Ch Radioactivity. Henry Becquerel, using U-238, discovered the radioactive nature of elements in 1896.

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1 Ch Radioactivity Henry Becquerel, using U-238, discovered the radioactive nature of elements in Radioactivity the process in which an unstable atomic nucleus emits charged particles and energy (a nucleus is unstable because of the ratio of protons to neutrons). Radioisotope any atom containing an unstable nucleus that spontaneously changes into a more stable isotope. Nuclear decay atoms of one element change into atoms of a different element 1

2 Types of Nuclear Decay 1. Alpha Decay radioisotope emits an alpha particle to form a more stable element. Alpha particle = 2 protons & 2 neutrons What element has 2 protons and 2 neutrons? Helium We represent an alpha particle as: 4 He The 4 represents the atomic mass (protons + neutrons). The 2 represents the atomic number (protons) 2

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4 Alpha decay reactions can be written as chemical reactions: Note that the final product, Radon, has 2 less protons and a total of 4 less in its atomic mass (2 protons and 2 neutrons). However, if you total the atomic numbers on the right side of the arrow, they equal the atomic number value on the left side of the arrow its balanced! Alpha particles are the least penetrating type of nuclear radiation. The particles can't travel very far (only a few cm.) and be stopped by a sheet of paper or clothing. 4

5 Work this alpha decay problem: 5

6 2. Beta Decay release of a negatively charged particle (electron) by an unstable nucleus. The electron comes from the breakdown of a neutron into a proton and an electron. The electron is then emitted as the beta particle. Beta particle = 1 electron We represent a beta particle as: 6

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8 We write a beta decay reaction as follows: Note that the number of protons increases because of the change from 1 neutron into a proton (and an electron.) However, the mass stays the same because the loss of 1 neutron is offset by the gain of a proton. Because they are smaller and faster than alpha particles, beta particles have more penetrating power. They can pass through paper but can be stopped by a thin sheet of metal. 8

9 Work this beta decay problem: 9

10 3. Gamma Decay A penetrating ray of energy emitted by an unstable nucleus. The release of this energy packet (photon) allows the nucleus to move down to a more stable energy level. Gamma ray = no mass, no charge, just lots of energy. A gamma particle is written as: γ A gamma reaction would be written as: 10

11 Gamma rays are much more penetrating than either alpha or beta particles. It takes several cm. of lead or several meters of concrete to stop gamma radiation. It often occurs in addition to alpha or beta decay. Gamma particles are very similar to x-rays but have more energy and originate from a nucleus. 11

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13 Effects of Nuclear Radiation Background radiation nuclear radiation that occurs naturally in the environment. Comes from radioisotopes in air, water, rocks, plants, and animals as well as cosmic rays from outer space. Most background radiation is at low levels that are safe to humans. If radiation levels exceed safe levels, atoms can be ionized. In humans, if cells are exposed to nuclear radiation, bonds holding together proteins and DNA may break and the molecules no longer function properly. 13

14 Alpha, beta & gamma particles are all forms of ionizing radiation. Alpha = skin damage similar to a burn, not dangerous unless ingested or inhaled. (Radon gas is a naturally occurring source of alpha particles that is formed when uranium rock undergoes radioactive decay. It can cause cancer in humans because it can be inhaled.) Beta = can penetrate the skin Gamma = can penetrate deeply into the body thus damaging internal organs. 14

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16 Detecting Nuclear Radiation You can't see, hear or feel radioactivity. Current devices used to detect nuclear radiation: 1. Geiger counter = uses a gas-filled tube to measure ionizing radiation. When radiation enters the tube, it ionizes the atoms of the gas. These ions produce an electric current which is then measured. 2. Film badges = A piece of photographic film wrapped in paper. Film is developed and the amount of exposure indicates the amount of radiation exposure for the person wearing the badge. 16

17 Rates of Nuclear Decay Because most materials contain at least trace amounts of radioisotopes, we can estimate how old something is based on its rate of nuclear decay. Half-life = the time required for one half of a sample of a radioisotope to decay. Each radioactive element decays at a specific rate and this rate is constant. 17

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19 Iodine-131 has a half-life of 8.07 days. If you started with a sample of pure iodine-131, after 8 days about 50% of it would have undergone radioactive decay. After 16 days, another ½ of the remaining 50% that was radioactive would undergo radioactive decay (so only 25% would still be radioactive). Beginning 100% is radioactive 8.07 days 50% is still radioactive 16 days 25% " 24 days 12.5% " 32 days 6.25% " 40 days 3.125% " A simple formula can often be used to determine half-lives: # of half-lives elapsed = Total time of decay Time in one Half-life 19

20 Work this: The half-life of iridium-182 is 15 minutes. After 45 minutes, how much iridium-182 will remain in the sample? # of half-lives = 45 min/15 min # of half-lives = 3 Therefore, 100% 50% 25% 12.5% 12.5% of the original sample will still be iridium

21 You may be given the actual sample size and asked to determine how much of the sample is still radioactive. For example: You began experimenting on a 60 gram sample of iridium-182. After 75 days, how much of the sample will still be radioactive? # of half-lives = 75 days/15 days = 5 half-lives 60 g 30 g 15 g 7.25 g g g 21

22 In radiocarbon dating, the age of an object is estimated by comparing the object's carbon-14 levels (half-life of 5730 years) with carbon-14 levels in the atmosphere. Carbon-14 levels in the atmosphere can vary so its not completely accurate. To get a true age, scientists compare the carbon-14 levels in a sample to carbon-14 levels in objects of known age. Objects older than 50,000 years old can not be dated using carbon-14 dating because there is too little carbon-14 present to detect. Potassium-40, uranium-235, and uranium-238 are used to date rocks because they have much greater half-lives. 22

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24 Artificial Transmutation Transmutation = conversion of atoms of one element to atoms of another Artificial transmutations = done by bombarding atomic nuclei with high energy particles such a protons, neutrons, or alpha particles. (Nuclear decay = natural transmutation) Ernest Rutherford performed first artificial transmutations. He exposed gas to nuclear radiation and discovered that nitrogen atoms bombarded with alpha particles would form oxygen-17 isotopes and a hydrogen molecule. 24

25 What would be the correct formula for this reaction? N + He O + H Would it be possible to turn lead into gold?!? Why aren't we doing this?!? 25

26 Transuranium elements elements with atomic numbers greater than 92. All are radioactive and are generally not found in nature. These can be synthesized by the artificial transmutation of a lighter element. Neptunium = 1st transuranium element synthesized (U-238 was bombarded with neutrons, formed U-239 which then underwent beta decay to form N-239) Most transuranium elements are synthesized for research. However, some are used for common applications: Americium-241 is in smoke detectors Plutonium-238 is used to produce energy for space probes 26

27 Transmutations often require particles to be moving at very high speeds so devices called particle accelerators are used. Particle accelerators can accelerate charged particles to a speed close to the speed of light. They then collide with target molecules and produce new substances. (these accelerators are also used to study nuclear structure and led to the discovery of the quark what's a quark?) 27

28 Nuclear Medicine Nuclear medicine is the use of small amounts of radioactive materials to either look inside the body or to treat diseases. Tracers = used to pinpoint problems in the body; should have short half-lives (Ex: Iodine 131 in the thyroid; Glucose molecules tagged with fluorine-18 for cancer) Some radioactive material can be used to destroy cancer cells if it can be localized. 28

29 Fission and Fusion Nuclear force a strong attractive force that binds protons and neutrons together in the nucleus. Electric force a force resulting from particles with like charges repelling each other Who wins nuclear or electric forces? If the protons are very close together, the attractive nuclear force is greater than the repelling electric forces. As the distance between protons increases (the case in larger atoms), the nuclear forces get weaker. A nucleus becomes unstable when the nuclear force can no longer overcome the electric force. **All nuclei with 83 or more protons are radioactive.** 29

30 Hahn and Strassman were attempting to bombard U-238 with neutrons to make bigger elements. Instead, the smaller element barium was produced. What process had they discovered? Nuclear fission the splitting of an atomic nucleus into 2 smaller parts 30

31 Nuclear fission produces 2 smaller atoms, free neutrons (usually 2-3), and LOTS OF ENERGY!! When fission of U-235 occurs, about.1% of the original mass of the reactants is "lost". Its not really lost just converted into energy. Einstein's mass energy equation explains it: E = mc 2 (E = energy, m = mass, c = speed of light) 31

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33 Chain Reactions Chain Reactions = During nuclear fission of U-235, 2-3 neutrons are released. If there is more U-235 around, these neutrons can then bombard the additional U-235 causing more fission reactions to occur. Uncontrolled chain reactions = used in nuclear weapons to produce huge amounts of destructive energy (also what happened at the Chernobyl Power Plant) Controlled chain reactions = some of the extra neutrons are absorbed by nonfissionable materials (this is how we control fission in nuclear power plants) 33

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35 Critical mass = the smallest possible mass of a fissionable material that can sustain a chain reaction. We currently get about 20% of our power from nuclear power plants. The fuel used in these plants is U

36 Advantages of nuclear power: 1. Large quantities of energy are produced. 2. No air pollution Disadvantages of nuclear power: 1. Radioactive wastes are produced that must be stored in isolation for many years. 2. Threat of a runaway reaction 36

37 Nuclear Fusion Fusion a process in which the nuclei of 2 atoms combined to form a larger nucleus. Along with the new product, tremendous amounts of energy are created. 37

38 The sun is powered by the nuclear fusion of two hydrogen atoms into a helium atom. (FYI: 600,000,000 tons of hydrogen undergo fusion each second in the sun!) Advantages: 1. Tremendous amounts of energy produced 2. Produces very little radioactive waste. Disadvantages: 1. Requires temperatures >10,000,000 o C 2. Because matter would be in the plasma form, we must develop vessels to contain it. 38

39 Most likely, fusion reactors will use deuterium and tritium (both hydrogen isotopes) to produce helium. 39

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