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Chapter 23 Nuclear Chemistry

Chapter 23 Nuclear Chemistry. Nuclear Chemistry. Images of a human heart before and after stress detecting gamma rays from radioactive Tc-99m . Atomic Composition. Protons (+1) electrical charge mass = 1.672623  10  24 g mass = 1.007 atomic mass units ( amu ) Electrons

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Chapter 23 Nuclear Chemistry

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  1. Chapter 23Nuclear Chemistry

  2. Nuclear Chemistry Images of a human heart before and after stress detecting gamma rays from radioactive Tc-99m

  3. Atomic Composition • Protons • (+1) electrical charge • mass = 1.672623  1024 g • mass = 1.007 atomic mass units (amu) • Electrons • negative electrical charge • relative mass = 0.0005 amu • Neutrons • no electrical charge • mass = 1.009 amu

  4. 11B 10B Isotopes • Isotopes are atoms of the same element (same Z) but different mass numbers (A). • Boron-10: 5 protons and 5 neutrons: • Boron-11: 5 protons and 6 neutrons:

  5. Radioactivity • The isolation and characterization of radium & polonium by Marie Curie was one of milestones of chemistry. • It is a credit to her skills as a chemist that she was able to isolate only a single gram of radium from 7 tons of uranium ore. Marie and Pierre Curie

  6. Natural Radioactivity • -particles can be stopped by paper. • -particles require at least a cm of lead (Pb). • -particles require at least 10 cm of lead (Pb). • Energy:  >  > 

  7. Penetrating Ability

  8. Nuclear Reactions • Ernest Rutherford isolated Radium forms Radon gas while studying alpha particle emission. • 1902 Rutherford and Soddy proposed radioactivity was the result of the natural change of the isotope of one element into an isotope of a different element.

  9. Nuclear Reactions Alpha emission • Nucleons must be conserved in any nuclear reaction. • In  emission, the mass number (A) decreases by 4 and the atomic number (Z) decreases by 2.

  10. Nuclear Reactions Beta emission In  emission, the mass number (A) remains unchanged and the atomic number (Z) decreases by 1.

  11. Radioactive Decay Series

  12. 207 207 Other Types of Nuclear Reactions Positron (positive electron) emission • Positrons have the mass of an electron, but positive charge. They are the antimatter analog of an electron. • Positron emission arises from “electron capture”. • An inner shell electron is absorbed by the nucleolus converting a proton into a neutron along with an emitted positron.

  13. Stability of Nuclei • H is most abundant element in the universe. • H represents 88.6% of all atoms • He represents 11.3% of all atoms • Together 99.9% of all atom & 99% of mass of the universe.

  14. Isotopes • Hydrogen: • 11H, protium • 21H, deuterium • 31H, tritium (radioactive) • Helium, 42He • Lithium, 63Li and 73Li • Boron, 105B and 115B • Iron • 5426Fe, 5.82% abundant • 5626Fe, 91.66% abundant • 5726Fe, 2.19% abundant • 5826Fe, 0.33% abundant

  15. Stability of Nuclei • 209Bi with 83 protons and 126 neutrons is the heaviest naturally occurring non-radioactive isotope. • There are 83 x 126 = 10,458 possible isotopes. • Why do so few exist in nature?

  16. Stability of Nuclei • Up to Z = 20 (Ca) stable isotopes often have the same # of neutrons and protons. Only H and He-3 have more protons than neutrons. • Beyond Ca, the ratio of neutrons to protons is >1. • As Z increases, the n:p ratio deviates further from 1:1 • Above Bi all isotopes are radioactive. Fission leads to smaller particles, the heavier the nucleus the greater the rate. • Above Ca: elements of EVEN Z have more stable isotopes than ODD Z elements. • The more stable isotopes have an EVEN number of neutrons.

  17. Out of > 300 stable isotopes: N Even Odd Z 157 52 3115P Even Odd 50 5 21H, 63Li, 105B, 147N, 18073Ta 199F Stability of Nuclei

  18. N Even Odd Z 157 52 Even Odd 50 5 Stability of Nuclei • The trend suggests some PAIRING of NUCLEONS • There are “nuclear magic numbers” • 2 He 28 Ni • 8 O 50 Sn • 20 Ca 82 Pb

  19. Band of Stability and Radioactive Decay Isotopes with low n/p ratio, below band of stability decay, decay by positron emission or electron capture

  20. Binding Energy, Eb • The energy required to separate the nucleus of an atom into protons and neutrons. • For deuterium, 21H • 21H 11p + 10n Eb = 2.15  108 kJ/mol • Eb per nucleon = Eb/2 nucleons • = 1.08  108 kJ/mol nucleons

  21. Calculate Binding Energy For deuterium, 21H: 21H 11p + 10n Mass of 21H: = 2.01410 g/mol Mass of proton: = 1.007825 g/mol Mass of neutron: = 1.008665 g/mol ∆m: = 0.00239 g/mol From Einstein’s equation: Eb = (∆m)c2 = 2.15 x 108 kJ/mol Eb per nucleon = Eb/2 nucleons = 1.08  108 kJ/mol nucleons

  22. Binding Energy/Nucleon

  23. Half-Life • The HALF-LIFE of an isotope is the time it takes for 1/2 a sample to decay from its initial amount. • The rate of a nuclear transformation depends only on the “reactant” concentration. • The decay and half-life for a nuclear reaction follows first order kinetics.

  24. Half-Life After each successive half-life, one half of the original amount remains.

  25. Kinetics of Radioactive Decay Activity (A) = Disintegrations/time = (k)(N) where N is the number of atoms Decay follows first order kinetics: The half-life of radioactive decay is t1/2 = 0.693/k

  26. Radiocarbon Dating Willard Libby (1908-1980) Libby received the 1960 Nobel Prize in chemistry for developing carbon-14 dating techniques. He is shown here with the apparatus he used. Carbon-14 dating is widely used in fields such as anthropology and archeology.

  27. Radiocarbon Dating Radioactive C-14 is formed in the upper atmosphere by nuclear reactions initiated by neutrons in cosmic radiation: 14N + 10n 14C + 1H The C-14 is oxidized to CO2, which circulates through the biosphere. When a plant dies, the C-14 is not replenished. But the C-14 continues to decay with t1/2 = 5730 years. Activity of a sample can be used to date the sample.

  28. Artificial Nuclear Reactions • New elements or new isotopes of known elements are produced by bombarding an atom with subatomic particles such as a protons or neutrons, or even a heavier particles such as 4He and 11B. • Reactions using neutrons are called n, reactions because a -ray is usually emitted. • Radioisotopes used in medicine are often made by n, reactions.

  29. Artificial Nuclear Reactions • An Example of a n, reaction is production of radioactive 32P. • 32P is used in studies of phosphorous uptake in the body.

  30. Transuranium Elements Elements beyond 92 (transuranium) are made via n, reactions.

  31. Transuranium Elements & Glenn Seaborg 106Sg

  32. Nuclear Fission

  33. Nuclear Fission Fission chain reaction has three general steps: Initiation: Reaction of a single atom starts the chain (e.g., 235U + neutron) Propagation: 236U fission releases neutrons that initiate other fissions Termination. Consumption of the fissionable material is completed

  34. Nuclear Fission & Lise Meitner 109Mt

  35. Nuclear Fission & Power • Currently about 103 nuclear power plants in the U.S. and about 435 worldwide. • 17% of the world’s energy comes from nuclear.

  36. Units for Measuring Radiation • Curie: 1 Ci = 3.7  1010distintegrations/s (dps) • SI unit is the becquerel: 1 Bq = 1 dps • Rad: measures amount of energy absorbed 1 rad = 0.01 J absorbed/kg tissue • Rem: “roentgen equivalent man” based on amount and type of radiation. • Quantifies biological tissue damage, usually represented “millirems”.

  37. Effects of Radiation

  38. Effects of Radiation

  39. Nuclear Medicine: Imaging

  40. Nuclear Medicine: Imaging Technetium-99m is used in more than 85% of the diagnostic scans done in hospitals each year. Synthesized on-site from Mo-99. 99m43Tc decays to 9943Tc giving off a  -ray. The half-life of the radioisotope is 6.01 hrs. Once ingested, the Tc-99m concentrates in areas of high activity such as the thyroid.  -ray imagining detects its presence.

  41. Nuclear Medicine: Imaging Imaging of a heart using Tc-99m before and after exercise.

  42. BNCTBoron Neutron Capture Therapy • 10B isotope (not 11B) has the ability to capture slow neutrons • In BNCT, tumor cells preferentially take up a boron compound, and subsequent irradiation by slow neutrons kills the cells via the energetic 10B 7Li neutron capture reaction (that produces a photon and an alpha particle) • 10B + 1n 7Li + 4He + photon

  43. Food Irradiation • Food can be irradiated with  rays from 60Co or 137Cs. • Irradiation retards the growth of bacteria, molds and yeasts. • Irradiated milk has a shelf life of 3 mo. without refrigeration. • USDA has approved irradiation of meats and eggs.

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