 Okay, so today we are going to start items chapter, okay, items is the second chapter in modern physics, right and you might have already learned about items in atomic structure in chemistry, right, so this is you can say a repetition of similar concept in physics, right, but then the kind of numericals will be there in chemistry in atomic structure, it will be slightly different from what numericals we have seen in physics, the more emphasis will be given on let us say the physical parameters, velocity, energy, you know momentum, all those things, so there will be slight difference when it comes to the numerical part of it, but when it comes to concept you might have seen this already, Bohr's model for example, you have learned about Bohr's model already, right, so similar thing, okay, okay, so see when scientists all these guys who were researching about what nature is and how nature works, they have this understanding initially that if they understand more and more about universe, the better their understanding will be about the nature, okay, so they started exploring about planets, Kepler has spent around 20 years of his life studying just the planets and then universal law of gravitation came, a lot of telescope were built and they started to understand about you know the planets then about the solar system, about galaxies then they were like okay there are multiple galaxies and could be there is a possibility that multiple universes are there, so there was no end towards you know till where they have to study the behavior, okay, so it completely amazed their understanding about the nature and then they started you know breaking down things and they have hypothesized that if they find out what is the smallest possible thing based on which every structure is built upon then it is just a repetition of same thing again and again in any matter, so if they understand what is the smallest possible unit of a matter then they can extrapolate the property to the bulk matter, fine, so from there on they started to find out that the effort was what is the smallest fundamental unit, okay, so that is how this notion of atoms came in, they thought that okay it is atom that is the smallest possible unit and later on based on few experiments rather forward and everybody else Chadwick discovered neutrons, so it came out that inside the atom also there are small particles, electrons, neutrons, protons, fine, so for a very long time there was an understanding that only electron, neutrons or protons they are the smallest fundamental particles, but recently there was a discovery that you know even the electrons or for the matter proton can also be divided, there are pi bosons and bosons, so there was no end towards the finding of the smallest possible particle also, fine, in fact when there was a discovery of smaller particle they started behaving very weirdly, you cannot understand their behavior using the conventional physics or classical physics, we have learned in the dual nature of radiation and matter how matter behaves, how light will behave, you have to apply some quantum physics over there, right, similarly here also, you know in atoms chapter also since we are dealing with very very small particles they behave in a different manner, they do not follow the usual physics, fine, you have to have a different set of rules and regulation to understand what is going on, fine, so there is a bit of quantum physics in this chapter also, alright and in this chapter although the name of the chapter is atoms, the majority of the time we will be talking about the behavior of electron in the atom and in fact most of the time we will be talking about hydrogen or hydrogen like atoms electron, what is hydrogen like atom, only one electron, helium name, helium plus, lithium 2 plus, if lithium is there knock out 2 of its electron, one electron is left, fine, so in order to study just single electron atom also, there is lot of effort so multi electron atoms to understand that you may have to go for higher, you know higher for auto physics, you may have to take physics for your BSc or NSC or whatever it is, fine, but in our syllabus just ideal like atoms are there which has a single electron, fine, so I think it is the same thing is there in chemistry also, you have atomic structure but most of the time you are talking about the behavior of electron, it is energy in the anionth orbit and things like that, isn't it, fine, so in this chapter also similar thing, okay, now the chapter starts with Rutherford's experiment, fine, Rutherford was the guy who has, you know, conceptualized the experiment, he never conducted the experiment, it was his two students who have conducted the experiment and came up with the observation, fine, the two students name was this, I don't know how to pronounce, how do you pronounce this, how do you pronounce this, any letter can be silent, you never know, fine, so these two students of Rutherford have conducted Rutherford experiment which was conceptualized by Rutherford, fine, so let us see what is that Rutherford experiment is, okay, we will draw its setup and then we will talk about why the setup is like this and what are the findings of this experiment, okay and what does it mean, this is also called Goldfoil experiment, fine, so this is the experimental setup I was talking about, here you can see that there is a source of alpha particle, it is a radioactive source which can emit the radiation, right, it emits alpha radiation and you have lead bricks, what lead bricks will do is there is a alpha particle that goes, deviates little bit, lead bricks will just cancel, it absorbs, they are the good absorber of alpha particle, fine, so only through the gap of the lead bricks alpha particle can move and if you make sure that happens then you will see that the stream of alpha particles will get developed, okay, then you have a very thin Goldfoil, 10 is for minus 8 meter is the thickness, in what you are trying to achieve here is that you want to make very very thin foil so that it is like one atom on top of it, it is stacked, okay, you want to achieve that but of course that is not possible but you want to make it as thin as possible, fine, so why Gold is used, any idea, Gold has very good malleability and ductility, they can be made very very thin shades out of the Gold, okay, that is why, okay and one more reason why Gold is used is that you are bombarding alpha particle, fine, since you are bombarding alpha particle, alpha particle is heavy enough, it is helium nucleus, it has 2 protons and 2 neutrons, fine, it is heavy enough and if you put some lighter element this thing, okay, it is like you know helium is a ping pong ball and if you put an element there in front of it which is malleable and ductile which is greater than alpha particle whose mass but not so great, so it is like striking ping pong ball with a cricket ball, fine, but right now since Gold foil, the atom Gold has lot of mass in it, it has lot of inertia, so now it is like this, you are striking a ping pong ball with a football, fine, so if it happens like this then football will not move, whatever the change in momentum, apparent change in momentum that will happen to the alpha particle only, fine, so that is another reason why Gold foil is used, okay, now when the experiment was conducted, there was no assumption made, they do not know what they are trying to get, okay, they are just trying to validate probably the plumperding model of atom, you remember what is plumperding model, what it is, so it is like a watermelon the atom was hypothesized as a watermelon in which positive and negative charges are distributed, fine, they are distributed everywhere, fine, but they are just trying to see, you know, some, you know, trying to validate what is there, what is proposed, now if this plumperding model is correct, what do you think the observation should be, what do you think it should come back, it should either all the particles will come back or there will be suppose there is a deviation, then alpha particle is charged, right, it has plus 2 charge, now the atom also has charges in it, right, so they will also try to deviate the alpha particle, okay, now which direction they will deviate, there is no preferred direction, they should deviate the alpha particles equally at all angles, fine, whatever happens should be unbiased, right, but what is observed is something which is completely different, what is observed is more than alpha particle just pass through, 99.99% they just pass through, as if it is nothing there, getting it, few particles get reflected and probably one in billion or million they reflect back, fine, this angle theta is scattering angle, fine, now how will you detect how many particles have been deflected and what is number of particles that have reached in the screen, you have suppose a gallon meter over here, if gallon meter catches a charge, gallon meter will show a deflection, fine, so you can just check the current rate at which charge is coming to you, that is the current, fine, you see the current is very high here and here current is very less, okay, so like that you can give a rough estimation of the intensity, a number of, number is proponent of current you can say, getting it, so based on that observation, there was a graph that was plotted, on x axis you have angle of scatter theta, on y axis, number of scattered particles, can you guess how the current is very low, fine, can you guess how the graph will be, if theta is zero, number will be more or less, more, theta is less means it going straight, that are more in number, then theta will tend to 180 degree means what, 180 degree means it is coming back, that will be very less, okay, so you will expect a graph like this and this is what it is experimentally also, okay, any doubt, nothing, now let us try to see what does this experiment mean to us, most of the alpha particle just pass through, as if nothing is there, what does it mean, if I say that the material is nothing but a stack of atoms, as in it is one atomic layer thickness is there, getting it, right now there will be at least hundreds or thousands of atoms in a straight line, fine, so even if there are thousands of atoms in a straight line, not exactly in a straight line, may be in a haphazardly matter, then also it is going straight as if nothing is there, imagine if it is made up of only one atom at a time, that will be more empty, it will be emptier, right, so you can extrapolate this backwards, fine, so your, this thing graph will be even steeper like that, if had it been a monomole, mono atomic, fine, now if you assume that it is made up of atoms only, stack one above the other, of course there should not be any gap between two atoms, otherwise the gold foil will break, isn't it, so you can assume it like this, the atoms are stacked like that, fine, then also the alpha particle is getting enough space to just pass through, what does it mean, it means that the atom is empty, mostly the atom is empty, that is the first observation, immediately that comes in mind that atom is empty, do you know that the percentage of empty space between a nucleus and the first shell of the electron is 10,000 times more than percentage of empty space within earth and sun, that empty space, fine, so alpha particle is able to just pass through as if nothing is there, it means atom is mostly empty, so it immediately rejects plumper in model, fine, then what, see what happens, there is a deflection, why deflection will happen, deflection could happen because of some attraction or repulsion, fine, now suppose it deflects like this, what will come in your mind, it gets attracted or repulsed something, okay, this alpha particle may get repulsed or attracted by some charge, so there is some charge inside the nucleus that is doing it, fine, now you can say that there is attraction, this can happen because of attraction, but this thing coming back at different angles, this cannot happen because of attraction, there has to be some repulsion going on, fine, and one in million or one in billions straight away comes back, as if there is a very heavy particle situated there, once you measure the velocity of this alpha particle coming back, you will see that its velocity is almost same as what it was thrown with, the kinetic energy is almost the same, it means that as if it is hitting a wall and coming back, fine, so if you consider that there is a very very heavy small this thing inside the atom which is positively charged, then it makes sense, you cannot say the entire atom has a positive charge, because mostly it is empty, fine, and there has to be something very heavy that is able to deflect it back, but only one in million is getting deflected, so that means the size is very very small, fine, so this particular experiment has given us a model picture of atom, that the atom has a nucleus at the center which is very very small and very heavy, very heavy does not mean its mass, I am talking about its density, it is very very dense, fine, and one more thing is that atom is neutral, fine, the atom is neutral, so if you say that there is a positively charged at the center, there has to be negatively charged particles, fine, so equal number of positive negative charges are there, but at the center only positive charges are there, and it is very heavy, okay, but till now we have not discovered neutrons, this experiment would not tell anything about neutrons, Chadwick had discovered the neutrons, what he did was he bombarded beryllium atoms, he bombarded beryllium atoms with probably alpha particles only, and some radiation came out, and then he put it under the influence of electric and magnetic field, it was undeflected, as if it has no charge, so it could be photon also, so then he compared the momentum with the wavelength what it should have been, had it been photon but did not match, and whatever was the experiment conducted, it was coming out to be like as if it is a matter wave, it is made up of matter, so and this matter is coming out of atom, inside the atom, and it is not named electron, it is not proton, it is neutral, it must be sitting inside the nucleus, okay, of course it is not only one of the observation that led to this conclusion that it is inside the nucleus, but this is how it developed, fine, so Chadwick has discovered the neutron and from there on, this complete picture of the atom came in place, okay, many times this Rutherford experiment is referred with the name of these two guys, that this experiment something is happening.