 Hello, everyone. This is the first lecture in this course on nuclear radio chemistry. And in this first lecture, I will introduce the subject of radioactivity. While most of you might have already read this part in your science classes in high school, intermediate or even BSE, I thought it would be good to start with the introductory lecture to bring all the students on the same status so that you understand what I am going to discuss next. So, this lecture, I will put in terms of asking some questions or at the end of this lecture, the students will be able to answer the questions that I have shown here. What is radioactivity? Why do radioactivity direct evidence decay? Why are some isotopes radioactive while others are not? What is natural radioactivity and what is artificial regret? So, these questions you should be able to answer after attending this particular lecture. Let us come to the first question. What is radioactive? So, radioactivity essentially involves a spontaneous emission of ionizing radiations like alpha, beta, gamma from the nucleus of an atom. So, it is a nuclear phenomenon. So, you can say radioactivity is an atomic phenomenon and it is independent of the chemical state of an atom. Whether you take a metal, whether you take an oxide, whether you take a liquid form or gaseous form, it is independent of the chemical state. Like for example, the parameters of radioactive decay are the half-life. So, the half-life is the constant of a radioisotope. It does not depend upon chemical state of that. Before we go to the next part, other aspects of radioactivity, I thought it would be good to introduce some of the nomenclatures we will be discussing in this lecture. Like we have radioisotopes. So, isotopes are atoms of an element having the same atomic number which we will call as Z, but different mass numbers. So, mass number is denoted by A, A atomic number by Z. And mass number is nothing but the sum of the proton number and neutron number of the nucleus. For example, carbon-12 and carbon-13 are the two isotopes of carbon. Carbon-12 has six protons and six neutrons. So, mass number is 12. Carbon-13 has six protons and seven neutrons. So, the mass number is 13. So, these are the isotopes of carbon. Now, radioisotopes are those isotopes which are radioactive. For example, tritium and carbon-14, these are radioactive and they are the like tritium is isotope of hydrogen, carbon-14 is isotope of carbon. Another term that we will be frequently using is isobars. Isobars are atoms which have same mass number, but different atomic number. For example, carbon-14 and nitrogen-14 have the same mass number 14. So, they are the isobars and they have different atomic numbers 6 and 7 respectively for carbon-14 and nitrogen-14. Now, before we go further, I thought let us discuss some of the initial history of this subject of radioactivity, namely the discovery of radioactivity. Most of you probably know already that the phenomenon of radioactivity was discovered by Henry Becquerel in France in 1896. Ronzen discovered x-rays in 1895 and inspired by this discovery, Henry Becquerel started understanding the fluorescence emitted by uranium compounds when the light is sign on them. Now, you know the fluorescence phenomena, when a fluorescent substance is exposed to light, it emits its own correctance with luminescence, we call it fluorescence. So, while studying the fluorescence of uranium compounds, Henry Becquerel observed that even the photographic plate which was not kept in which was not which was kept in the dryer inside by a uranium mineral called pitch blend and so there is no light coming from outside. But he found that the photographic plate was blackened which was kept in the neighborhood of a uranium mineral pitch blend. And so he came to conclusion that uranium mineral is emitting highly penetrating radiations which is coming out of the packing etc. and affecting the photographic plate. But the name radioactivity in fact was coined by Madame Curie who was responsible for the discovery of two more radioactive elements like radium and polonium along with perecurie. So, perecurie and Madame Curie discovered two elements, radium and polonium which were present in the pitch blend. In fact, what they found based on the half life of uranium 238 which is of the order of 10 for 9 years, the specific activity that means the activity per gram which found to be much more than what you expect from the half life of uranium 238. And so Madame Curie predicted that this uranium pitch blend contains some more elements which are more radioactive means you have higher specific activity than radium. And so when we did the chemistry, separated elements which are responsible for higher activity, see how that these are the daughter products of uranium and they were named as radium atomic number 88 and polonium atomic number 84. So, the term radioactivity actually was coined by Madame Curie and they double price for this particular discovery of radioactivity was shared by Henry Becquerel, P.L. and Madame Curie in 1903. So, the subject of nuclear science and technology began in the end of 19th century and there have been many more such discoveries is subsequently in the 20th century we will discuss them later. Immediately after the discovery of radioactivity, the Rutherford's group at Manchester also became very active and he along with Shoddy formulated the laws that govern the different radioactivity. You know that the prominent ones are the alpha decay, the beta decay, gamma decay actually is not the decay of a nucleus from its ground state. I will discuss that more in details, but the alpha decay essentially involves the emission of a doubly charged helium atom from the nucleus of the parenter. So, like 238 uranium when it is undergoing alpha decay, the mass number is decreased by 4 and the atomic number is decreased by 2 with that is the configuration of the helium. So, uranium-38 becomes 234 thorium. In the case of beta decay, now beta decay can be beta minus or beta plus like for example, 239 uranium undergoes beta minus decay to become 239 nectunium. So, the in the case of beta decay the mass number remain the same, but atomic number is increased by 1. On the other hand, in beta plus decay, the mass number remain the same, but atomic number is decreased by 1. You can see here delta A is 0, delta D is minus 1. The fundamental equations representing the beta minus and beta plus decay are given there. The neutron going to proton and in the process an electron is limited and also an anti-neutronome. Whereas for the progetron decay the beta plus decay proton is converted into a neutron and the process a progetron and a neutron is limited. And we will discuss later on in details the beta decay theory because of the three body interactions the beta spectra continues unlike the alpha spectra. The gamma decay essentially you can see here the excited state of a nucleus emits gamma like excited atom emits x-rays, excited molecules emit even visible radiations, excited nuclei emit gamma rays. So, gamma rays is not essentially the decay of the ground state of the nucleus it is the decay of an excited state. Now, let us come to the fundamental questions like why do radioactive atoms decay? So, the decay of a radioactive atom is essentially because of the thermodynamic instability of the atom. So, the disintegration of the atomic nucleus occurs due to their inherent thermodynamic instability. What do you mean by thermodynamic instability? That means if the mass of the parent atom is more than is some of the masses of the products. So, it is like you know a potential. Potential means you know when the water from a dam if you open the floodgates the water gases down the stream because the water at the dam has a high potential and so the moment there is a channel there is a pathway for the decay water will rush down. Similarly, if a nucleus has higher mass than some other nucleus and there is a pathway what is the pathway alpha, beta. So, these are the pathways for decay of an atom then because of the thermodynamically instability of the atom the mass the nucleus having higher mass will decay to that and the lower mass. Just for example, the phosphorus 32 emits a beta decay to sulphur 32. Now, if you calculate the masses of these isotopes, so the mass of phosphorus 32 in atomic mass unit is 31.973908. I will discuss later on why we use such a precise number for the mass because when you convert the mass into energy using the formula E equal to mc2. We are multiplying the delta m by 931. One atomic mass unit is 931 MeV. So, if you have the precise atomic masses in terms of MeV we need to multiply we need to have them in the very large significance. The mass of sulphur 32 is 31.972074. And so this difference between the two you can see the difference lies in the third decimal point in the atomic masses. So, that is 0.001834 when you multiply by 931 then you get the energy of beta decay 1.705 that is the q beta for decay of phosphorus 32 into sulphur 32. And this energy is shared between the three particles sulphur 32, beta minus and MeV. Just to give you a feel this we will be using the energy in electron volts, kilo electron volts and million electron volts. One electron volt is the energy acquired by an electron when it is accelerated to a potential of one volt. That is the one electron volt and then you can have kilo electron volt, million electron volt and so on. And one electron volt corresponds to 1.602 10 power minus 12 hertz or 1.602 10 power minus 19 joules. Now, you must have read already that the chemical reactions like A going to B they are mostly they can be reversible. Some of them are irreversible also like burning of carbon C plus 4 to CO2 irreversible. But by large you can say that majority of the chemical reactions are reversible. So, when you go from A to B you can also come back from B to A. But the relative decay if you call this as a reaction then they are irreversible. You can go from A to B you cannot come back from it because the mass of B is less than that of A. Now, the mass and energy of nuclei are sometimes you know we have interchange the mass and energy. So, the mass of a nucleus we call in terms of mass bracket z comma A where z is the atomic number. A is the mass number and this is equal to mass of the proton m h into number of protons that is the atomic number z plus mass of neutron into number of neutrons n into m n minus the binding energy. So, this concept of binding energy has come from when you combine z protons and n neutrons to form a nucleus having mass number having mass m z comma A then a energy equivalent to B is released. So, when the protons and neutrons combine to form a nucleus energy is released and that energy released is called binding energy. So, the energy released when z protons and n neutrons combine to form the nucleus is called its binding energy. In other words, binding energy is the energy required to break a nucleus into its constituent nucleons. And using the term nucleons we will see later that protons and neutrons are together called nucleons. So, higher the binding energy of a nucleus lower the mass and it is more stable. You can see from the equation of nuclear mass if B is more then m mass of the nucleus will be smaller. So, that will give you an idea that those nuclei which have high binding energy they are more stable. Now, the next question is why are some isotopes radioactive while others are not? What decides a particular isotope to be radioactive or not? So, I illustrate this question using this section. I just give you an example of different isotopes of carbon carbon 11 12 13 14 you can see here. The atomic number of carbon is 6 and the neutron number from carbon 11 to carbon 14 goes from 5, 6, 7 and 8. Carbon 12 and carbon 13 are stable. You add one more neutron to carbon 13 from carbon 14 and it is unstable it undergoes beta minus decay to nitrogen 14. Similarly, so it is undergoing beta minus decay. Carbon 11 which is having one neutron less than carbon 12 undergoes beta plus to boron 11. So, you can see here that if you alter the neutron to proton ratio of this stable isotope, you have a high chance that next isotope will be radioactive. Another example like you, cobalt has only one isotope that is stable that is 59 cobalt having 27 protons and 32 neutrons. You add one neutron to cobalt 59 and you form cobalt 16 and this becomes radioactive. You remove one neutron from cobalt 59 we form cobalt 58 and it becomes radioactive. So, cobalt 58 is undergoing electron capture decay we will discuss that later on. So, you can see here if I alter the neutron to proton ratio of this nucleus becomes radioactive. So, the atoms are stable if their neutron to proton ratio is in a particular range. That means when you have certain number of protons you require certain number of neutrons to stabilize the nucleus and anything beyond that range on both sides neutron rich side or neutron deficient side nucleus becomes unstable. So, I hope this explains that why are some isotopes radioactive while in the sun now. Now I come to what is natural radioactivity? So, when you say natural radioactivity means a radioactive isotope is present in nature in earth crust. Now, you know that the age of the earth the earth was formed few billion years ago. Around 5 billion years ago and therefore, if there were when the earth was formed there were some radioactive isotopes formed at that time in the earth. Then they are decaying but they are still present because of their long half. For example, 1.238 has got a half life of 4.47 billion years and it undergoes alpha decay to thorium 234. Now, you can use the group displacement law to determine the proton number, neutron number and so on. So, it undergoes alpha decay to thorium 234 which has half life of 24 days then goes to protonium 234 by beta minus decay and then to 234 uranium by beta minus. And again a series of alpha decay and beta decay take place and finally it ends up with 206 lead. Now, one common thing you will find in this series called in a natural radioactivity series that the mass numbers 238, 234, 234, 230, 236 and so on. So many isotopes are formed in this series all of them have a particular formula. You can write this in terms of 4n plus 2, n is an integer. For example, 206 you can write as 4 into something what is that 51, 4 into 51, 204 plus 2. Similarly, so all of them are 4 into some integer plus 2. That is why it is called as a 4n plus 2 series. All members of this natural series belong to the family 4n plus 2. Now, how to calculate the number of alpha and beta imitated in this radioactive series? So, I have given you a simple formulation though. The number of alpha you know that the mass number is decreased by 4 in alpha decay whereas in beta decay there is no change in mass number. So, from 238 to 206 the mass number decreased by 32. So, 32 by 4 is 8. That means in this radioactive series from 238 to 206 lead 8 alphas are in it. So, how to calculate now the betas? The 8 alphas will decrease the atomic number by 8 into 2, 16. So, from 92 we get 76 by alpha decay. If there were no beta decay then we should have got atomic number 76, but we have get 82. That means there have been 6 beta minus decay because in beta minus decay atomic number increases by 1. So, 6 beta minus decay will give you 8. So, 92 minus 16 is 76, 76 plus 6 is 8. So, in this natural radioactive series there are emission of 8 alphas and 6 betas. That is how we can calculate in any natural series what is the number of alpha in beta. I give you another examples 4n plus 3 to 35 uranium leading to 207 lead. You can see all of them follow 4n plus 3 formula where n is an integer. And there is another series 4n to 32 thorium going to 208 lead 4n all of them are multiples of 4. So, these are the isotopes I have given the half-lives on the right hand side to 38 uranium, 4.7, 47 by 9 years, 35 uranium, 7.0, 8 years to 32 thorium, 1.4 by 10 years. Now you can see the half-lives are either compatible or more than the age of earth and that is the reason why these isotopes are still present in the earth. Another explanation you can see here 235 uranium which is the fissile isotope that content it much less. It is only 0.7 percent now in the natural uranium because its half-life is one order of magnitude less than 238 uranium. So, it has decreased much more than 238 and it is decreasing faster. A point and when the earth was formed 235 content must have been quite high this may be of the order of 238 uranium. There are other isotopes like potassium 40 going to argon 40 by beta minus decay again because of its long half-life 1.25 for 9 years again of the order of age of the earth. But in addition to these natural resources there are two more isotopes which have short half-life but they are present in nature. These are carbon 14 and tritium. Carbon 14 has a half-life of 5700 years whereas tritium has a half-life of 12.3 years. So, how is it that they are present in earth even now? The reason is that they are being continuously formed in the atmosphere by interaction of cosmic neutrons with the nitrogen. So, atmosphere contains nitrogen 78 percent is nitrogen and nuclear reaction of neutron with nitrogen 14 gives you carbon 14 plus tritium. Similarly, carbon 14 plus proton whereas neutron practically nitrogen 14 gives you tritium plus carbon 12. And because of these two reactions continuously happening in atmosphere and this atmospheric carbon and tritium can get into the water bodies by rains and you will see here that all the water bodies will have tritium. So, lateral water contains tritium and even our body living organisms any living organism will contain carbon 14. So, while we say radioactivity is harmful if we measure our body radioactivity you will have potassium for people our body contains potassium our body contains carbon our body contains hydrogen and therefore these three isotopes are present in our body ups. But we do not have any lost and harmful due to this radiance. But the important point I wanted to highlight here is that these isotopes the long-lived ones which have the half-life of the order of age of the earth as well as carbon 14 and tritium are very much useful in many studies. For example, you want to determine the age of the rocks by determining the content of 238 uranium and 208 lead 206 lead. You can in fact find out when was that time this rock was formed. Similarly, the fossils, fossils contain carbon. So, carbon 14 is decay you can use the activity of carbon 14 find out the age of the fossil. Similarly, water bodies, water bodies contain hydrogen and therefore tritium from the measurement of activity of tritium in a water body. You can find out if it has got disconnected from the normal live water the rain water and so on. So, it is not being equilibrated by the the normal water. So, these are the way you know you can find out when was this particular water body or the fossils or the rocks form. Now, I come to the last part of my today's this module that is artificial reductants. Just now we saw several isotopes which are naturally occurring. But the number of isotopes which are formed by artificial means is now much more than the reductants. And this phenomenon of artificial reductivity was discovered in 1934 by I.B. Jury and Frederick Julliard. I.B. Julliard is a daughter of Madame Julliard. Now, the reaction that they used was bombardment of aluminum-27 with alpha particles giving rise to electron and phosphorous-30. This phosphorous-30 is undergoing beta plus decay to sulfur-30. Now, I will tell you the story very interesting story here is that the same reaction was used by Chadwick for the discovery of neutrons. So, this group also had actually seen neutrons but they were unable to interpret the results. And in 1932 Chadwick discovered neutron but this group were also studying the same reaction but they missed the discovery. But then they are in for another discovery that is by artificial means to produce radioisotopes. So, today while there are only 274 stable isotopes, the number of radioactive isotopes is more than 2. And the half-life of these radioisotopes can be as high as 10 per 15 years as low as microsecond. So, the artificial activity has provided you with a tool to have different applications. So, these isotopes can be produced by different means like in accelerators where we accelerate charged particles like alpha particles, protons or avianes or in nuclear reactors where we have neutrons at the time and produce them and use them in different fields like healthcare, industry, agriculture, food technology, water and environmental systems. So, this particular field of nuclear science and technology opened up once we knew that we can induce radioactivity in stable isotopes. And therefore, a new era began in the early 20th century. Subsequently, of course, the nuclear fission was discovered that is another way of producing radioisotopes. But this phenomenon of other radioactivity gave rise to a tool to produce isotopes of our interest. Think of an isotope and you can produce it in different facilities like accelerators or reactors. So, I will stop here and in the next lecture I will take about the radioactive decay law and so on. Thank you very much.