12TH GRADE NUCLEAR CHEMISTRY RADIOACTIVITY TABLE OF CONTENTS 01 03 Nuclear chemistry Nucleons and Nuclides Detection of Radioactivity The Geiger-Muller counter 02 Radioactivity What is radioactivity? 04 Types of radiation Alpha, Beta, and Gamma radiation 05 07 Induced Radioactivity Natural and Artificial Radioactivity Nuclear Reactions The Atomic Bomb, Nuclear, and Ordinary Chemical Reactions and Equations 06 Spontaneous and Induced Nuclear Reactions Differences 08 Electricity Generation of Electricity by Nuclear Reactions 09 11 Effects of Radioactivity Hazards associated with nuclear radiations Stability of Nuclides and Half Life Factors affecting the stability of Nuclides and Calculations 10 Application of Radioactivity Carbon Dating, etc. INTRODUCTION Radiation is energy that travels in the form of waves or particles and is part of our everyday environment. People are exposed to radiation from cosmic rays, as well as to radioactive materials found in the soil, water, food, air, and also inside the body. Radiation is energy in the form of waves or streams of particles. All life has evolved in an environment filled with radiation. 01 NUCLEAR CHEMISTRY Nuclides and Nucleons Nucleons and Nuclides Nucleons refer to the protons and neutrons that are found in the nucleus or nuclear particles which are protons and neutrons. A nuclide is an atom with a particular or specific number of protons and neutrons. The word “Nuclide'' was coined by an American Chemist named Truman P. Kohman in 1947. The term nuclide or nuclide is taken from the word nucleus. We describe a nuclide by the composition of its nucleus, the number of protons, the number of neutrons, and the energy content. Example: carbon-13 with 6 protons and 7 neutrons or 13 6πΆ Radioactivity is the spontaneous decay/disintegration of the nucleus of an unstable. nucleus/atom (of an element) RADIOACTIVITY with the release of any one, two or all of. alpha, beta and gamma What is Radioactivity? radiations. 02 03 DETECTION OF RADIOACTIVITY The Geiger-Muller Tube THE GEIGER-MULLER TUBE HOW DOES IT WORK? A Geiger counter has two main parts—a sealed tube, or chamber, filled with gas, and an information display. Radiation enters the tube and when it collides with the gas, it pushes an electron away from the gas atom and creates an ion pair. A wire in the middle of the tube attracts electrons, creating other ion pairs and sending a current through the wire. The current goes to the information display and moves a needle across a scale or makes a number display on a screen. These devices usually provide "counts per minute," or the number of ion pairs created every 60 seconds. If the loudspeaker is on, it clicks every time an ion pair is created. The number of clicks indicates how much radiation is entering the Geiger counter chamber. You hear a clicking sound as soon as you turn on the speaker because there is always some radiation in the background. This radiation comes from the sun, natural uranium in the soil, radon, certain types of rock such as granite, plants and food, and even other people and animals. 03 TYPES OF RADIATION Alpha, Beta, and Gamma Radiation CLASSIFICATION OF RADIATIONS ALPHA RADIATION BETA RADIATION GAMMA RADIATION “ DIFFERENCES Property Nature α ray β ray Positive charged Negatively charged 4 particles, 2π»π nucleus particles (electrons). γ ray Uncharged ~0.01a, electromagnet ic radiation Charge Mass 2+ –27 6.6466 × 10 kg 1–31 9.109 × 10 kg 0 0 Range ~10 cm in air, can be stopped by 1mm of Aluminium Up to a few m in air, can be stopped by a thin layer of Aluminium Several m in air, can be stopped by a thick layer of lead Natural Sources By natural By radioisotopes 236 radioisotopes e.g. 92π e.g. 68 29πΆπ Excited nuclei formed as a result of Gamma decay THE THREE TYPES OF PARTICLES Alpha Beta Gamma Has the least penetration power and can ionize other atoms. Its speed is about one-tenth of the speed of light. It has a moderate penetration power and can ionize other atoms, but not as good as alpha particles. Its speed is about 90 % of the speed of light. It has the highest penetration power and the least ability to ionize other matter. Its speed is equal to the speed of light. 05 INDUCED RADIOACTIVITY Natural and Artificial Radioactivity NATURAL AND ARTIFICIAL RADIOACTIVITY Natural radioactivity is the spontaneous disintegration/decay of a nucleus to produce radiations/particles (and energy). The elements with atomic numbers 8292 are found to radiate spontaneously in nature so they are known as natural radioactive elements. Whereas the elements that are produced in the laboratory by the bombardment of particles are called artificial radioactive elements. These are generally elements with atomic numbers less than 82. Induced radioactivity, also called artificial radioactivity or man-made radioactivity is the process of using radiation to make a previously stable material radioactive. The radioactivity of isotopes that have been artificially produced through the bombardment of naturally occurring isotopes by subatomic particles or by high levels of x-rays or gamma rays. 06 Spontaneous and Induced Nuclear Reactions DIFFERENCES Natural radioactivity is the process of radioactivity that takes place naturally whereas artificial radioactivity is the process of radioactivity that is induced by man-made methods. Therefore, the key difference between natural and artificial radioactivity is that natural radioactivity is the form of radioactivity takes place on its own in nature whereas when it is induced by man in laboratories, it is called artificial radioactivity. Furthermore, natural radioactivity is spontaneous while artificial radioactivity is non-spontaneous. Hence we need to initiate the radioactivity to get the artificial radioactivity. 07 NUCLEAR REACTIONS The Atomic Bomb, Nuclear, and Ordinary Chemical Reactions and Equations CHEMICAL AND NUCLEAR REACTIONS Two notable types of nuclear reactions are nuclear fission reactions and nuclear fusion reactions. Nuclear fission is the process of splitting a heavy nucleus, such as uranium or plutonium, into two smaller nuclei of nearly the same mass. During this process, the unstable radioactive nucleus is split into two smaller nuclei. Nuclear fission can occur spontaneously in some cases or can be induced by the bombardment of the nucleus with a variety of particles (e.g., protons, neutrons, or alpha particles) or by gamma rays radiation. During this process, there is a strong repulsion force between the protons. Nuclear fusion is a combination of smaller nuclei to form a large nucleus with the release of large amounts of energy and radiation. Nuclear fusion is the process by which two light atomic nuclei combine to form a single heavier one while releasing massive amounts of energy. Fusion reactions take place in a state of matter called plasma — a hot, charged gas made of positive ions and free-moving electrons with unique properties distinct from solids, liquids, or gases. The sun, along with all other stars, is powered by this reaction. To fuse in our sun, nuclei need to collide with each other at extremely high temperatures, around ten million degrees Celsius. CHEMICAL REACTIONS A chemical reaction is in which the bonds are broken within reactant molecules, and new bonds are formed within product molecules in order to form a new substance. Chemical reactions are all around us, from the metabolism of food in our body to how the light we get from the sun is the result of chemical reactions. Before beginning with chemical reactions, it is important to know about physical and chemical changes. DIFFERENCES Nuclear reactions involve a change in an atom's nucleus, usually producing a different element, along with the emission of radiations like α, β, and γ rays, etc. Chemical reactions, on the other hand, involve only a rearrangement of electrons and do not involve changes in the nuclei. So the nuclear reaction is a nuclear phenomenon and the chemical reaction is an extranuclear phenomenon. Different isotopes of an element normally behave similarly in chemical reactions as their extra-nuclear electronic configurations are the same. The nuclear chemistry of different isotopes varies greatly from each other. Rates of chemical reactions are influenced by external effects like temperature, pressure, and catalysts. Rates of nuclear reactions are spontaneous and unaffected by such factors. Nuclear reactions are independent of the chemical form of the element. This means both in elemental and compound states, the same amount of radio-element shows similar radioactivity. Energy changes accompanying nuclear reactions are much larger. This energy comes from the destruction of mass. In a nuclear reaction, mass is not strictly conserved. Some of the mass is converted into energy, according to the equation E = mc2 and the order of energy evolved during a nuclear reaction is much higher than that of a chemical reaction. THE ATOMIC BOMB The immense destructive power of atomic weapons derives from a sudden release of energy produced by splitting the nuclei of the fissile elements making up the bombs' core. The U.S. developed two types of atomic bombs during the Second World War. The first, Little Boy, was a guntype weapon with a uranium core. Little Boy was dropped on Hiroshima. The second weapon, dropped on Nagasaki, was called Fat Man and was an implosion-type device with a plutonium core. The isotopes uranium-235 and plutonium-239 were selected by atomic scientists because they readily undergo fission. Fission occurs when a neutron strikes the nucleus of either isotope, splitting the nucleus into fragments and releasing a tremendous amount of energy. The fission process becomes self-sustaining as neutrons produced by the splitting of atom strike nearby nuclei and produce more fission. This is known as a chain reaction and is what causes an atomic explosion. When a uranium-235 atom absorbs a neutron and fissions into two new atoms, it releases three new neutrons and some binding energy. Two neutrons do not continue the reaction because they are lost or absorbed by a uranium-238 atom. However, one neutron does collide with an atom of uranium235, which then fissions and releases two neutrons and some binding energy. Both of those neutrons collide with uranium-235 atoms, each of which fission and release between one and three neutrons, and so on. This causes a nuclear chain reaction. THE ATOMIC BOMB The atomic bomb, also called atom bomb, weapon with great explosive power that results from the sudden release of energy upon the splitting, or fission, of the nuclei of a heavy element such as plutonium or uranium. When a neutron strikes the nucleus of an atom of the isotope uranium-235 or plutonium-239, it causes that nucleus to split into two fragments, each of which is a nucleus with about half the protons and neutrons of the original nucleus. In the process of splitting, a great amount of thermal energy, as well as gamma rays and two or more neutrons, is released. Under certain conditions, the escaping neutrons strike and thus fission more of the surrounding uranium nuclei, which then emit more neutrons that split still more nuclei. This series of rapidly multiplying fissions culminates in a chain reaction in which nearly all the fissionable material is consumed, in the process generating the explosion of what is known as an atomic bomb. Einstein (and some others) noted that if you measure the mass of a radium nucleus and watch it decay into lighter products, then measure the combined mass of all those decay products, there is some “missing mass” Δm in the latter. Then they measured the initial kinetic energies of the decay products and added those up to get E . Playing around with the numbers, they found that E/Δm≈0.9×1017 m 2 /s 2, which someone noticed was the square of the speed of light. Then a little light bulb went on over Einstein’s head. THE ATOMIC BOMB The next step was noticing that U-235 not only occasionally fissions spontaneously, liberating a couple of neutrons, but fissions immediately if one of those neutrons slows down and captures to make U-236. There was enough U235 around (in very small concentrations in natural uranium) to do an experiment at the University of Chicago where the neutrons from spontaneous U-235 decay were “moderated” by carbon “control rods” so that they would cause a “chain reaction” in the remaining U-235. This “controlled fission” (the first nuclear reactor) was achieved before the first uncontrolled fission reaction (bomb) was set off at Alamogordo, NM. To do that they just had to slam a bunch of bits of U-235 together hard enough to get them to do their own “neutron moderating”. NUCLEAR REACTIONS BALANCED EQUATIONS (NUCLEAR REACTIONS) A balanced chemical reaction equation reflects the fact that during a chemical reaction, bonds break and form, and atoms are rearranged, but the total numbers of atoms of each element are conserved and do not change. A balanced nuclear reaction equation indicates that there is a rearrangement during a nuclear reaction, but of nucleons (subatomic particles within the atoms’ nuclei) rather than atoms. Nuclear reactions also follow conservation laws, and they are balanced in two ways: The sum of the mass numbers of the reactants equals the sum of the mass numbers of the products. The sum of the charges of the reactants equals the sum of the charges of the products. If the atomic number and the mass number of all but one of the particles in a nuclear reaction are known, we can identify the particle by balancing the reaction. For instance, we could determine that 178π is a product of the nuclear reaction of 147π and 4 1 2π»π if we knew that a proton, 1π», was one of the two products. NUCLEAR EQUATIONS Balancing Equations for Nuclear Reactions The reaction of an α particle with magnesium-25 ( ππ πππ΄π) produces a proton and a nuclide of another element. Identify the new nuclide produced. Solution The nuclear reaction can be written as: ππ π π π¨ πππ΄π + ππ―π βΆ ππ― + ππΏ where A is the mass number and Z is the atomic number of the new nuclide, X. Because the sum of the mass numbers of the reactants must equal the sum of the mass numbers of the products: 25 + 4 = A + 1, or A = 28 Similarly, the charges must balance, so: 12 + 2 = Z + 1 , and Z =13 Check the periodic table: The element with nuclear charge = +13 is aluminium. Thus, the product is ππ πππ¨π. Check Your Learning The nuclide πππ πππ° combines with an electron and produces a new nucleus and no other massive particles. What is the equation for this reaction? ANSWER: πππ πππ π πππ° + −ππ βΆ πππ»π MORE NUCLEAR EQUATIONS The first naturally occurring unstable element that was isolated, polonium, was discovered by the Polish scientist Marie Curie and her husband Pierre in 1898. It decays, emitting α particles: πππ πππ π πππ·π βΆ πππ·π + ππ―π The first nuclide to be prepared by artificial means was an isotope of oxygen, O-17. It was made by Ernest Rutherford in 1919 by bombarding nitrogen atoms with α particles: ππ π ππ΅ + ππ―π βΆ ππππΆ + πππ― James Chadwick discovered the neutron in 1932, as a previously unknown neutral particle produced along with C-12 by the nuclear reaction between Be-9 and He-4: π π ππ©π + ππ―π βΆ ππππ·π + πππ The first element to be prepared that does not occur naturally on the earth, technetium, was created by bombardment of molybdenum by deuterons (heavy hydrogen, 21π»), by Emilio Segre and Carlo Perrier in 1937: ππ π ππ― + πππ΄π βΆ 2 πππ + ππ πππ»π The first controlled nuclear chain reaction was carried out in a reactor at the University of Chicago in 1942. One of the many reactions involved was: πππ π πππΌ + ππ πππ π βΆ ππ πππ©π + πππ³π + 3 ππ 08 ELECTRICITY Generation of Electricity by Nuclear Reactions GENERATION OF ELECTRICITY Nuclear energy can be used to create electricity, but it must first be released from the atom. In the process of nuclear fission, atoms are split to release that energy. A nuclear reactor, or power plant, is a series of machines that can control nuclear fission to produce electricity. The fuel that nuclear reactors use to produce nuclear fission is pellets of the element uranium. In a nuclear reactor, atoms of uranium are forced to break apart. As they split, the atoms release tiny particles called fission products. Fission products cause other uranium atoms to split, starting a chain reaction. The energy released from this chain reaction creates heat. The heat created by nuclear fission warms the reactor's cooling agent. A cooling agent is usually water, but some nuclear reactors use liquid metal or molten salt. The cooling agent, heated by nuclear fission, produces steam. The steam turns turbines, or wheels turned by a flowing current. The turbines drive generators or engines that create electricity. Rods of material called nuclear poison can adjust how much electricity is produced. Nuclear poisons are materials, such as a type of the element xenon, that absorb some of the fission products created by nuclear fission. The more rods of nuclear poison that are present during the chain reaction, the slower and more controlled the reaction will be. Removing the rods will allow a stronger chain reaction and create more electricity. 9/10 APPLICATION AND EFFECTS OF RADIOACTIVITY Hazards and Examples of radioactivity applications. HAZARDS AND APPLICATION Radioactive substances can be dangerous if one is exposed to their radiation for a long time. Some of the radiations are highly penetrating and if the body is over-exposed to them, they can destroy the cells in tissues and upset the natural chemistry of the body. Examples include: – Leukaemia – Skin burns At high doses, ionizing radiation can cause immediate damage to a person's body, including, at very high doses, radiation sickness and death. At lower doses, ionizing radiation can cause health effects such as cardiovascular disease and cataracts, as well as cancer. Nuclear radiation can impact the environment in three primary ways: improper disposal of nuclear waste, direct exposure via disasters, and the mining process of uranium. While nuclear power plants do not emit very much pollution, they do produce radioactive waste as a byproduct. Some plants dispose of nuclear waste – particularly waste with lower levels of radiation than is harmful to human health – using landfills or by releasing it into lakes and rivers. Unknown leaks of nuclear waste can also find their way into the environment, as can damage permanent underground housing facilities for nuclear waste. Disasters provide a similar danger to the environment and surrounding ecosystems, simply on a larger and more destructive scale. Accidents can happen, and the impact of an accident and a nuclear power plant can catastrophic consequences for human health and the environment. Disasters can directly expose those in the vicinity to high levels of radiation; wind and water can carry radiation long distances, and radiation can remain in the soil for many years. Nuclear power requires the use of uranium, which companies must mine from the ground to obtain. Uranium mining provides a slew of environmental impacts. Some facilities dispose of the byproducts of uranium mining, known as tailings, in the surrounding area of the mine. These not only expose the area to radiation, which can spread through the air or leach into the water but also pose the risk of heavy metal contamination as well. APPLICATION • • • • Carbon Dating: A special application of this type of radioactivity age method is carbon-14 dating, This application has proven to be useful, especially to physical anthropologists and archaeologists. Additionally, it has helped researchers to better determine the chronological sequence of past events by enabling them to date fossils and artifacts from 500 to 50,000 years old. In agriculture, radiation and radioisotopes are also used in the nutritional studies of trace elements, mechanism of photosynthesis, plant protection including the action of insecticides, metabolisms in plants, uptake of fertilizers, ions mobility in soils and plants, and food preservation. Therapeutic applications of radioisotopes typically are intended to destroy the targeted cells. This approach forms the basis of radiotherapy, which is commonly used to treat cancer and other conditions involving abnormal tissue growth, such as hyperthyroidism. For the most part, radioactivity has the most important industrial applications in power generation as a result of the release of the fission energy of uranium. Other applications include the use of radioisotopes to measure/control the thickness/density of metal and plastic sheets. Industrialists also find uses of radioactive substances in the following works: Firstly, to stimulate the cross-linking of polymers. Secondly, to induce mutations in plants to develop harder species. Thirdly, to preserve certain kinds of foods by killing microorganisms that cause spoilage. Lastly, in tracer applications, radioactive isotopes are employed. For instance, in automobile engines, we find the uses of radioactive substances measuring the effectiveness of motor oils on the wearability of alloys for piston rings and cylinder walls. 11 STABILITY OF NUCLIDES AND HALF LIFE Factors affecting the stability of Nuclides and Calculations FACTORS AFFECTING THE STABILITY OF NUCLIDES • • • • Binding Energy: The heavier the nucleus, the greater the internal repulsive forces due to the greater number of protons, and less energy must be applied to remove a nucleon from the nucleus, hence the binding energy is lower. Thus for lighter nuclei binding energy is more. The greater the binding energy, the more stable the nucleus is. The magnitude of the mass defect is proportional to the nuclear binding energy. Neutron-Proton Ratio: In general, stable nuclei have an approximately equal number of neutrons as protons, and a strong excess of one or the other will result in an unstable nucleus. The ratio of neutrons to protons in a stable nucleus is thus around 1:1 for small nuclei (Z < 20). Half-Life: half-life, in radioactivity, is the interval of time required for one-half of the atomic nuclei of a radioactive sample to decay (change spontaneously into other nuclear species by emitting particles and energy), or, equivalently, the time interval required for the number of disintegrations per second of radioactive material to decrease by one-half. The half-life determines how quickly a radioisotope decays. A long half-life indicates higher stability than a short half-life. The faster it decays the more unstable it is. The ‘half-life’ of a radioactive nucleus is one of its main features with the nature of the radiations it emits. It determines how quickly it will decay and for how long we need to worry about its radiation. Half-lives can range from a fraction of a second to billions of years. CALCULATIONS INVOLVING HALF LIFE
