nuclear equations, radioactivity, and fission/fusion 1

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3 Facts About the Nucleus, Con’t  Every atom of an element has the same number of protons; equal to the atomic number  Atoms of the same elements can have different numbers of neutrons.  Isotopes  Different atomic masses  Isotopes are identified by their mass number  Mass number = number of protons + neutrons

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Nuclear Equations, Radioactivity, and Fission/Fusion 1 2 Facts About the Nucleus Very small volume compared to volume of the whole atom Essentially entire mass of atom Very dense Composed of protons and neutrons that are tightly held together Nucleons 3 Facts About the Nucleus, Cont Every atom of an element has the same number of protons; equal to the atomic number Atoms of the same elements can have different numbers of neutrons. Isotopes Different atomic masses Isotopes are identified by their mass number Mass number = number of protons + neutrons 4 Facts About the Nucleus, Cont The number of neutrons is calculated by subtracting the atomic number from the mass number. The nucleus of an isotope is called a nuclide. Over 90% of isotopes are radioactive. Therefore, their nucleus is called a radionuclide Each nuclide is identified by a symbol. Element mass number. 5 Radioactivity Radioactive nuclei (radionuclides) spontaneously decompose into smaller nuclei Is called Radioactive Decay We say that radioactive nuclei are unstable The parent nuclide is the nucleus that is undergoing radioactive decay; the daughter nuclide are the new nuclei that are made Decomposing involves the nuclide emitting a particle ( , , etc.) and/or energy All nuclides with 84 or more protons are radioactive 6 Transmutation Rutherford discovered that during the radioactive process, atoms of one element are changed into atoms of a different element transmutation. In order for one element to change into another, the number of protons in the nucleus must change. 7 Chemical Processes vs. Nuclear Processes Chemical reactions involve changes in the electronic structure of the atom. Atoms gain, lose, or share electrons. No change in the nuclei occurs. Nuclear reactions involve changes in the structure of the nucleus. When the number of protons in the nucleus changes, the atom becomes a different element. 8 Nuclear Equations We describe nuclear processes using nuclear equations. Use the symbol of the nuclide to represent the nucleus. Atomic numbers and mass numbers are conserved. Use this fact to predict the daughter nuclide if you know parent and emitted particle. U Th + He Th Pa + e Alpha decay: Beta decay: SAME ON BOTH SIDES Mass numbers: 238 Atomic numbers: 92 SAME ON BOTH SIDES Mass numbers: 234 Atomic numbers: 90 10 What Kind of Decay and How Many Protons and Neutrons Are in the Daughter? Alpha emission giving a daughter nuclide with 9 protons and 7 neutrons. 11 p + 9 n 0 11 What Kind of Decay and How Many Protons and Neutrons Are in the Daughter?, Continued Beta emission giving a daughter nuclide with 10 protons and 11 neutrons. 9 p + 12 n 0 12 What Kind of Decay and How Many Protons and Neutrons Are in the Daughter?, Continued Positron emission giving a daughter nuclide with 4 protons and 5 neutrons. 5 p + 4 n 0 13 Nuclear Equations In the nuclear equation, mass numbers and atomic numbers are conserved. We can use this fact to determine the identity of a daughter nuclide if we know the parent and mode of decay. 14 PracticeWrite a Nuclear Equation for Each of the Following: Alpha emission from Th-238. Beta emission from Ne-24. Positron emission from N-13. 15 Alpha emission from Th-238 Beta emission from Ne-24 Positron emission from N-13 PracticeWrite a Nuclear Equation for Each of the Following, Continued: 16 Detecting Radioactivity To detect when a phenomenon is present, you need to identify what it does: 1. Radioactive rays can expose light-protected photographic film. Use photographic film to detect the presence of radioactive rays film badges. 17 Detecting Radioactivity, Cont 2. Radioactive rays cause air to become ionized. An electroscope detects radiation by its ability to penetrate the flask and ionize the air inside. Geiger-Mller counter works by counting electrons generated when Ar gas atoms are ionized by radioactive rays. 18 Detecting Radioactivity, Cont 3. Radioactive rays cause certain chemicals to give off a flash of light when they strike the chemical. A scintillation counter is able to count the number of flashes per minute Able to measure alpha and beta particles only 19 Natural Radioactivity There are small amounts of radioactive minerals in the air, ground, and water. Its even in the food you eat! The radiation you are exposed to from natural sources is called background radiation. 20 Half-Life Each radioactive isotope decays at a unique rate. Some fast, some slow. Not all the atoms of an isotope change simultaneously. Rate is a measure of how many of them change in a given period of time. Measured in counts per minute, or grams per time. The length of time it takes for half of the parent nuclides in a sample to undergo radioactive decay is called the half-life. 21 Half-Lives of Various Nuclides NuclideHalf-lifeType of decay Th x yrAlpha U x 10 9 yrAlpha C yrBeta Rn secAlpha Th x 10 6 secAlpha 22 Half-Life Half of the radioactive atoms decay each half-life. 23 24 How Long Is the Half-Life of this Radionuclide? 25 PracticeRadon-222 Is a Gas that Is Suspected of Causing Lung Cancer as It Leaks into Houses. It Is Produced by Uranium Decay. Assuming No Loss or Gain from Leakage, if There Is 1024 g of Rn-222 in the House Today, How Much Will There be in 5.4 Weeks? (Rn-222 Half-Life Is 3.8 Days.) 26 PracticeRadon-222 Is a Gas that Is Suspected of Causing Lung Cancer as It Leaks into Houses. It Is Produced by Uranium Decay. Assuming No Loss or Gain from Leakage, if There Is 1024 g of Rn-222 in the House Today, How Much Will There be in 5.4 Weeks? ( Rn-222 Half-Life Is 3.8 Days.), Continued Amount of Rn-222 Number of Half-lives Time (days) 1024 g g g g g g weeks x 7 days/wk = 37.8 38 days Amount of Rn-222 Number of Half-lives Time (days) 16 g g g g g1038 27 Practice How Much of a Radioactive Isotope, Rn-222 (with Half-Life of 10 Minutes) Did You Start with if, After One Hour if You Have 2 g? 28 PracticeHow Much of a Radioactive Isotope, Rn- 222(with Half-Life of 10 Minutes) Did You Start with if, After One Hour if You Have 2 g?, Continued Amount of Rn-222 Number of half-lives Time (min) 128 g00 64 g g g330 8 g440 4 g550 2 g660 Fill in the Number of half-lives and Time columns first, then work backwards up the Amount column. 29 Nonradioactive Nuclear Changes A few nuclei are so unstable, that if their nuclei are hit just right by a neutron, the large nucleus splits into two smaller nuclei. This is called fission. Small nuclei can be accelerated to such a degree that they overcome their charge repulsion and smash together to make a larger nucleus. This is called fusion. Both fission and fusion release enormous amounts of energy. Fusion releases more energy per gram than fission. 30 Fission + energy!! 31 Fission Chain Reaction A Fission Chain Reaction is the process by which neutrons from one reaction cause the fission process to keep continuing Only small number of neutrons needed Many of the neutrons produced in the fission are either ejected from the uranium before they hit another U-235 or are absorbed by the surrounding U-238. Minimum amount of fissionable isotope needed to sustain the chain reaction is called the critical mass. 32 Fission Chain Reaction, Cont 33 Fusion H1H 3 1H1H 4 2 He 1 0n0n deuterium + tritiumhelium-4 + neutron

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