Chapter 10 Section 10.1 What is Radioactivity? What happens when an element undergoes radioactive decay? How does radiation affect the nucleus of an unstable isotope? How do scientists predict when an atom will undergo radioactive decay? 1
The Discovery of Radioactivity In 1896, Henri Becquerel left uranium salt on a photographic plate He hypothesized that the uranium salt had emitted some unknown invisible rays, or radiation, that had darkened the film. Two years later, Marie and Pierre Curie discovered two new elements: polonium and radium (radioactive). Nuclear Radiation Radiation: The process of emitting energy in the form of waves or particles. Where does radiation come from? Radiation is generally produced when particles interact or decay. The sun (solar) or radioactive isotopes of the elements (terrestrial). Radiation is going through you at this very moment! 2
Nuclear Radiation Background radiation comes from several sources. It is low-level radiation created by radioactive isotopes found in Earth s rocks, soils, and atmosphere Nuclear Radiation Radioactivity The process of nuclear decay is called radioactivity. The force is not large enough to hold a nucleus together tightly. Nuclei of certain isotopes are unstable. To become stable, they emit energy. Change into different isotope of same element or a completely different element. Release different types off radiation 3
Isotopes What s an isotope? Two or more varieties of an element having the same number of protons but different number of neutrons. Certain isotopes are unstable and decay to lighter isotopes or elements. Nuclear Radiation When an unstable nucleus decays, particles and energy called nuclear radiation are emitted from it. Alpha, beta and neutron radiation are particles. Gamma radiation is an electromagnetic wave. Radiation type Symbol Mass (kg) Charge Alpha particles (α) 6.646 x 10-27 +2 Video Types of radiation Video Beta particles (β) Gamma-rays (γ) Neutrons 9.109 x 10-31 none 1.975 x 10-27 -1,(+1) 0 0 4
Alpha Particles Nuclear Radiation Madeoftwo protons and two neutrons is emitted from the decaying nucleus. Alpha particle =helium nucleus Same symbol Has an electric charge of +2 and an atomic mass of 4 decreases the atomic number by 2 and the mass number by 4 Alpha Particles Nuclear Radiation Much more massivecompared to others They also have the most electric charge. The least penetrating form of nuclear radiation. Can be stopped by a sheet of paper. 5
Note: This is the atomic weight, which is the number of protons plus neutrons Alpha Particles (α) Radium R 226 88 protons 138 neutrons Radon Rn 222 86 protons 136 neutrons + n p p n α ( 4 He) 2 protons 2 neutrons The alpha-particle (α) is a Helium nucleus. It s the same as the element Helium, with the electrons stripped off! Nuclear Radiation Beta Particles A neutron decays into a proton Emits a fast moving electron (beta particle). 0-1e May be positively charged (+), positrons 0 +1e Not as massive as alpha particles, so they can pass through thicker substances 6
Beta Particles (b) Carbon C 14 6 protons 8 neutrons Atomic Number Changes Nitrogen N 14 7 protons 7 neutrons + e - electron (beta-particle) Neutron decays into a proton and emits an electron. Because the atom now has one more proton,it becomes the element with an atomic number (proton number) one greater than that of the original element. The mass number of the new element is the same because the total number of protons and neutrons does not change during beta decay Try this!! Identify the type of radiation? A B Beta particle C Alpha particle K Alpha particle D Ar + 38 38 0 19 18 + 1 Positron particle e 7
Nuclear Radiation Gamma Rays γradiation is electromagnetic radiation. The nuclear structure stays the same, it simply represents a loss of energy Happens in conjunction with Beta or Alpha emissions. Rather than emit another particle, excess energy is given off in gamma rays. Gamma Ray=High Energy Photon Nuclear Radiation Gamma Rays They have no massand no charge and travel at the speed of light. The most penetrating form of nuclear radiation. Lead and concrete, are required to stop gamma rays. 8
Gamma Rays Neon Ne 20 10 protons 10 neutrons (in excited state) Neon Ne + 20 10 protons 10 neutrons (lowest energy state) gamma The gamma from nuclear decay is in the X- ray/ Gamma ray part of the EM spectrum (very energetic!) 9
Nucleus Symbol Explained A nucleus can be represented by a symbol that includes its atomic number, mass number, and the symbol of the element it belongs to. The symbol for the nucleus of the stable isotope of carbon is shown: Nuclear Decay Nuclear Equation shows the radioactive decomposition of an element 14 6 C 14 7N + 0-1e Use the letter X to denote the unknown product. Note that the mass and atomic numbers of the unknown isotope are represented by the letters A and Z. 10
Alpha Emission Nuclear Decay 234 92 U 90Th + 238 4 2 He parent nuclide daughter nuclide alpha particle Numbers must balance!! Alpha particle = helium nucleus Alpha Emission Math Problem Mass number = sum of mass number of products. Atomic number = sum of atomic number of products. 234 92 U 90Th + 238 4 2 He Mass number 238 = 234 + 4 Atomic number 92 = 90 + 2 11
Beta Emission Nuclear Decay 131 53 I 54Xe + 131 0-1 e Positron Emission electron K Ar + 38 38 0 19 18 + 1 e positron Beta particle electron is given off Beta Emission Math Problem Mass number before and after the decay does not change. Atomic number of the product increases by 1, so element changes. 131 53 I 54Xe + 131 0-1 e Mass number 131 = 131 + 0 Atomic number 53 = 54 + (-1) 12
Positron Emission Math Problem Mass number before and after the decay does not change. Atomic number of the product decreases by 1, so element changes. K Ar + 38 38 0 19 18 + 1 e Mass number 38 = 38 + 0 Atomic number 19 = 18 + (+1) Nuclear Decay Electron Capture Electron is captured by isotope and turns into different element 0 47 Ag + -1e 106 106 46 Pd electron Mass number 106 + 0 = 106 Atomic number 47 + (-1) = 46 13
Nuclear Decay Practice Problems Identify the unknown product. 234 92 U 90Th + 238 4 2 He 131 53 I 54Xe+ 131 0-1 e K Ar + 38 38 0 19 18 + 1 e 0 47Ag+ -1e 106 106 46 Pd Transmutation Transmutationis the process of changing one element to another through nuclear decay. In alpha decay, two protons and two neutrons are lost from the nucleus. 14
Transmutation The new element has an atomic number (protons) two lessthan that of the original element. The new element has a mass (proton + neutron) number four lessthan the original element. In this transmutation, polonium emits an alpha particle and changes into lead. Radioactive Decay Rates It is impossible to predict the moment when any particular nucleus will decay, but it is possible to predict the time required for half of the nuclei in a given radioactive sample to decay. 15
Half-Life Radioactive Decay Rates The half-life (h) is the time it takes for half the atomsof a radioactive substance to decay. Radon Polonium ½ ½ ½ ½ Radioactive Half-Life Some radioisotopes decay to stable atoms in less than a second and some require millions of years. 16
Radioactive Half-Life The nucleus left after the isotope decays is called the daughter nucleus. Bill Nye Half Life Original Element New Element 17
Radioactive Half-Life Half-lives vary widely among the radioactive isotopes. The half-lives of some radioactive elements are listed in the table. Radioactive Decay of a Sample of Uranium-238 18
Half-Life Math Problem For example, suppose we had 20,000 atoms of a radioactive substance. If the half-life is 1 hour, how many atoms of that substance would be left after: Half-lives #atoms remaining % of atoms remaining Time 0 1 2 20,000 10,000 5,000 100% 50% 25% 0 hours 1 hour 2 hours 3 2,500 12.5% 3 hours Radioactive Decay Rates You just won $1,000, but you can only spend half of it in month 1 half of the remainder in month 2, etc. After how many months would you be left with less than $1? What is the half life for this prize? Half live 0 1 2 3 4 5 6 7 8 9 10 11 Time 0 months 1 month 2 months 3 months 4 months 5 months 6 months 7 months 8 months 9 months 10 months Amount 1,000 500 250 125 62.5 31.25 15.625 7.8125 3.906 1.953 0.976 19
Radioactive Half-Life Practice Problems 1. How many grams of iodine 131 (half life-5 days) would be left after 20 days if you start with 25 grams? Answer: 1.56 g The half life is 5 days Number of half-lives passed Amount of Matter Time 0 Started with 25 g 0 { days} 1 How Much is left 12.5g 5 days 2 How Much is left 6.25 g 10 days 3 How Much is left 3.12 g 15 days 4 How Much is left 1.56 g 20 days 5 How Much is left How long will it take 600 grams of Plutonium 239 (half life 24,000 years) to decay to 18.75 grams? 120,000 yrs Number of half-lives passed The half life is Amount of Matter 24,000 yrs Time 0 Started with 600 g 0 yrs 1 How Much is left 300 g 24,000 yrs 2 3 4 5 How Much is left How Much is left How Much is left How Much is left 150 g 48,000 yrs 75 g 72,000 yrs 37.5 g 96,000 yrs 18.75 g 120,000 yrs 20
K-42 has a half-life of 15.5 hrs. If 13.125g of K-42 remains undecayedafter 62.0 hours, what was the original sample size? 210 g Number of half-lives passed The half life is Amount of Matter 15.5 hrs Time 0 Started with 210 g 0 hrs 1 How Much is left 105 g 15.5 hrs 2 How Much is left 52.5 g 31 hrs 3 How Much is left 26.25 g 46.5 hrs 4 How Much is left 13.125 g 62 hrs 5 How Much is left Your turn! 1. Thallium-208 has a half-life of 3 min. How long will it take for 120.0 g to decay to 7.50 g? 2. An isotope of cesium (cesium-137 has a half -life of 30 years. If 20 mg of cesium-137 disintegrates over a period of 90 years, how many mg of cesium-137 would remain? 3. If 60 g of Lithium-9 has a half-life of 100 years, how long will it take for lithium-9 to decay to 15 g? 21
The half life is Number of half-lives passed Amount of Matter Time 0 Started with 1 How Much is left 2 How Much is left 3 How Much is left 4 How Much is left 5 How Much is left Secton 2: Nuclear Fission and Fusion What holds the nuclei of atoms together? What is released when the nucleus of a heavy atom is split? What happens when the nuclei of small atoms are joined? Bill Nye Video (25) 22
Nuclear Forces The stability of a nucleus depends on the nuclear forces that hold the protons and neutrons (nucleus) together. The number of neutrons determines the stability of the nucleus. If it has too many neutrons, the nucleus becomes unstable. Nuclear Forces Nuclear Fission Splitting of atom into two or more smaller fragments, releasing neutrons and energy. Tremendous amounts of energy are released during fission Same amount of energy as 6.7 million TNT molecules do when they explode! 23
Nuclear Fission Nuclear Forces The products of a fission reaction usually include several individual neutrons in addition to the smaller nuclei. Neutrons released by fission can start a chain reaction, each cause the division of another nucleus Nuclear Fission Nuclear Forces Nuclear chain reaction - the continuous series of nuclear fission due to neutrons dividing other nucleus from the same sample. critical mass - the minimum amount of a substance than can undergo a fission reaction and can sustain a chain reaction. The chain reaction principle is used in the atomic bomb. Mouse trap Video 24
How it works 2 or more masses of U-235 are surrounded by an explosive. The explosion forces the masses together to form a critical mass and a fission chain reaction occurs rapidly. Control rods are inserted that absorb neutrons. Nuclear Fission Energy produced from fission is used to provide electrical energy to millions of homes and businesses. High levels of exposure cause radiation sickness. Radiation toxic waste Electricity Risk Video Animation 25
Nuclear Fusion Light nuclei combine at extremely high temperatures formingheavy nuclei and releasing energy. Nuclear Forces Nuclear Fusion Nuclear Forces The energy released from fusion is 3-4 times greater than the energy released from fission. The sun and other stars are natural fusion reactors. 26
Why aren t we using Fusion instead of Fission? Ignition temperatures are 100 million Kelvin, and no manmade container can hold this without melting. The reactants are hydrogen, a very abundant gas, and the product is helium, which is harmless. Runaway reactions aren t a problem. Not yet sustainable Fusion Reactors National Spherical Torus Experiment Tokamak Fusion Test Reactor Princeton University 27
Fission vs. Fusion F I S S I O N 235 U is limited danger of meltdown toxic waste thermal pollution F U S I O N fuel is abundant (Hydrogen) no danger of meltdown no toxic waste (Helium) not yet sustainable Section 3: Nuclear Radiation Today Where are we exposed to radiation? What are some beneficial uses of nuclear radiation? What factors determine the risks of nuclear radiation? How is the energy produced by nuclear fission used? 28
Beneficial uses of Nuclear Radiation Some common applications of nuclear radiation medical diagnosis and treatment smoke detectors manufacturing and agriculture Radioactive tracers- short lived isotopes that tend to concentrate in affected cells and are used to locate tumors. Radioactive Tracers show Alzheimer s Disease in a PET scan. Examples of tracers include carbon-11, iodine-131, and sodium-24. 29
Using Nuclear Reactions in Medicine Medicine Medical imaging Radioactive tracers can be ingested and then viewed in the body through imaging equipment. Cancer Treatment radiation kills cancerous cells more easily than healthy cells Tracer Showing Cancer Using Nuclear Reactions in Medicine Treating Cancer with Radioactivity Radiation can be used to stop some types of cancerous cells from growing. An intense beam of gamma rays is focused on the tumor for a short period of time. The gamma rays pass through the body and into the tumor. Healthy cells in the body that grow quickly are also damaged. 30
Using Nuclear Reactions in Medicine Smoke detectors A radioactive source ionizes the air between two electrodes. Thus current flows between them If smoke particles enter this space they stick to the ions and the current is reduced. This reduced current triggers the alarm Using Nuclear Reactions in Medicine Sterilization used to kill germs and sterilize food and plastic equipment Food Irradiation- Food irradiation is a process in which radiation is used to keep food fresh longer and kill germs. 31
Geiger Counter Geiger counters are devices that display (measurements of) radiation levels by detecting the presence of radioactive particles in an area. Radioactive Dating Carbon dating Once a living organism dies, it is no longer taking in any Carbon. C 14 is radioactive, and decays over time. By measuring the activity of C 14 in an object and comparing it with the amount of C 14 which was present initially you can estimate when the organism died 32
Radioactive Dating Used in aging of rocks and fossils found on Earth. The ages of these materials can be determined using radioactive isotopes and their half-lives. Carbon Dating Plants use carbon dioxide when they make food, so all plants contain carbon-14. 33