Radioactive Decay - Nuclear Decay Processes [PPT]

Sep 30, 2019
  • Nuclear Chemistry Radioactive Decay
  • The discovery of radiation In 1896 Henri Becquerel made an important discovery. He accidentally had placed a piece of uranium ore on top of an unexposed photographic plate. Later, when the plate was developed, the image of the rock was found on the plate. Based on further experiments, he concluded that the plate had been exposed by rays given off by the uranium.
  • Madame Curie discovers Radium and Polonium Following Becquerel's discovery, Marie Sklodowska Curie and her husband, Pierre Curie, attempted to isolate the "radioactive" material from the uranium ore. 6 In doing so they discovered two new elements, Radium and Polonium, both of which were more radioactive than the original ore.
  • Ernest Rutherford Rutherford investigated this new property of matter and discovered that, in the process of emitting radiation, atoms of one element became atoms of another element. Today, we describe the process of an atom of one element becoming an atom of a different element as transmutation,
  • Rutherford discovered the different types of radiation Rutherford was the first to identify and name two different types of radiation given off when an atom of one element underwent transmutation and became an atom of another element. The two types of radiation he found were: 'The alpha particle (a) •The beta particle (ß) A third type of radiation that was discovered later is called: 'Gamma radiation (y)
  • The properties of the different types of radiation The differences between the three types of radiation can be seen by passing them through an electric field. Lead block w radioactive source positive plate negative plate
  • Characteristics of an alpha particle The alpha particle (u) is deflected to some extent toward the negative plate. This indicated that it is positively (+) charged and has a fairly large mass. Lead block w radioactive source positive plate negative plate Today we know that an oc particle is the same as the nucleus of a He atom.
  • Characteristics of a beta particle Lead block w radioactive The beta particle (ß) is deflected toward the positive plate (+). It is also deflected more than the u particle This indicates that it is negatively (-) charged and has much smaller mass than the u particle. source positive plate negative plate ilß Today we know that a particle is the same as an electron,
  • Characteristics of gamma radiation Lead block w radioactive source Gamma radiation (y) is not deflected toward either the positive (+) or negative (-) plate. This indicates that it has no charge. positive plate negative plate Today we know that gamma rays are a type of electromagnetic radiation made up of photons (packets of energy).
  • Radiation and Table O The Regents Reference Tables provides us with a summary of the different types of radiation: Table O Symbols Used in Nuclear Chemistry Name alpha particle beta panicle (electron) gain Ina radiation neutron prolon positron Notation Ille or 201 e -l e +1 Symbol n on or
  • Penetrating power of radiation The ability of radioactive particles to pass through air and other materials is inversely related to their mass. Alpha particles the least penetrating, they travel only a few centimeters through air. They can be stopped by a single sheet of PaPer. Beta particles more penetrating, they travel several meters through air. They can be stopped by a sheet of Al or plastic. Gamma Rays most penetrating, thick sheets of lead or concrete are required to stop gamma rays.
  • Diagram showing penetrating ability Wood Beta oncre e Gamma
  • Where does the radiation come from? Rutherford suggested that the radiation resulted from the breakdown of the nucleus of an atom, resulting in radiation being given off, and the nucleus of the atom changing into a new element. For instance, the fact that U-238 undergoes alpha decay (emits an u particle) can be shown by this reaction: 234 238 90 92
  • Why does the atom break up? Remember that the nucleus of the atom is held together by the strong nuclear force. This force is normally strong enough to hold the protons and neutrons together. STABLE NUCLEUS Repulsive Attractive sti ong electlic force nuclear fm ce However, sometimes the force of repulsion due to the protons having the same charge overcomes the strong nuclear force and the atom breaks apart.
  • How does beta decay occur? Sometimes an atom will emit a particle when it breaks up. 14 6 O -1 14 Neutron In beta decay a neutron apparently "spits out" o an electron (the ß particle) and becomes a Proton + proton.
  • Balanced nuclear equations Nuclear reactions can be represented by equations. These reactions are governed by two "laws": The law of conservation of mass number — the sum of the mass numbers on the reactant sides must be equal to the sum of the mass numbers on the product side. 14 T. 14 6 + 14 14 This law applies to all nuclear equations !
  • Balanced nuclear equations The second law is: The law of conservation of charge (atomic #) — the sum of the atomic numbers on the reactant sides must be equal to the sum of the atomic numbers on the product side. -1 14 6 6 14 This law applies to all nuclear equations !
  • Predicting products in alpha decay The Law of Conservation of Mass Number and the Law of Conservation of Charge allows us to predict products in a nuclear reaction. For instance, suppose we wanted to predict the atom produced when Radon-222 undergoes alpha decay. The alpha decay of a Rn-222 atom: 222 86 4 He + -2
  • Predicting products Based on the two laws we can predict: The mass number of particle X must be 218. 222 86 218 The charge (atomic #) of particle X must be 84. 222 86 218 84 The symbol of the element can then be determined from the Periodic Table. 218 + PO 86
  • How about beta decay? It works the same way. Let's look at the beta decay of Strontium-90. 90 -1 38 Remember the sum of the mass numbers and atomic number on both sides MUST be the same. So atom X must be: 90 39
  • Using balanced nuclear equations to identify the type of radioactivity. Suppose we know that a particular atom undergoes radioactive decay and we are able to identify the atom that is produced. For instance, Iodine-131 is known to form Xenon-131 when it decays. What radioactive particle must it emit? 131 13k 54 53 Using the Laws of Conservation of Mass # and Charge, we can identify the type of radiation given off. Particle X must be a particle: -1
  • Another type of radioactive decay Some atoms undergo a decay process that produces a positron. A positron has the same mass as an electron, but is positively charged. Symbols for the positron include: positron +1 Positrons are a form of anti-matter. Antimatter is made up of particles with the same properties as normal matter, but are opposite in charge.
  • Positrons are also listed in Table O Table O Symbols Used in Nuclear Chemistry Name alpha particle beta particle (electron) t ron proton posi t Notation "Ille or 20 Symbol e -1 Ill e on or 11)
  • Positron Emission We can use our ability to balance nuclear equations to predict what will be given off when Potassium-37 undergoes positron emission. 37 19 There's only one atom that will work, and that's Argon-37. 37 19 37 18
  • Your turn ! Using the Laws of Conservation of Mass # and Charge, write balanced nuclear equations for the following nuclear reactions: 1. 2. 3. 4. 5. 6. Beta decay of Phosphorus-32 Alpha decay of U-238 Positron decay of Iron-53 32 238 92 53 26 Decay of Oxygen- 17 into Nitrogen- 17 170 42 Decay of Potassium-42 into Calcium-42 19 Decay of Plutonium-239 into Uranium-235 239 94 32 16 34 90 53 25 20Ca + 235 92
  • Half-lifes The rate at which a particular radioisotope decays is described by its half-life. half-life is defined as the time that it takes for one The half of a sample of a radioactive element to decay into another element. The half-life of a radioisotope is dependent only on what the radioisotope is.
  • Table N Selected Radioisotopes Table N provides us with a list of various nuclides, their decay modes, and their half-lifes. Using Table N, what is the decay mode and half- life for Radium-226? Nuclide J .vcs 5.8Fe 131 | 85K r 16N 1.9Ne 269 d 5730 y 175 ms 5.2B v 8,51 min 275 s 8.07 a 172 s 2.44 x v ß- 13 13 Nuclide Name carbon-14 ealei"1ii-37 137 hytlrugen-3 iine• 131 1K)tassium-.37 phosphonw-32
  • Using Table N Table N indicates that Radium-226 undergoes alpha decay. Based on this we can write a balanced nuclear equation to represent this reaction: Ra He + 222R11 226 86 88 This tells us that for every atom of Radium that decays an atom of Radon is produced.
  • Using Half-life Table N also tells us that Radium-226 has a half-life of 1600 years. Decay of 100 grams of Radiuun-226 120 •l CID I CID Stant 226 88 1600y 25 3200y 4SOOy 222 86 Starting with a 100g sample, after 1 half- life (or 1600 years), 50g remain. After another 1600 years, half of the 50g will remain (25g).
  • Carbon- 14 Dating The age of objects that were once alive can be determined by using the C-14 dating test. In this test, scientists determine how much C-14 is left in a sample and from this determine the age of the object. From Table N we can determine that C-14 undergoes ß -1 decay: 14 6 o 14
  • Where does the Carbon-14 come from? Cosmic Rays 14 coa Photosynthesis 14 Decay: C C-14 is created in the atmosphere by cosmic rays. It becomes part of living things through photosynthesis and the food chain. When the plant or animal dies, the C-14 begins to decay.
  • Using C-14 to Age Objects By comparing the amount of C-14 left in a sample to the amount that was present when it was alive, and using the half-life of 5700 years (Table N), one can determine the age of a sample. Decay of Carbon-14 1 20 % ofC-14 Remaining 40 Years • Unknovnu sample contanuino 40% of •nonnal' C-14 Radiactive decay nudicates an age of about -000 Y e al'S.
  • Uranium-238 Series The Uranium-238 Decay Series is used to determine the age of rocks. In this series, the ratio of the U-238 to the Pb-206 is used to determine the age of the rock. 238 234 230 226 222 218 214 210 206 202 80 Loss Loss Of The Uranium-238 Series 82 84 86 Atomic 88 90 number 92 94
  • *When I wos young I used to feel so Olive* so darnerousl In fact, would you I life @ I ejected on orpha particle-„.now look at me. a Spent old atom of Lead-206. Seems that all my life Since then has been nothing but decay, decay,
  • "Parent-daughter" Relationship Parentldaughter decaying process Is h*Jf4ife O rigirt of The rock 2nd half-life 3rd her-life rut üne
  • Aging moon rocks NASA astronauts have retrieved 842 pounds (382 kg) of moon rocks (in many missions), which have been closely studied. The composition of the moon rocks is very similar to that of Earth rocks. Using radioisotope dating, it has been found that moon rocks are about 4.3 billion years old.
  • Sample Half-life Problem 1 A 10 gram of sample of Iodine-13 lundergoes ß decay, what will be the mass of iodine remaining after 24 days? From Table N, the h life of iodine is determined to be approximately 8 days. That means that 24 days is equivalent to 3 half-lifes. The decay of 10 grams of 1-131 would produce: 10 grams —Y 5 grams 2.5 gralns 125 grams 2nd 1st 3rd 1.25 grams of 1-131 would remain after 24 days.
  • Sample Half-life Problem 2 A sample of a piece of wood is analyzed by C-14 dating. The percent of C-14 is found to be 25% of what the original C-14 concentration was. What is the age of the sample? First, let's analyze how many half-lives have taken place. 10000 5000 2500 2nd 1st Two half-lives have gone by while the sample decayed from the original C-14 concentration to 25% of that concentration. Based on Table N, the half-life of C-14 is 5730 years, so... x 5730 veal s 11,460 yem s
  • The End This is the end of the first slide show on nuclear reactions. You may continue learning about nuclear reactions by viewing the second show: Nuclear Chemistry: Fission and Fusion

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