is our source source of alpha particles source of alpha particles okay fine and later you he took a lead plate he took a lead plate And a hole was made onto a lead plate. It's because you know the alpha particle will start coming from this the source isn't it? And you know you must have a you know continuous path for a alpha particle. They should not spread here and there right? So just to make a continuous straight path for a alpha particle they were made to pass through this hole okay? So they were initially made to pass through this hole if this hole was not there you know the alpha particles would move randomly here and there but we don't want that for our experiment we want them to pass it in a straight line you know in a continuous way in an orderly manner so they were made to pass through this hole so it was a lead plate with hole okay so what is this this is our lead plate fine and now he took a gold foil okay so this is our gold foil gold foil is very thin sheet of gold is called as gold foil right so gold foil will have lots of gold atoms right so these alpha particles were made to pass through these gold foil right so something would happen when alpha particle pass through gold atoms right that's what we will discuss right and here they kept zinc sulfide plate is We all know what is the use of zinc sulfide screen or photographic plate, right? Say these alpha particles and all you can't see with your eyes. Say if you want to identify their direction and if you want to identify how they are passing through a gold foil, you know, we need to have a screen so that if the particles touches this screen, wherever the particles touches, there you will observe a glow, right? So wherever the particles are touching, there you will observe a glow so uh by looking at the glow you can identify that yes okay the alpha particles are moving in this direction or they are moving in straight direction or they are moving in this direction okay so definitely this alpha particle will pass through a gold foil and you know what the ruda ford and his you know his friends they have observed glow at a different position they started observing glow at this position and you know most of the rays passed straight that's the reason you know most of the times they have observed glow right at the center of zinc sulfide screen and some have deflected at smaller angles so they have observed glow here and few of them deflected here and you know he he kept a gold foil you know in 360 degrees Say luckily you know here also there is a screen, here also there is a screen. So here also they started observing a glow. That means you know the alpha particles are hitting the gold atoms and maybe they are coming back. They are coming back in the same direction. That's why they started observing glow even at this area. Right. Say you know this is what they observed. Now we will list out the observations. Once they have done this. particular experiment say they took a source of alpha particles alpha particles were coming and they were made to pass through the hole and then they were hitting the gold atoms which are there in a gold foil and they kept a zinc sulfide screen and at various positions the glow was observed it means that the alpha particles have went and hit at various positions on a zinc sulfide screen right So at what positions and how they deviated? Let us note it down in observations. Okay. So here we will learn about observations. So what are the observations we have learned? Say the first one is most of the alpha particles, most of the alpha rays, alpha rays passed, passed straight. passed straight through gold foil through gold foil gold foil has gold atoms right so i'll just write it as gold foil okay passed most of the alpha rays passed straight through the gold foil so how do you know that most of the particles passed straight because most of the glow you know has appeared exactly at the center of zinc sulfide in this area that means the alpha particles went straight Isn't it? So that was the first observation. And second observation is few alpha rays, few alpha rays deflected, deflected. at small angles, deflected at small angles. So, you know, a few of the alpha particles, they didn't go straight. Instead of that, they went and hit here or they went and hit here. That means, say their path is deflected. See, if there was no deviation, they would have gone straight, right? But somehow the path has been disturbed. They have deviated. So deviation means what? You know going in other direction than expected right? Deviate, deviation is called deflection. Clear? So few of the alpha rays have deflected at smaller angles right? This is another observation they have observed. And the third and important observation is very few, very few of Alpha particles. This is very very important point. Very few alpha rays. 1 in 20,000. Very few. Okay. Very less chances. 1 ray in 20,000 rays only. So that many like very few alpha rays completely bounced back. Completely bounced back. Okay. Bounced back. Right, so what do you mean by this? See, they didn't go straight on hitting a gold foil, many of them went straight, few of them, you know, deflected to a smaller angle and very few, one in 20,000 have completely come back. It has hit the gold atoms and it's coming back. Alpha ray is coming completely back. How did they come to know? Backside also, they kept a zinc sulfide screen. So, they have... observed glow every now and then even in the backside. Okay, so that's how they found that very few alpha rays have completely bounced back. So these are the three observations Rutherford observed during alpha ray scattering experiment. Okay, so now looking at these observations, what is the conclusion he has given? So he gave a justification, he gave an answer why many of them went straight, why few of them have deflected in smaller angles and why very few have completely bounced back. For that he has given a reason, that is what we call it as conclusion. So now let us list it out the points under conclusion. See guys to understand the conclusion better just observe this diagram. See if this is a gold foil right so this gold foil would have lot of gold atoms isn't it. So lots of gold atoms could have been there. See many of the rays went straight in the sense you know there was no disturbance for these alpha particles right. Say that means that you know most of the space in the atom. See, this is one gold atom, this is another gold atom, third gold atom, fourth gold atom. See, that means most of the space in the atom is empty, right? Say, most of the rays went straight in the sense there was no disturbance in the atom, right? This is a positively charged particle that is moving through the atom. See, that positively charged particle has not deviated. It went straight in the sense what? Nothing is disturbing it. There is no repulsion. There is no attraction around this particular race, right? So most of the race went like that. That means that most of the space in the atom is empty. So this point is confirmed by this particular observation. Okay, so what is the first conclusion that I can write? Most of the race went straight. This point confirms that most of the space is empty. in atom space in atom is empty okay so this is really very very important point and then what is the second observation few alpha rays deflected at smaller angles that means maybe somewhere at the center very small nucleus might have been there we all know nucleus is a positively charged nucleus is positively charged So the rays whichever were slightly near to the nucleus must have deflected right must have deflected. See the ray which is slightly moving near to the nucleus must have deflected right. So whichever the rays they were very much near to the nucleus. Why it will deflect? Because rays were having positively charged particles. They are helium nuclei positive charge. And nucleus is also positive charge since positive positive charge when they come near repulsion right repulsion occurs. So that's the reason the rays instead of going in a straight direction they deflected to a smaller angles. Okay so it means that say why most of the rays have sorry few of the rays have deflected. Say few rays deflected because few rays deflected. Deflected in small angles. Yes or no? Small angles because these rays were going much near to nucleus. Okay? To nucleus. Since nucleus. is positively charged it's having a positive charge okay so repulsion repulsion occurs occurs between the rays and nucleus so the rays must have deflected in smaller angles so this is what the conclusion that is given by observation of this particular point okay so now very very important point see very few rays you 1 in 20,000 rays completely bounced back. That means these rays, the rays which have directly hit the nucleus, okay, must have completely come back. Okay, so the rays which have directly hit, say these rays, they were just near to the nucleus, so they deflected. But the one which is straight away hitting, you know, the straight head-to-head accident if it occurs, you know, it's a very, very strong accident, isn't it? Okay. So when a positively charged particle completely hitting straight onto a positively charged particle, then definitely the repulsion would be maximum. So the ray is completely bouncing back. So here, what is the point I can write here? Very few rays, very few rays bounced back, bounced back. is because is because the rays the rays have directly directly hit the nucleus directly hit the nucleus that's the reason it is completely bouncing back Okay, so this is all about the different conclusions given by observing these things. And now let us go for the postulates of Rutherford's theory. Like by observing these conclusions, what are the different points he told? What are the different, you know, postulates he told about an atom is what I will give you next. See you guys. After observing the observations and conclusions, the first point he tells about an atom is, you know, as he said that most of the space in atom is empty. Space in atom is empty. Okay, so atom is something very much spherical. Atom is very much spherical, but most of the space is empty. And you will have... a nucleus nucleus is located at center located at center and is okay this nucleus is very small it is very very small okay right the nucleus is very very very small compared to the size of an atom you know what You know J.J. Thompson already have given an idea about size of an atom, right? He said that size of an atom is around 10 to the power of minus 10 meter. Whereas the size of nucleus if I talk about it is 10 to the power of minus 15 meter. You see the difference? See the nucleus size is very small. Say one comparison I can give you. Imagine you have a big cricket stadium. Cricket stadium is very much big. It has a radius of around 5 km. Big cricket stadium from center wherever you go it takes 5 km. 5 km distance is there. The radius of that stadium is 5 km. And if you keep a small ball at the center. You compare the size of a ball with size of a stadium. right? So that is what the analogy I can bring here. See the ball is nothing but the nucleus and the entire thing is your the cricket stadium is your atom. Okay. See ball is there exactly at the center of a stadium, cricket stadium. Okay. See here the ball is like a nucleus and that stadium is atom. You can just compare how small the nucleus is compared to the size of an atom, right? you have to visualize you have to bring that imagination right say uh when in a large cricket stadium small ball at the center so ball is your nucleus that stadium is your atom so that is what the best analogy i can uh bring in front of you just to uh make you understand how small the nucleus is compared to the size of an atom okay fine and uh you know most of the mass you Most of the mass of an atom is concentrated at the center. Concentrated at the center. So that means the entire mass of an atom is due to the nucleus. So the nucleus mass itself is the mass of an atom. So whatever the mass the nucleus has, that itself is the mass of an atom. That's what this point means. And now second point he talks about the electrons and he says that electrons will revolve around the nucleus. So the electrons always revolve around the nucleus. He says electrons revolve. Around nucleus. Around nucleus. Okay. So this is another point he says about an atom. And he also tells that electrons revolve in circular orbits. They revolve in circular orbits. Okay. So this is another point. And the last one is both electrons. Electrons and nucleus. are held by electrostatic force of attraction. Electrostatic force of attraction. So what is electrostatic force of attraction by the way? It's nothing but an attractive force that is there between positive and negative things. If this is a positive, this is a negative, you know there is a certain level of attraction between positive and negative. Such an attraction is called electrostatic force of attraction. Since electrons are negatively charged, nucleus is positively charged, right? Protons are present inside the nucleus. So nucleus is positively charged. Between electrons and nucleus, you have that positive negative attraction. That attraction is what we call electrostatic force of attraction. Okay. So these are the various postulates given by Rutherford. So very... So, you know, we got an idea about an atom now. It's nothing. It's spherical in shape. Nucleus is very small, located at the center. And around the nucleus, electrons revolve in a circular path. Okay. So, electrons revolve in a circular path, he said. Right. So, this is all about an atom. And he compared his model of an atom with solar system. See. All the planets will revolve around sun, right? Sun is at the center and all planets revolve around the sun. So just like that around the nucleus, electrons. Electrons were compared to the planets, okay? Nucleus is compared to the sun. So electrons in a circular path, they revolve around the nucleus. So that was the things told by Rutherford. But still there is a limitation in Rutherford's model of an atom. Let us discuss about the drawbacks or limitations of Rutherford's model. See guys, you must understand one important limitation. Actually, there are many drawbacks for Rutherford's model. So the major limitation is what he told was this was a nucleus and electron revolves in a circular path. It revolves continuously around the nucleus, he said. Say electron is a charged body. So. And whenever a charged body continuously moves with a certain speed, it will lose energy. It will lose energy. Okay. So when this electron continuously revolves around the nucleus with certain speed, slowly, slowly it will lose energy, loses energy and it may fall on a nucleus. So that particular electron may come and fall on a nucleus. So when electron falls on a nucleus, the atom collapses. the atom dies okay so this is the major drawback of the ruda ford's model okay so what can we write here when charged body charged body so which is a charged body here electron okay when charged body continuously continuously moves with certain speed I'm writing all these in a very simple words so that you can able to understand. Okay. So continuously moves with a certain speed. It loses energy. It loses energy. And finally, finally falls on nucleus. Falls on nucleus. Okay. So this leads to. This leads to collapsing of atom. This leads to collapsing of atom. Right. So this is the major limitation one must understand. The other one is he failed to arrange the electrons. Say in one atom you will have many electrons. Right. Say if you take a sodium you will have 11 electrons. So how these 11 electrons are arranged? You know. that's where he failed clear so he could not explain could not explain about about electronic structure of an atom electronic we call that as electronic structure electronic structure means what arrangement of electrons when many electrons are there how those many electrons you are arranged is what we call arrangement of electrons. Okay, so these are the two major limitations. And he could not tell how the atoms combine with each other to form various molecules or compounds. You know, if we go on listing, many limitations are there. But these are the two important ones one must remember. So that's all about Rutherford's model of an atom. So in today's class, we discussed Thomson's model, limitations. Rutherford's model, limitations. So next atomic model is Bohr's model of an atom. But! Before going to the Bohr's model, we have to study few concepts because it is based on those concepts Bohr has given his model. So we also have to study those concepts. They are studied under the heading developments towards Bohr's model. So first we will talk about the developments towards Bohr's model and then we will talk about the postulates of Bohr's model. So this much. is about today's class hope you enjoyed the class if you have enjoyed the class just post a comment in the comment section so that i feel more satisfied i can come up with more and more things for you uh so we'll meet up in the next video till then take care all the best

See in my previous videos I have discussed about the discovery of subatomic particles, discovery of electron, proton, neutron and discovery of charge to mass ratio of electron right and we have seen few questions on that particular aspect and coming to today's topic we are starting with atomic models. See once the subatomic particles are discovered Then it is very much necessary to understand how these particles are arranged. So where are protons present?

Where are electrons present? And how is our neutrons located inside the atom? You know this idea is very very important to understand.

So these atomic models will help us in giving that idea. I mean to say they will help us in making us understand about the arrangement or the presence or the location of protons neutrons and electrons inside the atom okay so different scientists have come up with different ideas actually for example we will study here you know thomson's model of an atom later we will go for rutherford's model of an atom after that bohr's model of an atom right so like that various atomic models we will study So, the chapter is all about the studying atomic models. Say the first atomic model is Thomson's atomic model. So this atomic model you must have studied in your schooling also, isn't it? Thomson's model of an atom.

See here, you know what? We study various atomic models from that we get lot of knowledge and we get lot of ideas like how electrons protons can be present. But finally all these are not accepted models.

Thomson's model, Rutherford's model and later you will study Bohr's model. You know these three models are not accepted models. Okay and finally we are going to study quantum's model of an atom and that is widely accepted model. So till date we are accepting quantum's model of an atom. Okay.

Say to understand about quantum's model of an atom it's very much essential to understand about these basic models first. Okay. So to understand that real accepted quantum's model this idea is very very important. That's the reason we are studying all these atomic models.

okay so initially i'm starting with thompson's model of an atom say this is a very simple model of an atom and thompson says that thompson says that you know what uh the protons are uniformly distributed okay in the atom and first and foremost he says that atom is spherical okay very important point he says atom is spherical Okay, so and you know the radius of an atom will always be around 10 to the power of minus 10 meters. So this is what the size of an atom. Okay, size of an atom is how much? Around 10 to the power of minus 10 meters. So this is actually the radius.

This is a value of radius of an atom. Okay, fine. Say he said it is spherical no? So its radius will be of this much and radius itself we call it as size isn't it?

Atomic size means what? It is the distance between the center and the outermost shell we say and that distance is exactly equal to the radius. So here radius itself is nothing but size of atom.

So he said it is spherical and it has a radius of around 10 to the power of minus 10 meter. And then what he says is in this spherical shape, in this spherical shape, protons, protons are uniformly distributed. Protons are uniformly distributed.

Okay, so they are spread all over the spherical shape. So protons are what? Positive charges, right?

So protons are nothing but our positive, positive charges. So these positive charges are spread all across the atom. So they are spread all over the space of an atom. Okay, so this is what we mean by protons are uniformly distributed. That means they have spread all across that space, that spherical space, right?

And fine. And where are the electrons present? He says that electrons are embedded in this spherical shape here and there. Okay, so he says electrons, electrons are embedded.

End. They are embedded in the sense they are present here and there. Right.

So just like a watermelon. You know what? Thomson's model of an atom is also called as watermelon model. Watermelon model. Or it is also called as plum pudding model.

So why do we call it as watermelon or plum pudding model? If you cut watermelon, how does it look? you will have full red color, right?

And black seeds are distributed here and there. So just like that, what is that red color compared to red mass is nothing but positive charges. So positive charges are distributed like that all over the atom, just like the red part of watermelon.

And what are those black color seeds? Black color seeds are nothing but your electrons. So here and there, the electrons are embedded, okay?

So he compared the black seeds as electrons and he compared that red mass which is there in a watermelon as the positive spheres that means they are our protons so protons are nicely distributed all over the area and here and there electrons are spread that that that's what we call embedded okay so this is how he gave justification uh for the arrangement of protons and uh you know And you know by Thomson's time neutrons were not discovered. Okay so Thomson's model, Rutherford's model, Bohr's model will never talk about the neutrons because neutrons were much recently discovered. They were discovered in 20th, near to the 20th century. Okay so they didn't give any idea about neutrons. So this is what all about the Thomson's model of an atom.

and one more thing he told there is there is electrostatic electrostatic force of attraction force of attraction between protons and electrons okay so between protons b means protons and e means electrons okay so it is that electrostatic force holding protons and electrons to stay together isn't it so that's what is main idea about thompson's model of an atom and observe one thing here the mass is distributed uniformly uh in the atom okay say the mass of an atom is due to the mass of each and every part right so that's what we mean by mass is uniformly distributed right so one more point try to note it down mass mass of an atom is uniformly distributed. Okay. So if you want to find out the mass of total atom, you know, it depends on each and every part of atom.

It's not that the entire mass is concentrated at one place. No. So that's what we say. The mass of an atom is uniformly distributed.

Right. So these were the important points that you have to remember on Thomson's model of an atom. But anyway this is not the accepted model. Say you know if protons and electrons are present the atom cannot be stable for longer time.

And he did not actually electrons are movable particles. Say they won't stay in one place actually they keep moving. uh you know throughout the place you know he didn't give justification for the uh movement of electrons right so that's the reason it had lot of limitations but still uh he could say that atom is spherical both positive negative charges are there and they are as a whole the atom is neutral because the number of positive charges is always equal to the negative charges uh you know he gave idea about the neutrality of an atom He says the atom is neutral.

Yes, that is acceptable. But the way he said, you know, the protons are distributed all over and here and there electrons are embedded. You know, that's not a good idea, right? So that is the greatest limitation of Thomson's model of an atom.

And now let us go for the Rutherford's model of an atom, which is very, very, very important. See guys, coming to Rutherford's model. He did an experiment called alpha ray scattering experiment.

Alpha ray scattering experiment. Okay, so this is really a very important experiment which you have been studying right from your schooling, right? So once again, we will try to revise this experiment. And try to remember, I told you in the previous classes also, alpha rays are nothing but helium nuclei.

It's nothing but helium. nuclei. So what do you mean by helium nuclei? It is this.

So helium will have two electrons and two protons. No, if you remove those two electrons, you will have only two positive charges, which we will represent as He plus 2. So this helium nuclei, which is totally a positive in charge, right? So that helium nuclei particles are called as alpha rays.

So one point is very clear. These helium nuclei. that is these alpha particles are positively charged particles okay fine so the experiment is something like this and he took a source of alpha particles say the one a particular element or a particular chemical reaction which can produce alpha particles is what we call the source of alpha particles okay so there are so many you know elements which can emit alpha particles right so such a source was taken initially so this is our source source of alpha particles source of alpha particles okay fine and later you he took a lead plate he took a lead plate And a hole was made onto a lead plate. It's because you know the alpha particle will start coming from this the source isn't it?

And you know you must have a you know continuous path for a alpha particle. They should not spread here and there right? So just to make a continuous straight path for a alpha particle they were made to pass through this hole okay?

So they were initially made to pass through this hole if this hole was not there you know the alpha particles would move randomly here and there but we don't want that for our experiment we want them to pass it in a straight line you know in a continuous way in an orderly manner so they were made to pass through this hole so it was a lead plate with hole okay so what is this this is our lead plate fine and now he took a gold foil okay so this is our gold foil gold foil is very thin sheet of gold is called as gold foil right so gold foil will have lots of gold atoms right so these alpha particles were made to pass through these gold foil right so something would happen when alpha particle pass through gold atoms right that's what we will discuss right and here they kept zinc sulfide plate is We all know what is the use of zinc sulfide screen or photographic plate, right? Say these alpha particles and all you can't see with your eyes. Say if you want to identify their direction and if you want to identify how they are passing through a gold foil, you know, we need to have a screen so that if the particles touches this screen, wherever the particles touches, there you will observe a glow, right? So wherever the particles are touching, there you will observe a glow so uh by looking at the glow you can identify that yes okay the alpha particles are moving in this direction or they are moving in straight direction or they are moving in this direction okay so definitely this alpha particle will pass through a gold foil and you know what the ruda ford and his you know his friends they have observed glow at a different position they started observing glow at this position and you know most of the rays passed straight that's the reason you know most of the times they have observed glow right at the center of zinc sulfide screen and some have deflected at smaller angles so they have observed glow here and few of them deflected here and you know he he kept a gold foil you know in 360 degrees Say luckily you know here also there is a screen, here also there is a screen. So here also they started observing a glow.

That means you know the alpha particles are hitting the gold atoms and maybe they are coming back. They are coming back in the same direction. That's why they started observing glow even at this area.

Right. Say you know this is what they observed. Now we will list out the observations. Once they have done this.

particular experiment say they took a source of alpha particles alpha particles were coming and they were made to pass through the hole and then they were hitting the gold atoms which are there in a gold foil and they kept a zinc sulfide screen and at various positions the glow was observed it means that the alpha particles have went and hit at various positions on a zinc sulfide screen right So at what positions and how they deviated? Let us note it down in observations. Okay. So here we will learn about observations. So what are the observations we have learned?

Say the first one is most of the alpha particles, most of the alpha rays, alpha rays passed, passed straight. passed straight through gold foil through gold foil gold foil has gold atoms right so i'll just write it as gold foil okay passed most of the alpha rays passed straight through the gold foil so how do you know that most of the particles passed straight because most of the glow you know has appeared exactly at the center of zinc sulfide in this area that means the alpha particles went straight Isn't it? So that was the first observation.

And second observation is few alpha rays, few alpha rays deflected, deflected. at small angles, deflected at small angles. So, you know, a few of the alpha particles, they didn't go straight.

Instead of that, they went and hit here or they went and hit here. That means, say their path is deflected. See, if there was no deviation, they would have gone straight, right? But somehow the path has been disturbed.

They have deviated. So deviation means what? You know going in other direction than expected right? Deviate, deviation is called deflection.

Clear? So few of the alpha rays have deflected at smaller angles right? This is another observation they have observed.

And the third and important observation is very few, very few of Alpha particles. This is very very important point. Very few alpha rays. 1 in 20,000. Very few.

Okay. Very less chances. 1 ray in 20,000 rays only. So that many like very few alpha rays completely bounced back. Completely bounced back.

Okay. Bounced back. Right, so what do you mean by this? See, they didn't go straight on hitting a gold foil, many of them went straight, few of them, you know, deflected to a smaller angle and very few, one in 20,000 have completely come back. It has hit the gold atoms and it's coming back.

Alpha ray is coming completely back. How did they come to know? Backside also, they kept a zinc sulfide screen.

So, they have... observed glow every now and then even in the backside. Okay, so that's how they found that very few alpha rays have completely bounced back.

So these are the three observations Rutherford observed during alpha ray scattering experiment. Okay, so now looking at these observations, what is the conclusion he has given? So he gave a justification, he gave an answer why many of them went straight, why few of them have deflected in smaller angles and why very few have completely bounced back. For that he has given a reason, that is what we call it as conclusion. So now let us list it out the points under conclusion.

See guys to understand the conclusion better just observe this diagram. See if this is a gold foil right so this gold foil would have lot of gold atoms isn't it. So lots of gold atoms could have been there. See many of the rays went straight in the sense you know there was no disturbance for these alpha particles right. Say that means that you know most of the space in the atom.

See, this is one gold atom, this is another gold atom, third gold atom, fourth gold atom. See, that means most of the space in the atom is empty, right? Say, most of the rays went straight in the sense there was no disturbance in the atom, right?

This is a positively charged particle that is moving through the atom. See, that positively charged particle has not deviated. It went straight in the sense what?

Nothing is disturbing it. There is no repulsion. There is no attraction around this particular race, right?

So most of the race went like that. That means that most of the space in the atom is empty. So this point is confirmed by this particular observation.

Okay, so what is the first conclusion that I can write? Most of the race went straight. This point confirms that most of the space is empty.

in atom space in atom is empty okay so this is really very very important point and then what is the second observation few alpha rays deflected at smaller angles that means maybe somewhere at the center very small nucleus might have been there we all know nucleus is a positively charged nucleus is positively charged So the rays whichever were slightly near to the nucleus must have deflected right must have deflected. See the ray which is slightly moving near to the nucleus must have deflected right. So whichever the rays they were very much near to the nucleus.

Why it will deflect? Because rays were having positively charged particles. They are helium nuclei positive charge. And nucleus is also positive charge since positive positive charge when they come near repulsion right repulsion occurs.

So that's the reason the rays instead of going in a straight direction they deflected to a smaller angles. Okay so it means that say why most of the rays have sorry few of the rays have deflected. Say few rays deflected because few rays deflected. Deflected in small angles.

Yes or no? Small angles because these rays were going much near to nucleus. Okay? To nucleus.

Since nucleus. is positively charged it's having a positive charge okay so repulsion repulsion occurs occurs between the rays and nucleus so the rays must have deflected in smaller angles so this is what the conclusion that is given by observation of this particular point okay so now very very important point see very few rays you 1 in 20,000 rays completely bounced back. That means these rays, the rays which have directly hit the nucleus, okay, must have completely come back. Okay, so the rays which have directly hit, say these rays, they were just near to the nucleus, so they deflected. But the one which is straight away hitting, you know, the straight head-to-head accident if it occurs, you know, it's a very, very strong accident, isn't it?

Okay. So when a positively charged particle completely hitting straight onto a positively charged particle, then definitely the repulsion would be maximum. So the ray is completely bouncing back.

So here, what is the point I can write here? Very few rays, very few rays bounced back, bounced back. is because is because the rays the rays have directly directly hit the nucleus directly hit the nucleus that's the reason it is completely bouncing back Okay, so this is all about the different conclusions given by observing these things. And now let us go for the postulates of Rutherford's theory.

Like by observing these conclusions, what are the different points he told? What are the different, you know, postulates he told about an atom is what I will give you next. See you guys. After observing the observations and conclusions, the first point he tells about an atom is, you know, as he said that most of the space in atom is empty.

Space in atom is empty. Okay, so atom is something very much spherical. Atom is very much spherical, but most of the space is empty.

And you will have... a nucleus nucleus is located at center located at center and is okay this nucleus is very small it is very very small okay right the nucleus is very very very small compared to the size of an atom you know what You know J.J. Thompson already have given an idea about size of an atom, right?

He said that size of an atom is around 10 to the power of minus 10 meter. Whereas the size of nucleus if I talk about it is 10 to the power of minus 15 meter. You see the difference?

See the nucleus size is very small. Say one comparison I can give you. Imagine you have a big cricket stadium.

Cricket stadium is very much big. It has a radius of around 5 km. Big cricket stadium from center wherever you go it takes 5 km. 5 km distance is there. The radius of that stadium is 5 km.

And if you keep a small ball at the center. You compare the size of a ball with size of a stadium. right? So that is what the analogy I can bring here.

See the ball is nothing but the nucleus and the entire thing is your the cricket stadium is your atom. Okay. See ball is there exactly at the center of a stadium, cricket stadium.

Okay. See here the ball is like a nucleus and that stadium is atom. You can just compare how small the nucleus is compared to the size of an atom, right?

you have to visualize you have to bring that imagination right say uh when in a large cricket stadium small ball at the center so ball is your nucleus that stadium is your atom so that is what the best analogy i can uh bring in front of you just to uh make you understand how small the nucleus is compared to the size of an atom okay fine and uh you know most of the mass you Most of the mass of an atom is concentrated at the center. Concentrated at the center. So that means the entire mass of an atom is due to the nucleus. So the nucleus mass itself is the mass of an atom. So whatever the mass the nucleus has, that itself is the mass of an atom.

That's what this point means. And now second point he talks about the electrons and he says that electrons will revolve around the nucleus. So the electrons always revolve around the nucleus.

He says electrons revolve. Around nucleus. Around nucleus.

Okay. So this is another point he says about an atom. And he also tells that electrons revolve in circular orbits. They revolve in circular orbits.

Okay. So this is another point. And the last one is both electrons. Electrons and nucleus. are held by electrostatic force of attraction.

Electrostatic force of attraction. So what is electrostatic force of attraction by the way? It's nothing but an attractive force that is there between positive and negative things. If this is a positive, this is a negative, you know there is a certain level of attraction between positive and negative.

Such an attraction is called electrostatic force of attraction. Since electrons are negatively charged, nucleus is positively charged, right? Protons are present inside the nucleus. So nucleus is positively charged.

Between electrons and nucleus, you have that positive negative attraction. That attraction is what we call electrostatic force of attraction. Okay.

So these are the various postulates given by Rutherford. So very... So, you know, we got an idea about an atom now.

It's nothing. It's spherical in shape. Nucleus is very small, located at the center.

And around the nucleus, electrons revolve in a circular path. Okay. So, electrons revolve in a circular path, he said. Right. So, this is all about an atom.

And he compared his model of an atom with solar system. See. All the planets will revolve around sun, right? Sun is at the center and all planets revolve around the sun.

So just like that around the nucleus, electrons. Electrons were compared to the planets, okay? Nucleus is compared to the sun. So electrons in a circular path, they revolve around the nucleus. So that was the things told by Rutherford.

But still there is a limitation in Rutherford's model of an atom. Let us discuss about the drawbacks or limitations of Rutherford's model. See guys, you must understand one important limitation.

Actually, there are many drawbacks for Rutherford's model. So the major limitation is what he told was this was a nucleus and electron revolves in a circular path. It revolves continuously around the nucleus, he said.

Say electron is a charged body. So. And whenever a charged body continuously moves with a certain speed, it will lose energy.

It will lose energy. Okay. So when this electron continuously revolves around the nucleus with certain speed, slowly, slowly it will lose energy, loses energy and it may fall on a nucleus.

So that particular electron may come and fall on a nucleus. So when electron falls on a nucleus, the atom collapses. the atom dies okay so this is the major drawback of the ruda ford's model okay so what can we write here when charged body charged body so which is a charged body here electron okay when charged body continuously continuously moves with certain speed I'm writing all these in a very simple words so that you can able to understand. Okay.

So continuously moves with a certain speed. It loses energy. It loses energy. And finally, finally falls on nucleus.

Falls on nucleus. Okay. So this leads to. This leads to collapsing of atom. This leads to collapsing of atom.

Right. So this is the major limitation one must understand. The other one is he failed to arrange the electrons. Say in one atom you will have many electrons.

Right. Say if you take a sodium you will have 11 electrons. So how these 11 electrons are arranged? You know.

that's where he failed clear so he could not explain could not explain about about electronic structure of an atom electronic we call that as electronic structure electronic structure means what arrangement of electrons when many electrons are there how those many electrons you are arranged is what we call arrangement of electrons. Okay, so these are the two major limitations. And he could not tell how the atoms combine with each other to form various molecules or compounds.

You know, if we go on listing, many limitations are there. But these are the two important ones one must remember. So that's all about Rutherford's model of an atom. So in today's class, we discussed Thomson's model, limitations. Rutherford's model, limitations.

So next atomic model is Bohr's model of an atom. But! Before going to the Bohr's model, we have to study few concepts because it is based on those concepts Bohr has given his model.

So we also have to study those concepts. They are studied under the heading developments towards Bohr's model. So first we will talk about the developments towards Bohr's model and then we will talk about the postulates of Bohr's model. So this much. is about today's class hope you enjoyed the class if you have enjoyed the class just post a comment in the comment section so that i feel more satisfied i can come up with more and more things for you uh so we'll meet up in the next video till then take care all the best

Transcript for:Class 11 Chemistry: Exploring Atomic Models

Transcript for:
Class 11 Chemistry: Exploring Atomic Models