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Nuclear Reactions vs. Normal Chemical Changes

Nuclear Reactions vs. Normal Chemical Changes

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Nuclear Reactions vs. Normal Chemical Changes

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  1. Nuclear Reactions vs. Normal Chemical Changes • Nuclear reactions involve the nucleus • The nucleus opens, and protons and neutrons are rearranged • The opening of the nucleus releases a tremendous amount of energy that holds the nucleus together – called binding energy • “Normal” Chemical Reactions involve electrons, not protons and neutrons

  2. The Nucleus • Remember that the nucleus is comprised of the two nucleons, protons and neutrons. • The number of protons is the atomic number. • The number of protons and neutrons together is effectively the mass of the atom.

  3. Isotopes • Not all atoms of the same element have the same mass due to different numbers of neutrons in those atoms. • There are three naturally occurring isotopes of uranium: • Uranium-234 • Uranium-235* • Uranium-238

  4. Nuclide Symbols

  5. Nuclide Symbols

  6. Isotopes Two Categories • Unstable – isotopes that continuously and spontaneously break down/decay in other lower atomic weight isotopes • Stable – isotopes that do not naturally decay but can exist in natural materials in differing proportions

  7. Radioactivity • It is not uncommon for some isotopes of an element to be unstable, or radioactive. • We refer to these as radioisotopes. • There are several ways radioisotopes can decay and give off energy known as radiation.

  8. 238 92 234 90 4 2 4 2 He U Th He +  Types of Radioactive Decay Alpha Decay Loss of an -particle (a helium nucleus)

  9. 131 53 131 54 0 −1 0 −1 0 −1 e I Xe e  +  or Types of Radioactive Decay Beta Decay Loss of a -particle (a high energy electron)

  10. 0 0  Types of Radioactive Decay Gamma Emission Loss of a -ray (high-energy radiation that almost always accompanies the loss of a nuclear particle)

  11. Types of Radiation • Alpha (ά) – a positively charged helium isotope - we usually ignore the charge because it involves electrons, not protons and neutrons • Beta (β) – an electron • Gamma (γ) – pure energy; called a ray rather than a particle

  12. Penetrating Ability

  13. Geologic Time • Radioactive Isotopes used in Geologic Dating • Parent Daughter half-life (y) • U-238 Lead-206 4.5 billion • U-235 Lead-207 713 million • Thorium 232 Lead 208 14.1 Billion • K-40 Argon-40 1.3 billion • R-87 Sr-87 47 billion • C-14 N-14 5730 • Half-life = time it takes for 1/2 of the parent mass to decay into the daughter mass

  14. Geologic Time 14Carbon Dating • Dating is accomplished by determining the ratio of 14C to non-radioactive 12C which is constant in living organisms but changes after the organism dies • When the organism dies it stops taking in 14C which disappears as it decays to 14N

  15. Forensic 14Carbon Cases • Dead Sea Scrolls – 5-150 AD • Stonehenge – 3100 BC • Hezekiah’s Tunnel - 700 BC

  16. Forensic 14Carbon Cases ● King Arthur’s Table in Winchester Castle, England 14C dated to 13th century AD ● Cave painting at Lascaux, France 14C dated to 14,000 BC ● Rhind Papyrus on Egyptian math 14C dated to 1850 BC

  17. Forensic 14Carbon Cases ● The Shroud of Turin was 14C dated 1260-1390 AD which suggests that it is a fake ● However, recent evaluation shows that the sample measured was from a medieval patch and/or that it was seriously contaminated with molds, waxes, etc ●New estimates date the shroud from 1300-3000 ybp bases on vanillin retention

  18. Forensic 14Carbon Cases Nuclear testing during 1955-63 put large amounts of 14C into the atmosphere which was incorporated into the enamel of human teeth. Because such testing stopped the 14C input ended and the 14C in the teeth decayed at a fixed rate allowing dating of the teeth

  19. Nuclear Reactions • Alpha emission Note that mass number goes down by 4 and atomic number goes down by 2. Nucleons (nuclear particles… protons and neutrons) are rearranged but conserved

  20. Nuclear Reactions • Beta emission Note that mass number is unchangedandatomic number goes up by 1.

  21. Write Nuclear Equations! • Write the nuclear equation for the alpha decay of radon-222 222Rn  218Po + 4He • Write the nuclear equation for the beta emitter Co-60. 60Co  60Ni + 0e 86 84 2 -1 27 28

  22. BellringerFill in Chart 4 He 2 helium atom without any electrons 0 e -1 high energy electron, no mass with a negative charge 0 γ 0 high energy ray with no mass and no atomic number mass of an electron but a positive charge 0 e +1

  23. Neutron-Proton Ratios • Any element with more than one proton (i.e., anything but hydrogen) will have repulsions between the protons in the nucleus. • A strong nuclear force helps keep the nucleus from flying apart. • Neutrons play a key role stabilizing the nucleus. • Therefore, the ratio of neutrons to protons is an important factor.

  24. Stable Nuclei The shaded region in the figure shows what nuclides would be stable, the so-called belt of stability.

  25. Neutron-Proton Ratios For smaller nuclei (Z  20) stable nuclei have a neutron-to-proton ratio close to 1:1

  26. Neutron-Proton Ratios As nuclei get larger, it takes a greater number of neutrons to stabilize the nucleus.

  27. Stable Nuclei • Nuclei above this belt have too many neutrons. • They tend to decay by emitting beta particles.

  28. Stable Nuclei • Nuclei below the belt have too many protons. • They tend to become more stable by positron emission or electron capture. • Positron = 0e +1

  29. Stable Nuclei • There are no stable nuclei with an atomic number greater than 83. • These nuclei tend to decay by alpha emission.

  30. Radioactive Series • Large radioactive nuclei cannot stabilize by undergoing only one nuclear transformation. • They undergo a series of decays until they form a stable nuclide (often a nuclide of lead).

  31. Nuclear Transformations Nuclear transformations can be induced by accelerating a particle and colliding it with the nuclide. These particle accelerators are enormous, having circular tracks with radii that are miles long.

  32. Measuring Radioactivity • One can use a device like this Geiger counter to measure the amount of activity present in a radioactive sample. • The ionizing radiation creates ions, which conduct a current that is detected by the instrument.

  33. Energy in Nuclear Reactions • There is a tremendous amount of energy stored in nuclei. • Einstein’s famous equation, E = mc2, relates directly to the calculation of this energy. • In chemical reactions the amount of mass converted to energy is minimal. • However, these energies are many thousands of times greater in nuclear reactions.

  34. Nuclear Fission • How does one tap all that energy? • Nuclear fission is the type of reaction carried out in nuclear reactors.

  35. Nuclear Fission • Bombardment of the radioactive nuclide with a neutron starts the process. • Neutrons released in the transmutation strike other nuclei, causing their decay and the production of more neutrons. • This process continues in what we call a nuclear chain reaction.

  36. Trafficking Nuclear Materials This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

  37. Smuggled Plutonium – can identify the reactor type in which the fuel was originally radiated and the type of plant where the material was subsequently reprocessed In 1997, two pieces of stainless steel contaminated with alpha-emitters were found in a scrap metal yard in Germany. Source was identified as a fast-breeder reactor in Obninsk, Russia Man-made Radioactive Isotopes This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

  38. The isotopic composition of plutonium can indicate INTENT In 1994, a small lead cylinder discovered in a garage in Tengen on the Swiss-German border was found to contain plutonium metal, isotopically enriched to 99.7% Weapons-grade Pu-239 Weapons-grade Plutonium This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License