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Nuclear Chemistry

Nuclear Chemistry. 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. Isotopes.

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Nuclear Chemistry

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  1. Nuclear Chemistry

  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. Radioactivity • It is not uncommon for some nuclides of an element to be unstable, or radioactive. • We refer to these as radionuclides. • There are several ways radionuclides can decay into a different nuclide.

  5. Types ofRadioactive Decay

  6. 238 92 234 90 4 2 4 2 He U U He +  Alpha Decay: Loss of an -particle (a helium nucleus)

  7. 131 53 131 54 0 −1 0 −1 0 −1 e I Xe e  +  or Beta Decay: Loss of a -particle (a high energy electron)

  8. 11 6 11 5 0 1 0 1 e C B e +  Positron Emission: Loss of a positron (a particle that has the same mass as but opposite charge than an electron)

  9. 0 0  Gamma Emission: Loss of a -ray (high-energy radiation that almost always accompanies the loss of a nuclear particle) - results from a loss of mass when particles splits from the nucleus

  10. 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.

  11. Neutron-Proton Ratios • Neutrons play a key role stabilizing the nucleus. • Therefore, the ratio of neutrons to protons is an important factor.

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

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

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

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

  16. 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).

  17. Half - Life

  18. Half - Life Definition: the amount of time that it takes for half of a radioisotope to naturally decay into another element

  19. I – 131 has a half life of - if 100 g of I- 131 were allowed to sit for 8.07 days 50 g would remain - if allowed to sit for 16 days, 25 g would remain What would the I-131 turn into? (Write the equation using table N.) 131I → 131Xe + -1e -  Half lives are not effected by temp or press

  20. How can we make calculations using half life? (let’s use C-14 as an example) - To determine the mass remaining, begin with the original mass of the sample and divide by two every step down - To determine the time start at 0 (zero) and add one half life

  21. How much radium – 226 will remain of a 10.0 g sample after 4800 years?

  22. Transmutation Def: a rxn in which the nuclei of an element is changed into the nuclei of another element • Natural radioactive decays • (Natural Transmutations), spontaneous

  23. Artificial transmutations • caused by the bombardment of nuclei with high energy particles • can bombard a nuclei with an alpha particle, neutrons, or protons • must be supplied with sufficient energy to overcome force of repulsion from positive nucleus 4He + 27Al 30P + 1n

  24. Artificial transmutations

  25. Identify the missing product: 14N + 4He  X + 1H

  26. Fission • rxnthat involves the splitting of heavy nuclei to produce lighter nuclei • produces tremendous amts of energy (much greater than normal chemrxn)   Conversion of 1.0 g of matter produces E = mc2 = 1.0 g x (3.0 x 108)2 = 9.0 x 1013 J compared to burning of 1.0 g of methane, about a billion times greater

  27. Fission • During fission a nucleus is bombarded with a neutron, which makes the nuclei unstable 1n + 235U 142Ba + 91Kr + 31n + energy

  28. Fusion • the combining of 2 lighter nuclei to produce a larger nuclei • occurs naturally on the sun, high temps needed to fuse together nuclei • 1H + 1H 2H + +1e • 1H + 2H 3He • 3He + 3He 4He + 21H

  29. Beneficial uses of radioisotopes 1. Uses in medicine a. I – 131 used in identifying thyroid problems b. Co – 60 used in treating cancer 2. Uses in Geology/Anthropology a. C – 14 to C – 12 ratio used to date geological organisms b. Pb – 206 and U – 238 used to date geological formations

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