Hello students, today we are going to start with the new chapter nuclear chemistry and radioactivity Actually this chapter is not a separate chapter, it is well combined with chemical kinetics and atomic structure Actually there are two parts of this chapter, two parts of nuclear chemistry One is theoretical part in which we define radioactivity, radioactive disintegration series and stability of the nucleus this portion that goes very well with atomic structure we will take this portion in the beginning after that there is another portion called numerical portion because radioactive decay of the nucleus follows the first order kinetics and where have we studied that first order kinetics? we have studied in chemical kinetics so that portion and its applications like rock dating and carbon dating all these portions we read after chemical kinetics so it is better understood so actually 3-4 video lectures in the beginning 4-5 video lectures 11th class students after atomic structure they can study I will tell you how much to study and last 3-4 video lectures or 2-3 video lectures you should read after chemical kinetics because in chemical kinetics first order concepts will be applicable So overall, I am covering at one place, where I have to bring demarcation, where I have to differentiate, that we can study this portion in 11th class along with atomic structure, and after this portion we can study along with the chemical kinetics, I will tell you. So I said some general things about the nuclear chemistry, so first write the heading, nuclear chemistry.
Nuclear chemistry, by the way, nucleus is a kind of black box for the scientist Not chemist, this is the work of physicist, this is the main topic of physics Study of the nuclear chemistry is part of physics you must have studied all this in 12th class in modern physics all this is also known in nuclear chemistry but we have to study a little portion in chemistry also so nucleus is a kind of black box and lot of research is going on to unearth or to understand the nature of the nucleus how neutrons protons nucleons are distributed inside the nucleus see extra nuclear chemistry is well understood extra nuclear chemistry means the portion outside the nucleus the portion outside the nucleus is electrons for the distribution of electrons you have read the bohor model and most advanced wave mechanical model or schrodinger wave model also you have read now it is about nuclear chemistry we have to read in detail what is our requirement for IIT JEE and advanced we will talk about that here first we will talk about a term radioactivity we will define a term radioactivity radioactivity spontaneous decay or disintegration of some of the nuclei or some nuclei of the elements without any external excitation, due to which some alpha, beta, gamma rays emit out and nucleus are unstable. This process of emission of the rays is called radioactivity. So, write some nuclei.
Some nuclei of some elements are unstable and undergo nuclear decay or disintegration spontaneously without any external means or excitation. Without any excitation, spontaneous means spontaneously some elements whose nucleus are unstable and they decay, disintegrate and that is called radioactivity. Emission of Nuclear particles like alpha emission, beta emission and gamma rays emission from Unstable nuclei is called radioactivity. so this is radioactivity, emission of the alpha particles nuclear particles like alpha emission, beta emission or gamma rays emission from unstable nuclei is called radioactivity so emission of alpha, beta, gamma rays is called radioactivity to gain the stability here we define a term nuclear stability Why is nucleus stable? students listen carefully to me this is clear like electrons get negative charge electrons repel each other protons get positive charge protons repel each other neutrons are uncharged so let's not talk about direct repulsion Now, we have already found out from atomic structure that size of the nucleus is negligible in comparison to the size of atom.
The size of atom is very big in comparison to the size of nucleus. And the electrons outside the nucleus, they spread into the large space to reduce the electron density and hence electronic repulsions so this means that electrons spread in a very large space so that electron density and repulsion are reduced so we understood that electronic repulsion is reduced and electrons are comfortable and stable But the nucleus is very small. The negative charge on the electron is equal to the total positive charge in the form of protons inside the nucleus.
Now the question arises that the electronic charge has spread to a large volume. Protonic charge means the positive charge on protons is in very small volume which we call as nucleus. So how is it stable there? There are strong coulombic repulsions between proton and proton. So how is it stable?
For that a theory is given, nuclear forces. These nuclear forces operate in nuclear distances only and they are very very strong forces, much stronger forces than the coulombic forces of repulsions between proton and proton. So how is it stable?
it is found that the strongest nuclear force is nearly 137 times stronger than the columbic force of repulsion however these nuclear forces vary with the inter atomic or inter particle distance so it means that high stability of most nuclei of stable elements is due to the nuclear forces is due to the strong, very strong Nuclear forces that operate in nuclear distance that operate within nucleus or at nuclear distance. nuclear distances may operate with the of the order of 10 to the power minus 15 meter or Fermi meters phantometer or Fermi this types of get the to high stability of the most nuclei of stable elements is due to the very strong nuclear forces that operate within nucleus or at nuclear distance of the order of 10 to the power minus 15 meter or fermi meter fermi or femtometer remember one fermi meter or femtometer is equal to femtometer or fermi that is equal to 10 to the power minus 15 meter So these are the evidences. Evidence of nuclear forces can be illustrated by alpha particle scattering experiment. If you remember, when I was teaching atomic structure, when I was teaching Rutherford alpha particle scattering experiment, even then I told that nuclear forces are operative at the Fermi distances or within the nuclear distances, like this. See, this is how nuclear force is known.
Overall potential energy varies like this. This is potential energy. See it varies like this.
Approximately like this. Like this. This is r distance of alpha particle. from center of nucleus of element If the distance of an element's nucleus to an alpha particle is very far then it means that alpha particle is also positively charged and nucleus is also positively charged so repulsion is done, potential energy will be positive and this potential energy increases so which region shows this?
columbic repulsion electrostatic repulsions but under these distance here these are nuclear distances in distances who becomes dominant and this nuclear forces become much dominant Nuclear forces become dominant in this zone. Nuclear forces of attraction, attractive forces. so these nuclear forces of attraction they operate between proton proton between proton neutron and neutron proton due to which net forces are attracted and potential energy becomes negative this was zero potential energy became negative see this negative potential energy negative potential energy indicates attraction this is known to everyone, that potential energy of repulsion is positive and potential energy of attraction is negative so this denotes attraction so these small distances under nuclear distances, attractive forces, nuclear forces become dominant so nuclear forces operate in nuclear distances nearly up man low SM unloaded say point 8 for me say to for me tuck is range me operate curtin these forces as a knee care attractive you get a point 8 for me if distance between proton and proton or between proton and neutron or between alpha particle and nucleon if 0.8 fermi meters will be less then very strong repulsive forces will be there If there are no repulsive forces, then protons and protons will not stick to each other in the nucleus.
Listen carefully, you are saying that when alpha particle was far away from the nucleus, then repulsive forces were there, like the Coulombic repulsion, Coulombic forces are operative in inverse square law and potential energy is inversely proportional to R. but as soon as it came near the nucleus, the force of attraction started on the alpha particle and as soon as it entered the nucleus who? alpha particle very strong attractive nuclear forces are operative and that operate within the distances of 0.8 fermimeter to 0.8 femtometer to 2 femtometer that indicates that strong attractive forces are there but this means that alpha particle and a proton or a neutron are having a force of attraction inside the nucleus, in the nuclear distances so this means that even inside the nucleus, the protons and protons protons and neutrons, neutrons and neutrons there must be some forces of attraction between them and they must be very strong, as you are saying nuclear forces so because of that nuclear force they are attracted to each other so now the point is that if they are attracted to each other then why don't they stick together two protons stick together, one neutron stick together, one neutron stick together, two neutrons stick together, three neutrons stick together and become single particle No!
they have repulsion less than 0.8 femtometer they have heavy repulsion which is very strong vehement than these attractions and it stops them from sticking what am i saying? do like this you will understand so this was potential energy vs distance of alpha particle from the center i will explain this with this experiment by the way this is so useful and important i don't know, maybe physics people will teach you all this Potential energy versus r. This time we are saying r.
Distance of neutron from center of nucleus of element. So this time, the neutron is not positively charged, it is a neutral particle so it will not face any repulsion from the neutron or proton inside so if it is very far away, then negligible forces will be there and attractive forces will be there so the potential energy will be negative like this but as soon as they come very close, then one they have a very strong attraction like this like this cup is made of this type this is zero so potential energy means these distances what does it indicate? that nuclear forces are operative Attractive forces, nuclear forces means attractive forces are operative within nuclear distance.
So, you can also call it nuclear radius. So, what can you call this? Nuclear radius. from where they have started to be separated so this distance shows that this is a potential energy diagram of neutron scattering experiment Did you understand? So pay attention.
So this kind of attraction shows that in these nuclear distances, a nuclear force, an attractive force operates which is much stronger than the repulsive forces. This is the reason for the stability of the nucleus. This is the reason for the strongest. strongest nuclear force is nearly 137 times stronger than a columbic force That's why he is able to cross the columbian barrier with the columbian force. On an average, in fact, nuclear force varies with distance and on an average, It is 40-50 times stronger than columbic forces.
the nuclear forces are much stronger maximum strongest nuclear force is nearly 137 times stronger than the columbic force but on an average 40 50 60 times stronger than the columbic forces so because of these strong forces neutron neutron neutron proton proton proton are held together in the nucleus yukawa he is a german physicist, so Yukawa proposed his meson theory for stability of nucleus according to which There is rapid exchange, there is rapid exchange of pi mesons between proton proton proton neutron neutron neutron and energy is released Yukawa got a Nobel Prize for this. So, Yukawa proposed, it was proposed in 1935. Proposed is meson theory for the stability of the nucleus according to which there is a rapid exchange of the pi mesons between proton proton proton neutron neutron neutron and energy is released that makes the system or nucleus stable. Pi meson may be electrically neutral positive or negatively charged charge of pi meson charge of pi plus pi minus meson by three types of pi meson you said pi neutral pi plus pi minus so pi mesons may be electrically neutral positive or negatively charged like pi neutral pi zero pi positive pi negative so charge of pi plus pi minus meson is same as charge on electron but Piemesson is much heavier then electron electron is heavier than electron nearly 70 times heavier so like that so this is the reason for the stability in one line what is the reason for the stability of the nucleus neutrons protons are there protons will repel so why nucleus is stable for most of the elements They say that nuclear forces operate within the nucleus which are much stronger than the coulombic forces of repulsion between protons. Due to which protons are stable inside each other and can live in the nucleus easily.
Here we will define a term mass defect and binding energy. Mass defective binding energy. It is experimentally found that atom of every element is slightly lighter in mass than the mass sum of its constituent particles so it is experimentally found that atom of every element is slightly lighter in mass than the mass sum of its constituent particles example low oxygen atom art proton art neutron art electron electron is negligible in mass so let it be what is experimentally found?
what is the observation? observation mass of O16 atom approximately 15.99 amu approximately it is exactly 5 digit after decimal but i am giving you an idea not a data mass of oxygen 16 atom approximately this much in amu mass sum of 8 proton plus 8 neutron plus 8 electron that is 16.13 amu this means there is a loss in mass this means that whenever constituent particles protons and neutrons come closer to form the nucleus there is a mass loss observed that is called mass defect and equivalent amount of energy is released called binding energy. so when protons and neutrons combine or protons and neutrons combine To form nucleus, there is a mass loss called mass defect and an equivalent amount of energy is released.
And an equivalent amount of energy is released called binding energy. This term is important. Mass defect. So protons and neutrons combine to form that nucleus.
There is a mass loss called mass defect and the equivalent amount of energy is called binding energy. So formula E is equal to delta M into C square. Einstein's formula where delta M is mass defect or mass loss and E is called binding energy.
Obviously, the more binding energy, the more energy will be released, so the remaining system will be of low energy. more stable. So this is a very important point. First of all remember this important point. If mass loss is 1 amu then nearly 931.4 mega electron volt of equivalent amount of energy is released.
This is very important for solving questions whether it is chemistry or physics. So 1 mass loss, 1 amu is what? Mass defect. we will remember this energy conversion and this is binding energy remember this remember this you will not be able to solve the question where you will do c square then you will do delta m then you will change the unit then you will convert it to electron volt then you will convert it to mega electron volt then you will have difficulty in the questions you have to remember whenever there is a mass defect of energy then you will have to solve of 1 amu then equivalent amount of the energy released is 931.4 mega electron volt you should remember this value remember this is important okay you just said that the more binding energy the stability will be more, binding energy alone is not the deciding factor binding energy per nucleon will be more because in the nucleus of different atoms number of neutron nucleons can be more than that 50 can be, 100 can be, 200 can be like uranium is 235, 238 also so then what is the measure? this is a very important point, we will remember it by noting it important Higher is the binding energy per nucleon.
Greater will be the stability of nucleus. of nucleus binding energy experimentally we measure so higher is the binding energy per nucleon per nucleon then greater will be the stability of nucleus do you understand nucleon? proton and neutron collectively they are called nucleons proton is also called as Nucleon any animal living in the nucleus means from proton and neutron so what will you call it? Nucleon so either proton or neutron is called Nucleon so this experimental graph is found like this this is binding energy per Nucleon B E Binding energy per Nucleon So, this is almost with mass number mass number A we use this symbol so, this graph is almost like this almost it is a vertical type like that and peak is found here around 60 here it is like iron and it is at very low like H1 H2 and this is found in graph like here here will be H4 this is C12 this is oxygen 16 all these like this here will be elements like after helium let's assume lithium comes here lithium like after carbon and between oxygen nitrogen came so that can become here nitrogen came n14 like this so it means it increases like this if it increases more suppose 50 100 150 200 around 235 here uranium 235 and 200 8 to 7 lead so this kind of this will be there so this means very light elements are expected to undergo fusion and heavier very heavy nucleus are expected to undergo fission so that they come into this zone which zone? high stability zone where binding energy per nucleon is maximum around iron around 60 mass number 60 around binding energy per nucleon becomes nearly maximum so this means very light elements where binding energy is very less they want to come in this range so this range is stable range where peak is so this means that lighter binding energy per nucleon is found to be maximum for mass number nearly 60. lighter nuclei are expected to undergo fusion.
so lighter nuclei to get into a stability zone under appropriate conditions fusion is not like that fusion is in stars you will read we will talk about nuclear fusion and nuclear fission so fusion is like in sun sun is a star so in sun hydrogen and helium are fused together and they convert into larger nuclei and release a huge amount of energy so this way you can say that graph is not that important but this term was important that higher is the binding energy per nucleon greater will be the stability of the nucleus so this is an important point so this was the introduction of nuclear stability so we defined radioactivity as N nuclear stability or some magic number and other things we will talk in the next video lecture that's it for now