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Chapter 20

Chapter 20. Nuclear Chemistry. HISTORY. Radioactivity was discovered by Henry Bequerel in 1896 by observing uranium salts emit energy. Madame Curie and her husband extended Bequerel’s work on radioactivity

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Chapter 20

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  1. Chapter 20 Nuclear Chemistry

  2. HISTORY Radioactivity was discovered by Henry Bequerel in 1896 by observing uranium salts emit energy. Madame Curie and her husband extended Bequerel’s work on radioactivity Curie was the first to use Radioactivity to describe the spontaneous emission of alpha, beta, and gamma particles from an unstable nucleus. Both Curies suffered the effects of radiation poisoning. Rutherford took over and bombarded gold with alpha particles

  3. Identification of Radiation

  4. Penetrating Effects

  5. Nuclear Chemistry • Nuclear chemistry is the study of reactions that involve changes in the nuclei of atoms. • Radioactive decay is the spontaneous disintegration of alpha, beta, and gamma particles. • Radioactive decay follows first-order kinetics.

  6. NUCLEAR STABILITY Kinetic stability describes the probability that a nucleus will undergo decomposition to form a different nucleus (radioactive decay) Thermodynamic Stability - the potential energy of a particular nucleus as compared with the sum of the potential energies of its component protons and neutrons. (Binding energy)

  7. Stability radioactive decay Light elements are stable if the neutrons and protons are equal, i.e. 1:1 ration, heavier nuclides require a ratio of 1:1.5 Magic numbers of protons and neutrons seem to exist, much like 8 electrons to make elements and ions Nobel.

  8. Magic Numbers Even number protons and neutrons are stable compared to odd ones Magic numbers (protons or neutrons) 2,8,20,28,50,82 and 126 For example tin has 10 stable isotopes, even number, but on either side of elemental tin indium and antimony have only two stable isotopes Nuclei with magic numbers of both protons and neutrons are said to be “doubly magic” and even more stable i.e. Helium-4 two protons and two neutrons and Pb-208 with 82 protons and 126 neutrons Could be shells for nucleons, like electrons

  9. Stability radioactive decay Number of stable nuclides related to numbers of protons and neurons

  10. Nuclear Stability

  11. Stability radioactive decay Nucleons are protons and neutrons The strong nuclear force keeps the nucleus together by overcoming the repulsive force of the protons. Neutrons are present to help dissipate the repulsive forces between the protons As the atomic number (number of protons) increases, so does the number of neutrons to shield the repulsion of the protons All nuclides with 84 or more protons are unstable and radioactive. This means the strong force is only strong enough neutralize the force of 84 protons.

  12. Decay Series

  13. The Half-Lives of Nuclides in the 23892U Decay Series

  14. Types of Radioactive Decay Decay processes Neutron-rich nuclei, converts a neutron to a proton, thus lowering the neutron/proton ration Neutron-poor nuclei, net effect of converting a proton to a neutron thus causing an increase neutron/proton ratio Heavy nuclei, Z>200 just unstable regardless of the neutron/proton ratio, just too many positive protons

  15. Decay Types Alpha particle emitters (Mass number changes) Nuclei with atomic mass number>200 The daughter nuclei contains two fewer protons and two fewer neutrons than the parent U-238, Th-230

  16. Types of Radioactive Decay Decay processes Neutron-rich nuclei, converts a neutron to a proton, thus lowering the neutron/proton ration Neutron-poor nuclei, net effect of converting a proton to a neutron thus causing an increase neutron/proton ratio Heavy nuclei, Z>200 just unstable regardless of the neutron/proton ratio, just too many positive protons

  17. Decay Types Beta particle decay Too many neutrons Atomic number increases, thus more protons Neutron splits into a proton and electron called transmutation. n → P + β Examples Th-234, I-131 1 o 1 0 -1 1 234 0 234 e Th Pa + -1 91 90

  18. Decay Types Electron Capture Neutron-poor nuclides Electron in an inner shell reacts with a proton 11P+ 0-1β → 10n AZX + 0-1β → AZ-1X’ + x-ray No change in mass number Example iron-55

  19. Decay Types Positron Emission Neutron-poor nuclides Same mass as an electron, but opposite charge, the positron emission is opposite beta decay 11P → 10n + 0+1β AZX → AZ-1X’ + 0+1β Example C-11

  20. Decay Types Electron Capture Neutron-poor nuclides Electron in an inner shell reacts with a proton 11P+ 0-1β → 10n AZX + 0-1β → AZ-1X’ + x-ray No change in mass number Example iron-55

  21. Decay Types Gamma Emission 00γ Many nuclear decay daughters are in an elevated, or excited, energy state These meta stable isotopes emit gamma rays to lower their potential energy This emission can be instantaneous, or delayed for sever hours Te-99m has a half life of about 6 hours 9843Tc* → 9843Tc + 00γ

  22. Decay Types Spontaneous Fission Very massive nuclei Z > 103 Usually large amounts of energy are released Usually neutrons are released Example: 25498Cf → 11846Pd + 13252Te + 4 10n

  23. Decay Types Various Types of Radioactive Processes Showing the Changes That Take Place in the Nuclides

  24. Radioactive Decay

  25. Radiochemical Dating n = t/t1/2 t - time, t1/2 - time for a half-life, and n - the number of half-lives At/Ao = 0.5n Ao - amount initially present, At - amount at time t, and n - the number of half-lives If we know what fraction of sample is left (At/Ao) and its half-life (t1/2), we can calculate how much time has elapsed.

  26. Radiocarbon Dating of Artifacts

  27. Calibration Curves

  28. Kinetics of Radioactive Decay Radioactive decay is a first order process, but using atoms instead of concentration Radioactive decay rates Activity is defined as the number of nuclei that decay per unit time A = -ΔN/Δt, the units are usually disintegrations per second or minute (dps), dpm The activity is directly proportional to the number of atoms, thus A(Rate)=kN From Che162 we know the first order rate law is lnN/N0 = -kt Also t1/2 ln1/2N0/N0 = -kt1/2 → t1/2 = 0.693/k

  29. Example problem Fort Rock Cave in Oregon is the site where archaeologists discovered several Indian sandals, the oldest ever found in Oregon. Analysis of the 14C/12C ratio of the sandals gave an average decay rate of 5.1 dpm per gram of carbon. Carbon found in living organisms has a C-14/C-12 ratio of 1.3 X 10-12, with a decay rate of 15 dpm/g C. How long ago was the sage brush in the sandals cut? The half life of carbon-14 is 5730 years. Note dpm is disintegrations per minute

  30. Sample Problem Solution First calculate the rate constant k from the half-life: k=0.693/5730 = 0.000121 yr-1 Substitute into the first order rate equation. ln(N/N0) = kt t = ln(N/N0)/k = ln(15/5.1)/0.000121 t = 8910 years oldsandals

  31. Practice A mammoth tusk containing grooves made by a sharp stone edge (indicating the presence of humans or Neanderthals) was uncovered at an ancient camp site in the Ural Mountains in 2001. The 14C/12C ratio in the tusk was only 1.19% of that in modern elephant tusks. How old is the mammoth tusk?

  32. Practice Radioactive radon-222 decays with a loss of one  particle. The half-life is 3.82 days. What percentage of the radon in a sealed vial would remain after 7.0 days?

  33. Nuclear Transformations Rutherford (1919) was the first to carry out a bombardment reaction, when he combined an alpha particle with nitrogen-14, creating oxygen-17 and a proton The next successful bombardment reaction was done 14 years later when Aluminum-27 to make phosphorus-30 and a neutron If the bombarding particle has a positive charge then repulsion by the nucleus hinders the process, thus particle accelerators are required. Cyclotron and linear accelerator pg850 Neutrons, do not suffer from the repulsive effect Synthetic elements have been made, called transuranium elements

  34. Cyclotron Nuclear reactions can be induced by accelerating a particle and colliding it with the nuclide.

  35. Cyclotron An Aerial View of Fermilab, a High Energy Particle Accelerator Cyclotron.

  36. The Accelerator Tunnel at Fermilab

  37. Linear Accelerator

  38. Linear AcceleratorCyclotron

  39. Detection and Uses of Radioactivity Geiger counter, high energy from radioactive substances ionizes the Ar, thus allowing a current to flow. The more ions the more current, thus more radioactive Scintillation counter, measures the amount of light given off by a phosphor such as ZnS, which is measured by a photometer Badges

  40. Geiger Counter 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

  41. Geiger Counter

  42. Thermodynamic Stability • This is done by comparing the mass of the individual protons and neutrons to the mass of the nucleus itself. The difference in mass is called the mass defect (Δm), which when plugged into E = mC2, or ΔE = ΔmC2 for change in energy • The mass of an atom is always less than the mass of the subatomic particles • Protium is the only exception, since there is no defect • The other isotopes of hydrogen deuterium and tritium have defects • Mass of neutron = 1.008665 amu • Mass of proton = 1.007276 amu • Mass of electron = 0.0005446623amu, note mass of electron is not really necessary in calculations since it subtracts out when finding the difference

  43. Subatomic Particles Particle Mass(g) Charge Electron(e) 9.11 x 10-28 -1 Proton(p) 1.67 x 10-24 +1 Neutron(n) 1.67 x 10-24 0 Particle 6.64 x 10-24 +2 Positron 9.11 x 10-28 +1

  44. Thermodynamic Stability • Just like a molecule is more stable that its atoms, an nucleus in more stable than its individual atoms. • Energy changes for nuclear process are extremely large when compared to normal chemical and physical changes, thus very valuable energy source. • Normal units are expressed per nucleon, in MeV (million electron volts) • MeV = 1.60 X 10-13 J OR amu = 931 MeV • All nuclei have different relative stabilities, see figure 18.9

  45. Sample problem: • Calcualte the changes in mass (in amu) and energy (in J/mol and eV/atom) that accompany the radioactive decay of 238U to 234Th and an alpha particle. The alpha particle absorbs two electrons from the surrounding matter to form a helium atom. Solution (Note: AMU = g/mole) Δm = mass prod. – mass react. Δm = (mass 234Th + mass 42He) - mass238U Δm = (234.43601 + 4.002603) - 238.050788 = -0.004584 amu or -4.584X10-6kg ΔE = mC2↔ ΔE =( -4.584X10-6kg)(2.998X108m/s)2 =-4.120X1011j/mole ΔE = -0.004584 amu X 931 MeV/amu Divide by the mass number to get energy per nucleon, called binding energy

  46. Practice What is the binding energy of 60Ni? The mass of a 60Ni atom is 59.9308 amu. The mass of an electron is 9.10939 x 10-31 kg and 1 amu is 1.66054 x 10-27 kg.

  47. Thermodynamic Stability Revisiting the graph on page 988 • Notice that Iron is the most stable nuclide • ∆E is negative when a process goes from a less stable to a more stable state • In nuclear reactions more stable nuclei can be achieved by combining nuclei (fusion) or splitting a nucleus (fission) • Lighter elements typically undergo fusion, while elements heavier than iron undergo fission.

  48. Thermodynamic Stability • For lighter elements, fusion processes lead to nuclei with greater binding energy, whereas heavy elements are formed through other processes.

  49. Artificial Elements Scientists have been transmuting elements since 1919 when oxygen-17 and hydrogen-1 were produced from nitrogen-14 and  particles. 147N + 42He 178O + 11H Artificial transmutation requires bombardment with high velocity particles. Alpha particles are positivly charged so how do they strike the nucleus, since the nucleus is positivly charged?

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