The Case of Melting Ice

Transcription

1 Nuclear Chemistry A is for Atom (15 minutes) part 1 (7:15) part 2 (7:29)

2 The Case of Melting Ice Frosty the Snowman lies melting in the funnels at your lab station. There were no eyewitnesses, but there are several suspects. All the suspects have holes in their alibis. You need to determine the exact time at which Frosty was put into the funnels to melt away, leaving no trace On a separate sheet of paper, immediately record the volume of Frosty s melted remains (water) in your graduated cylinder and note the time on the clock. Make a data table and at regular intervals (you decide how long) record the time on the clock and the volume of water in the graduated cylinder. Stop after about 30 minutes, unless Frosty has completely melted earlier.

3 Analysis: 1. What are the units for the rate at which Frosty melted? 2. Think about making a graph from your data. To determine which axis you will use for volume and which axis for time, recall that slope is rise (y-axis) over run (x-axis). Look at which units you decided to use for the rate of melting. 3. What volume will you start with at the origin of your graph? Why did you choose that number? 4. Estimate when you think Frosty met his demise. Explain how you got your estimate. 5. Using your answers to questions 1 through 4, set up your graph and graph your data. 6. Using your graph, find the exact time Frosty start to melt. How close is this time to the time you estimated in question 4? 7. Describe the shape of your graph. 8. What does your graph tell you about the rate at which Frosty melted and the rate of radioactive decay? 9. Write the equation for the beta decay of carbon Speculate: Do you think any isotopes but carbon-14 could be used for radio dating? Why do you think that?

4 Radioactivity Nucleons - two subatomic particles that reside in the nucleus known as protons and neutrons Isotopes - Differ in number of neutrons only. They are distinguished by their mass numbers U Is Uranium with an atomic mass of 233 and atomic number of 92. The number of neutrons is found by subtraction of the two numbers nuclide - applies to a nucleus with a specified number of protons and neutrons. Radionuclides - Nuclei that are radioactive Radioisotopes - atoms containing these nuclei radionuclides.

5 Types of Radiation Alpha particles Made up of 2 protons and 2 neutrons The nucleus of a He atom 2+ charge Symbolized as a Beta particles Fast moving electrons (or positrons) Symbolized as b Gamma Rays 1960 demo of giger counter and blocking radiation High energy electromagnetic radiation from an excited nucleus No mass; No charge Symbolized as g

6 Nuclear Equation Alpha Decay Uranium-238 undergoes alpha decay in attempt to be stable U Th He 4 2 He = 4 2 a Daughter nuclide: the nuclide formed from the decay Parent nuclide: the original nuclide undergoing decay

7 Nuclear Equations Beta Decay Consists of a stream of beta particles which are high speed electrons. Represented by the symbol 0-1 e, b Iodine-131 is an example of a radioactive isotope that undergoes Beta emission I Xe e -1 e = 0-1b Notice the atomic number increases. Beta emission results in the conversion of a neutron ( 1 0n ) to a proton ( 1 1 p) and an electron. 1 0 n 1 1 p e (the electron comes from the neutron being converted NOT from the electron cloud)

8 Nuclear Equations Positron Emission A positron is a particle that has the same mass as an electron but opposite charge. positron is represented as 0 +1e e = 0 +1b Carbon-11 is an example of a particle that undergoes positron emission C 11 5 B e Notice the atomic number goes down. Positron emission is the effect of converting a proton to a neutron 1 1 p 1 0 n e

9 Nuclear Equations Gamma Decay High energy photons (electromagnetic radiation of a short wavelength) Gamma radiation does not change the mass or atomic number and is represented as 0 0 It almost always accompanies other radioactive emission because it represents the energy lost when the remaining nucleons reorganize into more stable arrangements. Generally you do not show gamma rays when writing nuclear equations

10 Summary of Decay Types Mode of Decay Radiation emitted Change in Atomic # Change in Mass # Alpha emission 4 2 He = 4 2 a -2-4 Beta emission 0-1 e = 0-1b +1 0 Positron emission 0 +1 e = 0 +1b -1 0 Gamma emission 0 0 g 0 0 Explaining Radioactive Decay

11 Half Life No two radioactive isotopes decay at the same rate. Half-life, t 1/2 - the time required for half the atoms of a radioactive nuclide to decay. More stable nuclides have longer half-lives. Nuclide Polonium-214 Astatine-218 Polonium-218 Phosporus-32 Cobalt-60 Carbon-14 Uranium-238 Half-life microseconds 1.6 seconds 3.0 minutes days 5.27 years 5715 years 4.46x10 9 years

12 Nuclear Stability No single rule allows us to predict whether a nucleus is radioactive and might decay Neutron to proton ratio Strong nuclear force exists between neutrons and protons. The more protons packed together the more neutrons are needed to bind the nucleus together. Elements with atomic number Have equal protons and neutrons. Elements with higher atomic numbers have more neutrons to protons. The number of neutrons needed to create a stable nucleus increase more than the number of protons.

13 Energy Changes in Nuclear Reactions E = mc 2 A very familiar equation that shows mass and energy change are proportional. If a system loses mass, it loses energy (exothermic) and if it gains mass, it gains energy (endothermic). The c 2 shows a small mass loss can cause a large energy loss. This is why conservation of mass seems to hold in reactions. For example, combustion of one mole methane loses 9.9 x 10-9 grams. In nuclear reactions, this mass change are much greater, 50,000 times greater than methane combustion.

14 Nuclear Binding Energies Scientists discovered in the that the masses of nuclei combined are always less than these nucleons individually. Mass of 2 protons 2( amu) Mass of 2 neutrons 2( amu) Total = amu The mass of a Helium-4 nucleus is causing a mass defect of amu. The origin of this mass defect is some of the mass is converted to binding energy which binds the nucleons together in the nucleus. Energy then needs to be added to separate these nucleons to overcome this binding energy. This energy added to break the nucleons apart is called nuclear binding energy

15 Nuclear Fusion Low mass nuclei combine to form a heavier more stable nucleus. Occurs in the sun. Mass is decreases as it is converted to energy

16 Nuclear Fission A very heavy nucleus splits into more stable nuclei of intermediate mass. Releases a enormous amount of energy as mass is converted to energy. Scientific American Instant Egghead Fusion & Fission 2:33

17 Food Irradiation

18 Food Irradiation ml Cobalt-60 has a half-life of 5.3 years. This technology has been used routinely for more than thirty years to sterilize medical, dental and household products, and it is also used for radiation treatment of cancer. Radioactive substances emit gamma rays all the time. When not in use, the gamma ray source is stored in a pool of water which absorbs the radiation harmlessly and completely. To irradiate food or some other product, the source is pulled out of the water into a chamber with massive concrete walls that keep any rays from escaping. Medical products or foods to be irradiated are brought into the chamber, and are exposed to the rays for a defined period of time. After it is used, the source is returned to the water tank. Only certain radiation sources can be used in food irradiation. These are the radionuclides cobalt-60 or cesium-137 (used very rarely); X-ray machines having a maximum energy of five million electron volts (MeV); or electron machines having a maximum energy of 10 MeV. Energies from these radiation sources are too low to induce radioactivity in any material, including food.

19 Nuclear Medicine Radioisotopes used in Medicine Iodine-131 is commonly used to treat thyroid cancer, probably the most successful kind of cancer treatment. It is also used to treat non-malignant thyroid disorders. Boron Neutron Capture Therapy using boron-10 which concentrates in malignant brain tumors. The patient is then irradiated with thermal neutrons which are strongly absorbed by the boron, producing high-energy alpha particles which kill the cancer. This requires the patient to be brought to a nuclear reactor, rather than the radioisotopes being taken to the patient. A radioisotope used for diagnosis must emit gamma rays of sufficient energy to escape from the body and it must have a half-life short enough for it to decay away soon after imaging is completed. The radioisotope most widely used in medicine is technetium-99m, employed in some 80% of all nuclear medicine procedures. Applications/Radioisotopes/Radioisotopes-in-Medicine/#.UjpK6qMo7cs

20 Nuclear Fission -decay-a-sweet-simulation-of-a-half-life/