Decoding the Atom PDF

NUCLEAR CHEMISTRY AND ENERGY Nuclear Stability Radioactivity Transmutation, Nuclear Fission and Fusion Nuclear Chemistry is the study of reactions involving changes in atomic nuclei or the dense region consisting of protons and neutrons at the center of an atom. All nuclei c...

NUCLEAR CHEMISTRY AND ENERGY Nuclear Stability Radioactivity Transmutation, Nuclear Fission and Fusion Nuclear Chemistry is the study of reactions involving changes in atomic nuclei or the dense region consisting of protons and neutrons at the center of an atom. All nuclei contain two kinds of fundamental particles, proton and neutron, except Recall that the number of protons in the nucleus is called the atomic number (Z) of the element, the sum of the number of protons and number of neutrons is the mass number (A). Nuclide is a single type of Nucleus and it is identified by the notation In addition to writing the symbols for various chemical elements, we must also explicitly indicate protons, neutrons, and electrons. In fact, we must show the numbers of protons and neutrons present in every species in such an equation. For example, is referenced as carbon-14. It has 6 protons and 8 neutrons. Atoms with the same atomic number but different mass number are isotopes of the same element.  From Coulomb’s law we know that like charges repel and unlike charges attract one another. The stability of any nucleus is determined by the difference between coulombic repulsion and the short-range attraction. If repulsion > attraction, the nucleus disintegrates, emitting particles and/or radiation. If attractive forces prevail, the nucleus is stable. The principal factor that determines whether a nucleus is stable is the neutron-to proton ratio (n/p). n/p is close to 1 = low atomic number = stable = nonradioactive n/p > 1 = high atomic number = unstable = radioactive This means that a larger number of neutrons is needed to counteract the strong repulsion among the protons and stabilize the nucleus for an unstable nucleus. Unstable nuclei emit particles and/or electromagnetic radiation spontaneously this is known as radioactivity  n/p is close to 1 = low atomic number = stable = nonradioactive n/p > 1 = high atomic number = unstable = radioactive Nuclear decay occurs when the nucleus of an atom is unstable and spontaneously emits energy in the form of radiation A balanced nuclear reaction equation indicates that there is a rearrangement during a nuclear reaction in the nucleons (subatomic particles within the atoms’ nuclei) rather than atoms. Nuclear reactions also follow conservation laws, and they are balanced in two ways: 1.Conservation of Mass number: The sum of the mass numbers of the reactants equals the sum of the mass numbers of the products. 2.Conservation of Atomic number: The sum of the charges of the reactants equals the sum of the charges of the products. Practice: Find X.  NUCLEAR CHEMISTRY AND ENERGY Nuclear Stability Radioactivity Transmutation, Nuclear Fission and Fusion  NOTABLE SCIENTISTS Henry Antoine Becquerel Henry Antoine Becquerel ►First person to discover the natural evidence of radioactivity ►Radioactivity is the result of an atom trying to reach a more stable nuclear configuration.  NOTABLE SCIENTISTS Pierre and Marie Curie ►worked together in isolating polonium and radium. They were the first to use the term "radioactivity", and were pioneers in its study. ►Together with Becquerel, received the Nobel Prize in Physics in 1903 for their joint research in radiation phenomena.  RADIOACTIVE DECAY The process of radioactive decay, can be achieved via three primary methods: 1.Alpha decay - radioactive nucleus changes into a different more stable nucleus, with 2 fewer protons and 2 fewer neutrons, and a helium nucleus is emitted. The alpha particles lose energy in these collisions and they do not penetrate very far through materials, and can be stopped by just 30cm of air or a sheet of paper. 3.Spontaneous fission - the nucleus will split into two nearly equal fragments and several free neutrons. A large amount of energy is also released. Each individual radioactive substance has a characteristic decay period or half- life. A half-life is the interval of time required for one-half of the atomic nuclei of a radioactive sample to decay. It can range from thousands of years to milliseconds. Cobalt-60, which is used in radiation cancer therapy, has half-life of 5.27 years. Example 4: The half-life of is 11.0 s. If a sample initially contains 5.00 g of , how much remains after 44.0 s? n = number of half-lives n = 44.0 ÷11 n=4 Example 5: The half-life of 44Ti is 60.0 y. A sample initially contains 0.600 g of44Ti. How much 44Ti remains after 180.0 y? n = number of half-lives n = 180.0 ÷60.0 n=3  NUCLEAR CHEMISTRY AND ENERGY Nuclear Stability Radioactivity Transmutation, Nuclear Fission and Fusion  Two types of nuclear transmutation: a.Natural transmutation by stellar nucleosynthesis in the past created most of the heavier chemical elements in the known existing universe, and continues to take place to this day, creating the vast majority of the most common elements in the universe, including He, O and C. In the present, natural transmutation occurs when certain radioactive elements present in nature spontaneously decay by a process that causes transmutation, such as alpha or beta decay.  b.Artificial transmutation – when stable nuclei is bombarded with a particle and forced to become unstable and result in transmutation. It may occur in machinery that has enough energy to cause changes in the nuclear structure of the elements. Such machines include particle accelerators.  Particle accelerators made it possible to synthesize the so-called transuranium elements (elements with atomic numbers greater than 92). It uses electric and magnetic fields to increase the kinetic energy of charged species so that a reaction will occur. Nuclear fission is the process in which a heavy nucleus (mass number > 200) divides to form smaller nuclei of intermediate mass and one or more neutrons. In nuclear fission, atoms are split apart, which releases energy. During nuclear fission, a neutron collides with a uranium atom and splits it, releasing a large amount of energy in the form of heat and radiation. This process releases a large amount of energy because the heavy nucleus is less stable than its products. The first nuclear fission reaction to be studied was that of uranium- 235 bombarded with slow neutrons, whose speed is comparable to that of air molecules at room temperature.  Nuclear chain reaction is a self-sustaining sequence of nuclear fission reactions. The neutrons generated during the initial stages of fission can induce fission in other uranium-235 nuclei, which in turn produce more neutrons, and so on. In less than a second, the reaction can become uncontrollable, liberating a tremendous amount of heat to the surroundings. For a chain reaction to occur, enough uranium-235 must be present in the sample to capture the neutrons. Nuclear reactor -number of fissionable nuclei available is carefully controlled to prevent a “runaway” chain reaction. Atomic bomb –a very rapid growth in the number of fissions is sought. Fission weapons are normally made with materials having high concentrations of the fissile isotopes uranium-235, plutonium-239. Nuclear fusion -the combining of small nuclei into larger ones, is largely exempt from the waste disposal problem. For the lightest elements, nuclear stability increases with increasing mass number. If two light nuclei combine or fuse together to form a larger, more stable nucleus, an appreciable amount of energy will be released in the process. Nuclear fusion occurs constantly in the sun. The sun is made up mostly of hydrogen and helium. Because fusion reactions take place only at very high temperatures, they are often called thermonuclear reactions. In contrast to the fission process, nuclear fusion looks like a very promising energy source. Although thermal pollution would be a problem, fusion has the following advantages: (1)The fuels are cheap and almost inexhaustible and (2)The process produces little radioactive waste. If a fusion machine were turned off, it would shut down completely and instantly, without any danger of a meltdown. Both fission and fusion are nuclear reactions that produce energy, but the processes are very different. Fission is the splitting of a heavy, unstable nucleus into two lighter nuclei, while, Fusion is the process where two light nuclei combine together releasing vast amounts of energy.