Radioactivity & Nuclear Chemistry Mr. Matthew Totaro Legacy High School Chemistry
The Discovery of Radioactivity Antoine-Henri Becquerel designed an experiment to determine if phosphorescent minerals also gave off x- rays. 2
Becquerel s Plate 3
The Discovery of Radioactivity, Continued Bequerel discovered that certain minerals were constantly producing penetrating energy rays he called uranic rays. Like x-rays. But not related to fluorescence. Bequerel determined that: All the minerals that produced these rays contained uranium. The rays were produced even though the mineral was not exposed to outside energy. Energy apparently being produced from nothing? 4
Marie Curie Marie Curie used an electroscope to detect the radiation of uranic rays in samples. By carefully separating minerals into their components, she discovered new elements by detecting the radiation they emitted. Radium named for its green phosphorescence. Polonium named for her homeland. Since the radiation was no longer just emitted from of uranium, she renamed it radioactivity. 5
What Is Radioactivity? Radioactivity = release of tiny, highenergy particles from an atom. Particles are ejected from the nucleus. 6
Properties of Radioactivity Radioactive rays can ionize matter. Cause uncharged matter to become charged. Basis of Geiger counter and electroscope. Radioactive rays have high energy. Radioactive rays can penetrate matter. Radioactive rays cause phosphorescent chemicals to glow. Basis of scintillation counter. 7
Types of Radioactive Rays Rutherford discovered there were three types of radioactivity: 1. Alpha rays (ά): Have a charge of +2 c.u. and a mass of 4 amu. What we now know to be helium nucleus. 2. Beta rays (β): Have a charge of -1 c.u. and negligible mass. Electron-like. 3. Gamma rays (γ): Form of light energy (not particle like α and β). 8
Rutherford s Experiment ++++++++++++ β α γ -------------- 9
Penetrating Ability of Radioactive Rays α β γ 0.01 mm 1 mm 100 mm Pieces of Lead 10
Facts About the Nucleus Very small volume compared to volume of the atom. Essentially entire mass of atom. Very dense. Composed of protons and neutrons that are tightly held together. Nucleons. 11
Facts About the Nucleus, Continued Every atom of an element has the same number of protons; equal to the atomic number (Z). Atoms of the same elements can have different numbers of neutrons. Isotopes. Different atomic masses. Isotopes are identified by their mass number (A). Mass number = number of protons + neutrons. 12
Facts About the Nucleus, Continued The number of neutrons is calculated by subtracting the atomic number from the mass number. The nucleus of an isotope is called a nuclide. Less than 10% of the known nuclides are nonradioactive, most are radionuclides. Each nuclide is identified by a symbol. Element mass number. Uranium 238 13
Important Atomic Symbols Particle Symbol Nuclear symbol Proton p + Neutron n 0 Electron e - Alpha Beta α β, β Positron β, β + 1 1 4 2 H 1 1p 1 0 0 1 n e α 4 2 He β 0 1e 0 1 0 +1 + β 0 1e 14
Radioactivity Radioactive nuclei spontaneously decompose into smaller nuclei. 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 and/or energy. All nuclides with 84 or more protons are radioactive. 15
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. 16
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. 17
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. 18
Alpha Emission (Decay) An ά particle contains 2 protons and 2 neutrons. Helium nucleus. Loss of an alpha particle means: Atomic number decreases by 2. Mass number decreases by 4. 4 2 α 4 2He 222 88 Ra 4 2 He + 218 86 Rn 19
ά Decay 20
Beta Emission (Decay) A β particle is like an electron. Moving much faster. Produced from the nucleus. 0 1 When an atom loses a β particle, its: Atomic number increases by 1. β 0 1e Mass number remains the same. In beta decay, a neutron changes into a proton. 234 Th 0 e + 234 90 1 91 Pa 21
β Decay 22
Gamma Emission Gamma (g) rays are high-energy photons of light. No loss of particles from the nucleus. No change in the composition of the nucleus, however, the arrangement of the nucleons changes. Same atomic number and mass number. Generally occurs after the nucleus undergoes some other type of decay and the remaining particles rearrange. 0 0 γ 23
Positron Emission (Decay) Positron has a charge of 1+ c.u. and negligible mass. 0 Anti-electron. +1 + When an atom loses a positron from the nucleus, its: Mass number remains the same. Atomic number decreases by 1. Positrons appear to result from a proton changing into a neutron. 22 0 + Na e + 11 1 22 10 β 0 1e Ne 24
β + Decay 25
What Kind of Decay and How Many Protons and Neutrons Are in the Daughter? 11 p + 9 n 0 Alpha emission giving a daughter nuclide with 9 protons and 7 neutrons. 26
What Kind of Decay and How Many Protons and Neutrons Are in the Daughter?, Continued 9 p + 12 n 0 Beta emission giving a daughter nuclide with 10 protons and 11 neutrons. 27
What Kind of Decay and How Many Protons and Neutrons Are in the Daughter?, Continued 5 p + 4 n 0 Positron emission giving a daughter nuclide with 4 protons and 5 neutrons. 28
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. 29
Practice Write a Nuclear Equation for Each of the Following: Alpha emission from Th-238. Beta emission from Ne-24. Positron emission from N-13. 30
Practice Write a Nuclear Equation for Each of the Following, Continued: Alpha emission from Th-238. 238 U 4He + 234Th 92 2 90 Beta emission from Ne-24. 24 Ne 0β + 24Na 10-1 11 Positron emission from N-13. 13 β + 7 N 0 13 + 1 6 C 31
Decay Series In nature, often one radioactive nuclide changes in another radioactive nuclide. Daughter nuclide is also radioactive. All of the radioactive nuclides that are produced one after the other until a stable nuclide is made is called a decay series. To determine the stable nuclide at the end of the series without writing it all out: 1. Count the number of a and b decays. 2. From the mass nunmber, subtract 4 for each a decay. 3. From the atomic number, subtract 2 for each a decay and add 1 for each b. 32
U-238 Decay Series 33
Practice Write All the Steps in the U- 238 Decay Series and Identify the Stable Isotope at the End of the Series. α, β, β, α, α, α, α, β, α, β, α, β, β, α 34
Practice Write All the Steps in the U- 238 Decay Series and Identify the Stable Isotope at the End of the Series, Continued. α, β, β, α, α, α, α, β, α, β, α, β, β, α Great Great great granddaughter α Daughter β Granddaughter β granddaughter α U-238 Th-234 Pa-234 U-234 Th-230 α α α β α Ra-226 Rn-222 Po-218 At-218 Bi-214 β α β β α Po-214 Pb-210 Bi-210 Po-210 Pb-206 35
Practice Determine the Stable Isotope at the End of the U-238 Decay Series. α, β, β, α, α, α, α, β, α, β, α, β, β, α 36
Practice Determine the Stable Isotope at the End of the U-238 Decay Series, Continued. α, β, β, α, α, α, α, β, α, β, α, β, β, α 238 92 U 8 α 238-32 92-16? 6 β 206-0 76 + 6? 206 = 82 Pb 37
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. 38
Detecting Radioactivity, Continued 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-Müller counter works by counting electrons generated when Ar gas atoms are ionized by radioactive rays. 39
Detecting Radioactivity, Continued 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. 40
Natural Radioactivity There are small amounts of radioactive minerals in the air, ground, and water. It s even in the food you eat! The radiation you are exposed to from natural sources is called background radiation. 41
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. 42
Half-Lives of Various Nuclides Nuclide Half-life Type of decay Th-232 1.4 x 10 10 yr Alpha U-238 4.5 x 10 9 yr Alpha C-14 5730 yr Beta Rn-220 55.6 sec Alpha Th-219 1.05 x 10 6 sec Alpha 43
How Hot Is It? When we speak of a sample being hot, we are referring to the number of decays we get per minute. For samples with equal numbers of radioactive atoms, the sample with the shorter half-life will be hotter. That is, more atoms will change in a given period of time. 44
Half-Life Half of the radioactive atoms decay each half-life. 100 Radioactive decay Percentage of original sample 90 80 70 60 50 40 30 20 10 0 0 1 2 3 4 5 6 7 8 9 10 Time (half-lives) 45
60000 Decay of Au-198 half-life = 2.7 days Radioactivity (cpm.) 50000 40000 30000 20000 10000 0 0 2 4 6 8 10 12 14 16 18 20 22 Time (days) 46
How Long Is the Half-Life of this Radionuclide? 47
Practice Radon-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.) 48
Practice Radon-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 5.4 weeks x 7 days/wk = 37.8 38 days Amount of Rn-222 Number of Half-lives Time (days) 1024 g 0 0 512 g 1 3.8 256 g 2 7.6 128 g 3 11.4 64 g 4 15.2 32 g 5 19.0 Amount of Rn-222 Number of Half-lives Time (days) 16 g 6 22.8 8 g 7 26.6 4 g 8 30.4 2 g 9 34.2 1 g 10 38 49
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? 50
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?, Continued Fill in the Number of half-lives and Time columns first, then work backwards up the Amount column. Amount of Rn-222 Number of half-lives Time (min) 128 g 0 0 64 g 1 10 32 g 2 20 16 g 3 30 8 g 4 40 4 g 5 50 2 g 6 60 51
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. 52
Fission + energy!! 53
Fission Chain Reaction A chain reaction occurs when a reactant in the process is also a product of the process. In the fission process it is the neutrons. So you only need a small amount of neutrons to start the chain. 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. 54
Fission Chain Reaction, Continued 55
Fissionable Material Fissionable isotopes include U-235, Pu- 239, and Pu-240. Natural uranium is less than 1% U-235. The rest is mostly U-238. Not enough U-235 to sustain chain reaction. To produce fissionable uranium the natural uranium must be enriched in U- 235: To about 7% for weapons grade. To about 3% for reactor grade. 56
Nuclear Power Nuclear reactors use fission to generate electricity. About 20% of U.S. electricity. The fission of U-235 produces heat. The heat boils water, turning it to steam. The steam turns a turbine, generating electricity. 57
Nuclear Power Plants vs. Coal-Burning Power Plants Use about 50 kg of fuel to generate enough electricity for 1 million people. No air pollution. Use about 2 million kg of fuel to generate enough electricity for 1 million people. Produces NO 2 and SO x that add to acid rain. Produces CO 2 that adds to the greenhouse effect. 58
Nuclear Power Plant 59
Nuclear Power Plants Core The fissionable material is stored in long tubes, called fuel rods, arranged in a matrix. Subcritical. Between the fuel rods are control rods made of neutron absorbing material. B and/or Cd. Neutrons needed to sustain the chain reaction. The rods are placed in a material to slow down the ejected neutrons, called a moderator. Allows chain reaction to occur below critical mass. 60
Containment building PLWR Turbine Boiler Condenser Core Cold water 61
PLWR Core Control rods Hot water Fuel rods Cold water 62
Problems with Nuclear Reactors Fukushima Chernobyl 63
Nuclear Fusion Fusion is the combining of light nuclei to make a heavier one. The sun uses the fusion of hydrogen isotopes to make helium as a power source. Requires high input of energy to initiate the process. Because need to overcome repulsion of positive nuclei. Produces 10x the energy per gram as fission. No radioactive byproducts. Unfortunately, the only currently working application is the H-bomb. 64
Fusion 2 1H 1H deuterium + tritium + + 3 4 2He 0n helium-4 + neutron 1 65