 Good morning friends, so today we are going to study about structure of atom, so structure of atom as the name suggest in this chapter we are going to study about what is the structure of an atom, how the atom has been built and what are the various subatomic particles we have. So the first of all what is an atom, what is an atom, so atom is the word taken from a Greek word that is atomios, the meaning of this Greek word is indivisible, which means we cannot divide it further right, so the atom the definition is what, the definition the initial definition was what it is the smallest particle which cannot divide further that is the definition of atom, but this definition was proved to be wrong when various subatomic particles like electron, proton, neutron discovered. So that is why the definition of atom was wrong because if it is a smallest particle then how can other particles like other subatomic particles like electron, proton, neutron present within an atom, so that is the wrong definition we had initially and the definition of atom is also given if you see or if you study in NCRT it is given that it is a smallest particle which takes part in a chemical reaction, so chemical reaction when I am saying we are not considering here redox reaction, we are considering here the reaction in which the atoms are involved, so atoms are also the smallest particle which takes part in the chemical reaction. So regarding atom there was a scientist called John Dalton, he has given it is own atomic theory and that we call it as Dalton atomic theory, so what is Dalton atomic theory what is the main point of Dalton atomic theory we have that we will see first. The first thing is Dalton's atomic theory, the first point of this theory is what all the atoms or all matters in the universe are made up of very small particle called first point of Dalton's atomic theory. Second one atoms cannot be created or destroyed, the properties of atoms of different element different elements are different and for same elements are same that is the third point suppose we have oxygen atom, so the property of oxygen atom will be different and the property of nitrogen atom will be different, but all the atoms of oxygen will have the same property means what suppose you have to understand or you have to study about the property of oxygen molecule, then what we will do we will read out or we will study the property of oxygen atom because oxygen molecule is made up of oxygen atom only, so whatever the property of oxygen atom that will be the property of oxygen molecule more or less there would not be much change into that, so this is the theory of given by Dalton actually, so few points in this was wrong actually like this one atoms cannot be created or destroyed, we can actually break an atom into smaller particles also and this theory this point was proved to be wrong when the various subatomic particles discovered, so the next point we are going to understand here is the discovery of subatomic particles, discovery of subatomic particles, so first point or the first subatomic particles we are going to study here is electron, how the electrons are discovered, so the experiment for the discovery of electron that we will discuss here, discovery of electron will discuss in detail similar kind of experiment we have for the discovery of proton, so that will discuss and the theory is given in any physical chemistry book if you see atomic structure you can go through with that theory you will understand, you can understand after watching this video, so for the discovery of electron the experimental setup was something like this, this is the experimental setup for discovery of electron and this experiment we call it as cathode ray experiment, cathode ray experiment okay, so what happens into this first of all we have a discharge tube this is this cylindrical tube is we call it as discharge tube, discharge tube what is the discharge tube? It is a cylindrical glass tube glass tube which consist which consist two electrode the end which consist two electrode at the end, so this is the two electrode we have discharge tube cylindrical glass tube and two electrodes at the end okay, so the electrode and this this electrode it is connected with external voltage supply and the voltage we are using around 10000 volt here right and this is the vacuum pump which we use to maintain very low pressure inside the discharge tube which is 10 to the power minus 4 atmospheric in this order okay, so we have taken all the air present inside the discharge tube out with the help of vacuum pump and we fill this discharge tube with any gas here, so we have some gaseous particles present here in the discharge tube okay, any gas you can take that is not a constraint we have any gas you can take okay, so now what happens in this in this experiment what happens when we apply a very high voltage source here right then some invisible ray passes through passes from this electrode towards the positive electrode which is anode right and how do we know this this is an invisible ray invisible ray from cathode to anode it passes through okay and how do we know that because this part was coated with a fluorescent screen that is ZNS screen this part was coated with ZNS screen coated with ZNS screen and when this light is slice at the surface this part starts glowing and that is how we know that when we apply an external voltage source here an invisible ray from cathode travels towards anode that ray we call it as cathode ray why cathode ray because it is coming out from cathode or from this direction from cathode to anode that is why we are calling it as invisible ray okay coming out from cathode now since the cathode is negatively charged right so this ray was also considered to be or assumed to be negatively charged okay how do we know this that this ray is actually charged will have some charged particle or neutral that when this ray passes through an electric or magnetic field it get deflected okay deflection proves that that the ray is not neutral it has either positive or negative charged with it okay why negative only because it is it is moving towards the positive plate or it is coming from the negative plate that is why first of all since the deflection produced in the magnetic field it means it is charged what charge positive or negative that we understand by this experiment since it is moving towards the positive placed plate hence it is negatively charged so cathode ray this ray is cathode ray invisible ray coming out from cathode we call it as cathode ray so cathode ray consist of negatively charged particle okay cathode ray consist of negative charged particle called electrons cathode ray consist of negative charged particles called electrons okay so that is how the discovery of electrons takes place okay this experiment we call it as cathode ray experiment and the whole setup we have discussed okay right now the properties of cathode rays are what it is it produces mechanical effect okay mechanical effect means what I will tell you here the first one it produces mechanical effect it produces mechanical effect mechanical effect means what suppose you have a blade fan blade suppose like this this is the fan blade we have suppose and when you strikes with you know cathode ray on this blade all these are cathode when you strikes with the cathode ray then the energy transfers to this fan blade and it starts rotating like this this is mechanical effect the transfer of energy why transfer of energy takes place because cathode ray consist of particles called electrons right and electron has its own mass and since it is traveling so we will have some velocity over there so the kinetic energy of electron gets transferred into the rotational energy of fan okay that was the mechanical effect we have second point deflects cathode ray deflects in electric or magnetic field electric or magnetic field third point it is negatively charged negatively charged fourth one the properties of of cathode ray is same for all gases means whatever gas you are taking here right finally the electron only is coming out okay so electron of hydrogen electron of helium electron of methane all have the same properties okay so electrons properties of electron does not change with gases right that's why the properties of cathode ray why cathode ray because it consists of electrons cathode ray is same for all gases the fourth point is very important okay these are the few properties of cathode ray now now we have this understanding that atoms consist of negative charged particle negative charged particles called electrons now the atom as a whole is neutral in nature that we know already right so atom as a whole is neutral and if some negative charged particle is present within it it means to neutralize this negative charged particle we have some positive charged particle also present should also present right so in this by the this by this theory only we come to this conclusion that there must be a positive charged subatomic particles present within an atom which makes the whole atom overall neutral and hence it is stable so in that way only we come to this part that that is the discovery of proton which is a positive charged particle okay so now for the discovery of protons also we call it as anode ray experiment similar kind of experiment was done okay but we use here a perforated plate we use here a perforated plate right and what we observe in this that when the cathode ray was coming out I am not drawing the diagram again just I am trying to make you understand with this diagram only right and it was observed that when cathode ray was coming out from this from this cathode this side in this way at the same time some positive ray also some invisible ray also travels towards the negative electrode which is cathode so at this time what we absorb what we observe some negative charge some positive charged particles or positive ray also moves towards the negative charge electrode right this was assumed to be coming from the positive plate which is anode okay and since it is coming from anode towards the negative plate cathode we call it as anode ray right and anode ray coming from positive plate it is positive charged right this also produce deflection in electric and magnetic field since it is charged right so when we allow this way to pass through electric or magnetic field there is deflection in the in presence of electric and magnetic field and this deflection is opposite to the reflection of cathode ray hence it is said to be that cath anode ray has oppositely charged is then oppositely charged ray with respect to cathode ray and it is assumed to be coming from anode hence it is positively charged right so the particles of anode ray anode ray consists of particles called proton which is a positively charged all these points are same right it also produces some mechanical effect deflection electric and magnetic field it is instead of negatively charged this one is positively charged properties of cath anode ray will be different for all gases this is the very important difference we have properties of cathode ray is same for all gases whatever gas you take here the properties will be same of cathode but anode ray has different properties for different different gases suppose you are taking hydrogen right so hydrogen the proton is what h plus plus one electron so this is the proton we have here right similarly the positive charge species in case of helium is what when you take helium gas that will be he plus so h plus and he plus are two different cations right two different positive charge ions right that's why the properties of anode ray is different is different for different different gases this is the major difference we have in the two rays cathode rays and anode rays okay so I hope you have understood this experiment which is not that much you know important but to understand the you know experiment and discovery you should know this okay so you can go through the theory given in ncrt you will understand the experiment exactly okay the next thing is with the help of proton and electron when we try to find out the atomic mass of different different atoms the atomic mass was not coming out to be correct for all the atoms okay then we started thinking about the third type of the third type of subatomic particles present within an atom which must be neutral in nature okay and that third type is neutron okay for discovery of neutron the beryllium the beryllium atom is attacked by alpha particle beryllium atom is bombarded with alpha particle that is helium and then it converts into carbon and then we'll get one neutral element which is neutron so this one is neutron which is neutral in nature there's no charge present on this neutron we have only two charged subatomic particles that is electron and proton and this one is neutron this reaction is actually nuclear reaction nuclear reaction where nucleus are involved actually you must have thinking about that how this helium and beryllium converts into carbon so when the nucleus are involved this kind of change is possible so nuclear chemistry is not in our syllabus but this is one type of nuclear reaction okay so this is one thing now since the neutron is negatively sorry neutral in nature this produces no deflection in electric or magnetic field okay so when you allow this ray or this neutron to pass through an electric or magnetic field we don't observe any deflection because it is neutral in nature deflection is only possible when the particles are will have any kind of charge whether it is positive or negative charge okay now so this is a three subatomic particles we have various of the subatomic particles present like meson positron neutron neutrino neutrino etc all these are subatomic particles we have but mainly we have three that is electron proton and neutron and that we have discussed okay now the charge and mass charge and mass of this subatomic particles you have to keep in mind okay so like say like for example we have electron so the charge on electron is minus 1.6 into 10 to the power minus 19 coulomb mass of electron is 9.1 into 10 to the power minus 31 kg okay similarly we have proton charge on proton it is equal to that of electron but sign is positive magnitude is same mass of proton is 1.67 into 10 to the power minus 27 kg okay the third one is neutron neutron the charge on neutron as we have discussed it is 0 and the mass on neutron is equals to the mass of proton which is 1.67 into 10 to the power minus 27 kg this is the charge and mass we have sometimes you have to this they ask this question also the charge and mass ratio of electron proton and neutron okay they will find you the order so when you know these two values you can find out charge and mass ratio okay that we also call it as specific charge okay so with this value you can easily find out okay so till now we have discussed about the various subatomic particles the discovery of various subatomic particles now the next objective was or the you know or the task we have that how this subatomic particles are arranged within an atom okay so the arrangement of subatomic particles within an atom that we are going to understand now right so for that different different scientists various other scientists have given their you know their models their theory about how the subatomic particles are arranged within an atom okay so this theory of arrangement of subatomic particles within an atom we call it as atomic models okay so next we are going to understand what are the different atomic models we have what are the drawback why we have discarded some of the atomic models what atomic models we are following now okay so all these things we are going to discussed in the next we are going to discuss in the next session okay thank you