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Unit Five Part B

Unit Five Part B. Nuclear Chemistry. Radioactivity. There are two main types of radioactivity: Natural and Induced. Natural Radioactivity. Occurs in nature Usually large, unstable nuclei Occurs in three ways:  Decay (alpha particle)  Decay (beta particle)  Decay (gamma ray).

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Unit Five Part B

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  1. Unit Five Part B Nuclear Chemistry

  2. Radioactivity • There are two main types of radioactivity: Natural and Induced

  3. Natural Radioactivity • Occurs in nature • Usually large, unstable nuclei • Occurs in three ways: •  Decay (alpha particle) •  Decay (beta particle) •  Decay (gamma ray)

  4. Alpha Decay • A helium nucleus is released from the nucleus. ( ) • The mass decreases by 4 • The atomic number decreases by 2 (Because the He nucleus has 2p+ and 2no) • Alpha radiation can be stopped by a piece of paper. Cannot penetrate skin. Not dangerous.

  5. Alpha Decay Example Notice that the uranium has changed into a new element, thorium.

  6. Beta Decay • An electron is released from the nucleus when a neutron becomes a proton. • The mass is unaffected. (the mass of a neutron is roughly equal to the mass of a proton) • The atomic number is increased by 1. • Harder to stop and more dangerous.

  7. Beta Decay Example Notice that carbon has changed into nitrogen.

  8. Gamma Decay • Pure energy is released from the nucleus. • The mass and atomic number are unaffected. • Stopped by lead. The most harmful to living tissue.

  9. Gamma Decay Example No new element formed. Gamma radiation (energy) released.

  10. Induced Radioactivity • Particles are slammed together to cause transmutation of stable elements. (Nuclear Bombardment) • Discovered by Rutherford in 1919.

  11. Uranium-238 Decay Series

  12. Radioactive Decay of U-238 • Uranium-238 becomes Thorium-234 • Transmutation by Alpha Decay

  13. Radioactive Decay of U-238 Thorium-234 becomes Protactinium-234 Transmutation by Beta Decay

  14. Radioactive Decay of U-238 Protactinium-234 becomes Uranium-234 Transmutation by Beta Decay

  15. Radioactive Decay of U-238 Uranium-234 becomes Thorium-230 Transmutation by Alpha Decay

  16. Radioactive Decay of U-238 Thorium-230 becomes Radium-226 Transmutation by Alpha Decay

  17. Radioactive Decay of U-238 Radium –226 becomes Radon-222 Transmutation by Alpha Decay

  18. Radioactive Decay of U-238 Radon-222 becomes Polonium-218 Transmutation by Alpha Decay

  19. Radioactive Decay of U-238 Polonium-218 becomes Lead-214 Transmutation by Alpha Decay

  20. Radioactive Decay of U-238 Lead-214 becomes Bismuth-214 Transmutation by Beta Decay

  21. Radioactive Decay of U-238 Bismuth-214 becomes Polonium-214 Transmutation by Beta Decay

  22. Radioactive Decay of U-238 Polonium-214 becomes Lead-210 Transmutation by Alpha Decay

  23. Radioactive Decay of U-238 Lead-210 becomes Bismuth-210 Transmutation by Beta Decay

  24. Radioactive Decay of U-238 Bismuth-210 becomes Polonium-210 Transmutation by Beta Decay

  25. Radioactive Decay of U-238 Polonium-210 becomes Lead-206 Transmutation by Alpha Decay Lead-206 is stable. (phew!)

  26. Half-Life • The time it takes for half of the atoms in a given radioactive sample to decay into a more stable isotope. • This number is different for each kind of isotope of any kind of element. • Can be calculated because atoms decay at a predictable rate. • Half lives can range from fractions of a second to millions of years.

  27. Half-Life • Two formulas will help you solve half life problems: 1. Half-Life = Total time n 2. Final Mass = Total Mass 2 n (n = # decay cycles)

  28. Example Problems • The half-life of technetium is 6.00 hours. What mass of Tc-99 remains from a 10.0 gram sample after 24.0 hours. • First, calculate the number of half-life cycles that have occurred in the time given. n = Total time n = 24.0 hrs n = 4 Half-Life 6.00 hrs • Next, use the value of n to calculate the remaining mass of the sample. Final Mass = Total Mass = 10.0g = 10.0g 2n 24 16 = 0.625g Tc-99

  29. How about another one??? • A 50.0g sample of N-16 decays to 12.5g in 14.4s. What is its half-life? • Final mass = Initial mass 2n 2n = Initial mass 2n = 50.0g n = log(4 ) n = 2 Final mass12.5g log(2) • Half-Life = Total Time n Half-Life = 14.4s 2 Half-Life = 7.20s 50.0g 1 half = 25.0g 2 half = 12.5 g

  30. Sure, one more… why not? • There are 5.0g of I-131 left after 40.35 days. How many grams were in the original sample if its half-life is 8.07 days? Final Mass = Total Mass 2 n 1st : How many cycles have occurred? 40.35 / 8.07 = 5 cycles. 2nd: Rearrange the formula to solve for the original total mass. Total Mass = Final Mass x 2n

  31. So, solve it already!!! 3rd: Solve Total Mass = 5.0g x 25 Total Mass = 5.0g x 32 Total Mass = 160g

  32. Shall we check it???(of course) 160.0g Total Mass At the end of one half life = 80.0g (8.07days) At the end of two cycles = 40.0g (16.14 days) At the end of three cycles = 20.0g (24.21 days) At the end of four cycles = 10.0g (32.28 days) At the end of five cycles = 5.0g (40.35 days)

  33. Using Radioisotopes • Since decay occurs at a predictable rate, we can use the ratio of decayed to undecayed isotopes to… • Determine the age of Organic Matter with Carbon – 14 (Up to 30,000 yrs) • Determine the age of Rocks (and therefore other earth structures) with Uranium – 238 (Millions of yrs.)

  34. More uses for radioisotopes… • Tracers used to detect structure and function of organs (thyroid, gall bladder, GI tract, etc…) • Can also be used to track movement of silt in rivers and nutrient uptake in plants. • Cancer treatment • Food preservation • Sensors in Smoke Detectors • Starters in Fluorescent lamps • Nuclear fuel for power plants

  35. Detection of Radioactivity • Detected with a Geiger Counter. (When ions strike the cylinder of the Geiger counter, it emits an audible click.)

  36. Detection of Radiation • Dosimeter – measures the total amount of radiation that a person has received. Works because photographic film is sensitive to radiation. Usually is worn like a badge. The film is later developed and the exposure to radiation can be measured. • Unit used to measure radiation exposure in humans is the rem. (Stands for Roentgen Equivalent for Man) (Roentgen discovered X-rays.)

  37. Biological Effects of Radiation • Destruction of tissue especially blood and lymph which cells multiply rapidly. • Causes various cancers. • Direct damage to an organism is called Somatic Damage. • Damage that affects reproductive cells is called Genetic Damage. This leads to birth defects in offspring.

  38. Nuclear Fission • A large nucleus is split into two smaller nuclei of approximately equal mass. • The fission of 4.5g of U-235 will satisfy the average person’s energy needs for an entire year. (Equal to about 15 tons of coal.)

  39. Nuclear Fission • The total mass of the products in a fission reaction is slightly less than the mass of the starting materials. • Law of Conservation of Matter does not apply to fission reactions! • This small amount of “missing” mass is converted into a huge amount of energy. (E = mc2) c=300,000,000m/s

  40. Nuclear Fission

  41. Nuclear Fission • A fission reaction can produce a Chain Reaction because each reaction releases high speed neutrons, each capable of starting another fission reaction. • Chain reactions make the fission process sustainable for use in Nuclear Power Plants.

  42. Chain Reaction

  43. Nuclear Fusion • Two small nuclei join to form a large nucleus. • Some mass is converted into energy (even more than fission reactions) • Difficult to produce and control. To overcome the repulsion between nuclei, they must be heated to 40 million kelvins. For this reason, they are sometimes called Thermonuclear Reactions.

  44. Nuclear Fusion • Thermonuclear reactions create the energy produced by the sun and other stars. • Thermonuclear reactions are the source of the destructive power of a hydrogen bomb. • Not (yet?) sustainable for use in nuclear power plants.

  45. Nuclear Fusion

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