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Nuclear Chemistry Table of Contents 1.0 Nuclear Chemistry...3 2.0 Radioactive Decay...3 2.1 Determining the Half Life...3 3.0 Nuclear Transmutation...4 3.1 Fission...5 3.2 Fusion...5 3.3 Radioisotopes...5 3.4 Man-made Elements...6 4.0 Radiation...6 4.1 Alpha, Beta and Gamma Radiation...6 4.2 Nuclear Reactions (similarities and differences)...8 4.3 Fission and Fusion: Risks and Benefits...8 5.0 Balancing Nuclear Reaction Equations...9 6.0 Risks Associated with Radiation...10 6.1 Environmental Exposure...10 6.2 Nuclear Waste and Radioactive Pollution...10 7.0 Benefits of Radioactive Isotopes...10 7.1 Iodine-131, Carbon-14, Uranium 238 and Cobalt-60...11 8.0 Quiz...12
1.0 Nuclear Chemistry Most chemical reactions involve the exchange of electrons present in the valence orbital of chemical elements or molecules. Nuclear chemistry focuses on reactions that involve changes in the nucleus of chemical isotopes. That is, it focuses on changes in the number of protons and neutrons present in chemical species. While most chemical species have a stable chemical nucleus, some do not. The result of an unstable nucleus is the spontaneous breakdown or decay of the nucleus; this leads to the emission of radiation. Nuclear chemistry is also concerned about the rate at which the chemical species will decay and is termed the half life of the chemical species. An unstable species will decay in the hopes of attaining a more stable nucleus. Since the nucleus of a chemical species is what is responsible for the chemical characterization of that species, radioactive decay results in the conversion of one chemical element to another. The instability of the nucleus and the subsequent decay and release of radiation has been harnessed for its energy. The energy released during nuclear reactions is greater than typical chemical reactions. However, there are also risks associated with the harnessing of nuclear energy including nuclear accidents and environmental exposure. 2.0 Radioactive Decay All radioactive chemical species have a constant rate of decay. They do not all decay at the same time, but they decay at the same rate. This rate of decay is measured by determining the time necessary for the radioactivity to decrease by half its original count. Radioactivity was measured in Curie (Ci) and was so named as a result of pioneering work carried out by Marie and Pierre Curie. The current unit of measurement for radioactivity is the Becquerel (Bq), which is one (1) decay per second. 1Ci = 3.7 x 10 10 Bq 2.1 Determining the Half Life Half life is the time needed for exactly half the quantity of the radioactive isotope to decay. 3 Nuclear Chemistry
T = Time Based on the above graph, the half life is the time taken for the amount of radioactive material to decrease from 1 to 0.5. Therefore, the half life of the radioactive isotope would be T1. 3.0 Nuclear Transmutation Transmutation of chemical elements is the conversion of one nuclide into another. The process of nuclear transmutation is a result of radioactive decay or bombardment of a nuclide by radioactive energy. Since chemical elements are defined on the basis of nuclear composition (i.e. protons and neutrons) changes in the number of protons and neutrons change the chemical element. However, most chemical elements are stable when they contain the same number of protons and neutrons. Chemical Equation showing nuclear decay: 234 23892U 90Th + 4 2He Uranium Thorium + Helium Nuclear Chemistry 4
In the above equation Uranium 238 undergoes radioactive decay and is transmuted to thorium 234 and helium. 3.1 Fission Nuclear fission is a process by which the nucleus of an atom is split into smaller products of approximately equal size; the process also results in 2 or 3 neutrons being emitted. However, the sum total of the mass of all the fragments will not be equal to the original mass as some of that mass is lost as energy. Nuclear fission is the process exploited by man for the production of nuclear energy. For nuclear fission to occur a fissile (an isotope than can be split) isotope is bombarded by neutron particles. Below is an example of a fission reaction; this reaction shows the fission of Uranium 235. Uranium 235 is bombarded by neutrons, a process that converts it to uranium 236, which is an unstable isotope. This instability causes the uranium to split into multiple atoms, neutrons and energy. The neutrons released from the fission reaction then continue to bombard more uranium. This process is called a chain reaction and is essential in the production of nuclear energy. 235 92U + 1 1 0n Uranium + 1 Neutron 2 1 0n + 92 Kr + 142 Ba + Energy 2 Neutrons + Krypton + Barium + Energy The energy produced from this reaction is formed from the binding energy associated with the nucleus of the uranium atom and is released in the form of heat. 3.2 Fusion Nuclear fusion, unlike nuclear fission, involves the fusing of two smaller atoms to form a larger atom. This can result in either the release or absorption of energy. The sun utilizes fusion reactions to continually produce energy. The first step in solar fusion is the fusing of two hydrogen atoms to form deuteron and eventually produce helium. This process is a multistage fusion reaction, which starts with the reaction below. 1 H + 1 H 2 H + e + + v Hydrogen + hydrogen Deuteron + Positron + Neutrino 3.3 Radioisotopes A radioisotope is a version of a chemical element that has an unstable nucleus and, as a result, emits radiation as it decays in the process of forming a more stable element. 5 Nuclear Chemistry
Unstable isotopes have also been utilized for many purposes aside from energy production, one such area is medicine. Nuclear medicine uses radiation and radioactive isotopes to obtain diagnostic information about the functioning of a person s organs or, in some cases, to treat diseases. The use of radioisotopes is common practice and carried out by most hospitals as a routine diagnostic and treatment method. 3.4 Man-made Elements Man-made elements or synthetic elements are a group of elements so unstable that they have an extremely short half life. As a result, these elements could not be found naturally on earth. They may have existed at some point but, based on the age of the earth, they would have decayed long ago. As a result these elements will usually only be found in laboratories or nuclear reactors where they are created for specific purposes. One such element is Plutonium (Pu), which can be produced commercially by highly specialized and qualified laboratories. 4.0 Radiation The spontaneous decay of unstable isotopes can sometimes lead to the release of radioactive particles such as alpha, beta, positrons and gamma radiation. Radiation is the process by which energy travels through a medium or through space. Radiation can be categorized under two broad headings: ionizing and non-ionizing radiation. Ionizing radiation includes alpha, beta and gamma radiation, while non-ionizing radiation includes neutron, electromagnetic, thermal and light. Ionizing radiation contains enough energy to ionize some types of atoms by knocking off electrons, which ionizes the atom as a result. 4.1 Alpha, Beta and Gamma Radiation Alpha, beta and gamma radiation can be categorized on the basis of charge, mass, ionizing power and penetrating power. Nuclear Chemistry 6
ALPHA (α) BETA (β) GAMMA (γ) Paper Aluminum Lead The above diagram shows the penetrating power of the three forms of radiation, alpha radiation being the least penetrating and gamma being the most. Alpha decay is as a result of the decay of large nuclei and is composed of two protons and two neutrons bound together, or helium 4 atom (He 2+ ). 222 86Rn Radon 218 84Po + 4 2He Polonium + Alpha (α) Particle Beta decay is a result of the spontaneous changing of a neutron to a proton, an electron and other subatomic particles. The electron is ejected while the proton remains in the nucleus. The ejected electron is referred to as a beta particle. In effect, the nucleus loses a neutron and gains a proton. 3 H 1 Tritium 3 He 2 + 0 e -1 Helium 3 + Beta (β) Particle Gamma (γ) radiation differs from alpha and beta radiation in that it will penetrate matter and it is not composed of particles, as is the case for alpha and beta, but energy (electromagnetic radiation). Non-ionizing radiation is another form of radiation. However, unlike ionizing radiation, it does not carry enough energy to induce the ionization of atoms or molecules. This type of radiation includes radio waves, microwaves, infrared light and visible light. Non-ionizing radiation typically does not have sufficient energy to cause significant damage to biological organisms. 7 Nuclear Chemistry
4.2 Nuclear Reactions (similarities and differences) The concept of nuclear reactions is based on the premise that two or more nuclear particles will collide to produce products different from the original. They fall into the categories of fusion and fission type reactions. Table 4.1 Fission and Fusion Similarities Between Fission and Fusion Both nuclear Fission and Fusion produce far greater amounts of energy compared to chemical reactions. Fission and Fusion reactions have both been used to produce weapons of mass destruction. Both Fission and Fusion are dependent on there being sufficient quantities of the required atoms or nuclear particles to ensure collision occurs. Both Fission and Fusion reactions are chain reactions. Fission chain reactions occur as long as neutrons can interact with a fissile atom. Fusion chain reactions will occur once the required high temperature and pressure can be maintained. Differences Between Fission and Fusion Fission is the breaking or splitting of atomic nuclei. Fusion is the bonding of atomic nuclei or nuclear particles (neutrons and protons) Fission does not normally occur naturally. Fusion reactions can be seen in nature, it s the process that powers the starts. Fission will produce many highly radioactive particles. Fusion produces few radioactive particles Fusion reactions are more environmentally friendly than Fission reaction (which produce large amounts of radioactive waste) 4.3 Fission and Fusion: Risks and Benefits The fission and fusion reaction types both have advantages and disadvantages. Fission reactions utilize very little fuel to initiate, and the fuel required is inexpensive and available. Fission reactions also do not contribute to global warming. Global warming is a result of greenhouse gases, such as carbon dioxide and carbon monoxide, which fission reactions do not emit. However, the possibility of a nuclear meltdown is very real when utilizing fission to produce energy. If a nuclear meltdown occurs, the radioactive waste that will be released is hazardous to the environment. Nuclear waste will remain in the environment for many years and can contribute to significant increases in illnesses such as radiation poisoning and cancer. Fission reactions produce Nuclear Chemistry 8
large quantities of radioactive waste, which has to be properly disposed of and which can be used to produce nuclear weapons. There is also a high initial cost for start-up, as proper infrastructure needs to be in place for effective containment. Fusion power generation is considered to be the way of the future. However it also has its associated advantages and disadvantages. The fuel for fusion reactions is readily available. Fusion reactions require the heavier forms of hydrogen (Deuterium and Tritium), which are readily available. Deuterium is readily available and naturally occurring. Tritium is a naturally occurring radioactive form of Hydrogen (3H) and although not as available as Deuterium it is a byproduct of Deuterium reacting with a neutron. Fusion will not produce any harmful emissions, unlike the combustion of fossil fuels. Unlike fission, fusion reactions will not produce toxic or radioactive waste. Also, if there were a containment breach, the fusion reaction would not be capable of sustaining itself, therefore reducing the area of impact in the case of an accident. The major disadvantages of fusion reactions for power are: 1. The initial investment in energy that is needed to attain the temperature and pressure required is prohibitive. The result is that many countries are unwilling to invest further in developing the technology. 2. The scientists working on developing fusion as a power source have not been able to sustain the reaction sufficiently long enough for a net production of power. 5.0 Balancing Nuclear Reaction Equations Balancing nuclear equations is similar to balancing most chemical equations in that the quantity of reactants must balance to quantity of products. Analyzing the equations below, you will see that the Relative mass and number of protons is equal on both sides of the equation. 241 95Am + 4 2He Americium + Alpha Particle 2 1 0n + 243 97Bk 2 Neutrons + Berkelium 27 13Al + 4 2He Aluminum + Alpha Particle 30 15P + 1 0n Phosphorus + Neutron 6 3Li + 1 0n 0-1e + 4 2He + 3 2He 9 Nuclear Chemistry
Lithium + Neutron Beta Particle + Alpha Particle + Helium 239 93Np Neptunium 239 94Pu + 0-1e Plutonium + Beta Particle 6.0 Risks Associated with Radiation Biological exposure to radioactive material causes ionization within the molecules of the cells. The removal of electrons from the cell, as a result of ionization, causes a reaction between the ions and the atoms present in the cells. This can lead to a number of effects such as damaged tissue or DNA mutations. 6.1 Environmental Exposure Radiation in the environment is measured in units called rem (Roentgen Equivalent Man) and is a measure of the radiation actually absorbed by biological systems. The more rem an individual is exposed to, the more difficult it is to recover from it and the higher the risk of infection, cancer and immune deficiencies. 6.2 Nuclear Waste and Radioactive Pollution Nuclear waste is waste material containing radioactive material. This waste is mostly the result of nuclear energy production (nuclear fission). Although the radioactivity of nuclear waste will decrease over time, it still does require secure storage until it is no longer considered hazardous. One of the major concerns with nuclear waste is the fact that a small quantity can have a devastating effect on biological systems. The other issue being that nuclear waste takes centuries to decay to the point when it can be considered safe. A major concern of nuclear waste is the possibility of environmental contamination and, in particular water supply, contamination. This could cause radioactive waste to spread throughout the population unchecked for many years until finally showing itself in the form of cancer. 7.0 Benefits of Radioactive Isotopes Energy is not the only benefit of radioactive isotopes, in fact radioactive isotopes have been extensively used in medicine and industry. Nuclear Chemistry 10
In medicine, radioactive isotopes have been used for diagnostic purposes, such as molecular imaging, radio-tracers and non-invasive characterization of diseases and disease processes. In industry, radioactive isotopes are used for manufacturing to help improve quality and maintain product standard and plant safety. It is used to determine metal thinking, wear on ball bearings and radioactive tracing for leaking pipes. Radioactive isotopes have also been used for scientific research, such as carbon dating. 7.1 Iodine-131, Carbon-14, Uranium 238 and Cobalt-60 Iodine-131 ( 131 I) is a radioactive isotope produced predominantly as a result nuclear fission of uranium-235. It is hazardous, with a half life of approximately 8 days. During its decay, it emits beta and gamma radiation. However, it has applications in medicine, partially as a result of the importance of iodine in the human body. Iodine is present in food and is concentrated in the thyroid gland, where it is used for the proper functioning of the gland. As a result, Iodine-131 has found diagnostic and therapeutic uses in the detection and treatment of thyroid cancer and other thyroid conditions. Carbon-14 ( 14 C) is a radioactive isotope that occurs naturally on earth and is used heavily in archeology. It is produced when cosmic radiation converts atmospheric nitrogen to carbon. The process has been occurring for such a long time that it is now thought to be in equilibrium. Carbon-14 has a half life of 5,600 years and can be used to determine the age of most organisms. All life on earth will have trace amounts of carbon-14; when an organism dies it no longer takes up fresh carbon and starts to decompose. Carbon-14 will decay at a known rate and can be used to give a good approximation of the age of the organism by analyzing the ratio of carbon-12 to carbon-14. Uranium-238 ( 238 U) is the most common naturally occurring form of uranium; it is non-fissile and has a half life of approximately 4.5 billion years. It has found uses in industry such as radioactive shielding. Uranium-238 emits alpha particles, which can be stopped using any solid material, even paper. It also has a dense enough electron cloud to block beta and gamma rays. Uranium-238 will eventually decay and form the stable Lead-206 ( 206 Pb). The ratio of 238 U to 206 Pb is used to date geological formation through half life calculations. 11 Nuclear Chemistry
8.0 Quiz Fill in the blanks. 1. Solve the equations: 241 95Am + 4 2He + 243 97Bk 2. Solve the equations: 238 92U 234 90Th + 3. 1 H + 1 H 2 H + e + + v The above reaction is an example of a reaction. 4. List 2 similarities and 2 differences between nuclear Fission and nuclear Fusion Similarities: a) b) Differences: c) d) 5. What is the half life of 14 C? 6. What is the final decay product of 238 U? 7. Which radioactive isotope is used in the detection and treatment of human thyroid issues? 8. When writing and balancing nuclear equations, how would you represent: An Alpha Particle A Beta Particle A Positron Nuclear Chemistry 12