 So, now in this session we are going to discuss about atomic models like we have already discussed in the last class. So far we have discussed about the various subatomic particles. Now the task is how this subatomic particles are arranged within an atom. So, for this discussion or arrangement of various subatomic particles, various different theory were given by the scientists in that era, all those theory we call it as the models of an atom. So, we are going to understand next what are the various atomic models, what are the various atomic models. The first model we are going to discuss here is Thomson model, Thomson model, Thomson model. This model is given by the scientist called J.J. Thomson and according to this model atoms are assumed to be a positively charged solid sphere. Atoms are assumed to be a positively charged solid sphere within which this is suppose an atom we have positively charged solid sphere within which the electrons are embedded or placed within which the electrons are embedded or placed in such a way so that they experience minimum repulsion. So, all these are electrons actually electrons electrons electrons. So, according to Thomson model atoms are assumed to be a positively charged solid sphere within which the electrons are embedded in such a way so that they experience minimum repulsion. This model we also call it as watermelon model because watermelon is something is some similar to this only like the seeds are embedded in this manner only within the watermelon. So, that is why we call the watermelon model. Plum pudding model we also call it as watermelon, plum pudding model all these names are same, but this theory or this model was discarded later on because this particular theory was not proved to be correct with the various experiments like gold foil experiment or alpha particle scattering experiment. So, later on this model were discarded because it does not stand true with the various investigations like alpha particle scattering experiment or we also call it as gold foil experiment. So, what is alpha particle scattering experiment? What is gold foil experiment? That will see. So, this is done by all these experiment done by Rutherford right. So, and we also call it as Rutherford nuclear model Rutherford nuclear model Rutherford nuclear model Rutherford also like Thomson Rutherford also given his own atomic model right and this atomic model was based on an experiment which we call it as gold foil experiment. Gold foil experiment this experiment is also known as alpha particle, alpha particle scattering experiment, gold foil experiment and we also call it as alpha particle scattering experiment both are same thing. On the basis of this experiment only Rutherford has given its own model which we call it as Rutherford nuclear model and first of all we understand what is this experiment is all about and then we will see why this model was discarded. So, Rutherford experiment model or this gold foil experiment alpha particle scattering experiment is what we take a very thin gold foil here. The thicknesses of this gold foil is very thin it is around 0.0004 centimeter very thin gold foil right. This side will have an screen which is coated with zinc sulphide fluorescent screen this is ZNS screen which is fluorescent in nature. Here we have a source of alpha particle alpha particle. Now when you strikes with alpha particle on this gold foil what happens you see what happens when this gold foil when this light or alpha particle strikes at gold foil few of them alpha particle deviated by very small angle you can see this one deviated with very small angle or this one deviated with very small angle most of the alpha particle passes through the gold foil without any deviation without any deflection this is the second observation. First observation is what few of them deviated with a very small angle like this most of the alpha particle second observation is most of the alpha particle passes through the gold foil without any deviation. Third experiment third observation is what a very few of them retraces their path strikes at some point and then it comes back on the same path okay these are the three observation okay. So now the observation will write down the first point most of the alpha particle passes through without any deviation second one passes through without any deviation second one a few of them a few of them deviated deviates with a very small angle very small angle a very few of them of them retraces their path. This is the three observation we have right on the basis of this observation Rutherford given his own conclusion about the atomic atoms what is the possibility of atoms arrangement of subatomic particles within an atom and what is that conclusion the conclusion of this experiment is since the first point most of the alpha particle passes without any deviation it means what the maximum space within an atom is vacant empty maximum space within an atom is second point there are some positive charged particle positive charged particles present why because alpha particle is nothing but positively charged so when it strikes with the positive charge then only deflect okay third and the very most important point here the mass of an atom is concentrated in a very small region and when the alpha particle strikes at this region then it retraces their path it comes back on its own path right. So what we say mass of of an atom is concentrated in a very small region called this region is called nucleus okay this region is called nucleus so this is the very important point we have of this experiment we discovered about nucleus we come to know about this point that within an atom the mass is concentrated in a very small region and that we call as nucleus okay. So this third point is the very important point we have since the discovery of nucleus takes place from this experiment only that is why the model of Rutherford call it as Rutherford nuclear model right now on the basis of all these conclusion Rutherford proposed his own model okay so what is that model that we are going to see now based on gold foil experiment okay. Now the first point in this Rutherford nuclear model the first point what he suggest that electron revolves around the nucleus like like the planet revolves around the sun second point is what the electron revolves in a circular path revolves in a circular path third point is electron revolves in a circular path third point is the mass of an atom mass of an atom concentrated in a very small region called nucleus fourth point the force of attraction force of attraction between the nucleus force of attraction between the nucleus and the electrons balances balances the centrifugal force centrifugal force which acts on electrons right these are the main postulates of Rutherford nuclear model few points were correct like about the nucleus this point was correct this point circular path was wrong why it is wrong we will see that later okay but this is the few points given by the Rutherford which we call it as Rutherford nuclear model now what is centrifugal force of centrifugal force we have any object if it is moving in a circular path right any object if moving in a circular path so to be in this path there must be some force which keeps this object in this particular path right the object the force requires for an object to keep this object in circular path we call it as centripetal force centripetal force centripetal force is a force required for an object to move there to you know be in the circular motion right and this force is given by f is equals to m v square by r where v is the velocity of the object and r is the radius of the circular path suppose if this is r right the velocity of this object is v so we can write the force applied on this object if it is moving in the circular path is equals to m v square by r this force is necessary or required for this object to be in this to be on the circular path if the force is not equal to this then it won't be in on the same path it has to change its path right so this is centripetal force not every point you see where wherever the object is every point you see there is a tendency for this object to move out of the circular path like this it is going like this it is going so this force which is acting outward right on an object right it is also in hypothetical force this force which is acting outwards on an object which is in circular motion right this force we call it as centri-fugal force so it acts opposite to the direction of centripetal force centripetal force acts towards the radius along towards the center along the radius right but centri-fugal force always acts in the opposite direction outwards this force is outwards hypothetical force okay so when the electron coming back to this point when the electron moves in a circular path this force is acting outward so the electrostatic force of attraction between the nucleus and the electron which is this force in this direction balance this centrifugal force so that the electron revolves in a circular path okay that is what this point is okay but again this model was also discarded right because few of the you know the postulates were not correct with the electromagnetic theory okay what is that what is the drawback of Rutherford model that we see and why we discarded it that we will also see in the next session okay and then we will move to the very important Bohr's atomic model but to understand Bohr's atomic model there are few properties we have to understand regarding a matter a matter that is you know the wave nature or the particle nature so after this drawback we will discuss about the two the dual nature of light wave nature and particle nature and then we will move on to the Bohr's atomic model okay so this is it for today see you tomorrow take care bye bye