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Nuclear Chemistry Chapter 21 Nuclear Chemistry Nuclear Chemistry The Nucleus Remember that the nucleus is comprised of the two nucleons, protons and.

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Presentation on theme: "Nuclear Chemistry Chapter 21 Nuclear Chemistry Nuclear Chemistry The Nucleus Remember that the nucleus is comprised of the two nucleons, protons and."— Presentation transcript:

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2 Nuclear Chemistry Chapter 21 Nuclear Chemistry

3 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. (We will look at this in more detail later.)

4 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. Also considered to be the nuclear charge.

5 Nuclear Chemistry 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

6 Nuclear Chemistry There are three types of Radioactivity: 1.Spontaneous Decay 2.Nuclear Fission 3.Nuclear Fusion Radioactivity

7 Nuclear Chemistry Spontaneous Decay 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.

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9 Nuclear Chemistry Alpha Decay: Loss of an -particle (a helium nucleus) He 4242 U Th He

10 Nuclear Chemistry Alpha Decay: Note how to balance a nuclear equation (even easier than a chemical equation): U Th He

11 Nuclear Chemistry Alpha Decay: Note how to balance a nuclear equation (even easier than a chemical equation): U Th He Total on top = 238

12 Nuclear Chemistry Alpha Decay: Note how to balance a nuclear equation (even easier than a chemical equation): U Th He Total on top = 238

13 Nuclear Chemistry Alpha Decay: Note how to balance a nuclear equation (even easier than a chemical equation): U Th He Total on bottom = 92

14 Nuclear Chemistry Alpha Decay: Note how to balance a nuclear equation (even easier than a chemical equation): U Th He Total on bottom = 92

15 Nuclear Chemistry Alpha Decay: Note how to balance a nuclear equation (even easier than a chemical equation): U Th He Total on top = 238 Total on bottom = 92

16 Nuclear Chemistry Typical Balancing Problem: Note how to balance a nuclear equation (even easier than a chemical equation): Ra X ???? He

17 Nuclear Chemistry Typical Balancing Problem: Note how to balance a nuclear equation (even easier than a chemical equation): Ra X ???? He Total on top = 226 Total on bottom = 88

18 Nuclear Chemistry Typical Balancing Problem: Note how to balance a nuclear equation (even easier than a chemical equation): Ra X ???? He Total on top = 226 Total on bottom = 88

19 Nuclear Chemistry Typical Balancing Problem: Note how to balance a nuclear equation (even easier than a chemical equation): Ra X He Total on top = 226 Total on bottom = 88

20 Nuclear Chemistry Typical Balancing Problem: Note how to balance a nuclear equation (even easier than a chemical equation): Ra Rn He Total on top = 226 Total on bottom = 88

21 Nuclear Chemistry Beta Decay: Loss of a -particle (a high energy electron) 0 1 e 0 1 or I Xe e 0 1 n p + e

22 Nuclear Chemistry Beta Decay: Loss of a -particle (a high energy electron) 0 1 e 0 1 or I Xe e p, 78 n

23 Nuclear Chemistry Beta Decay: Loss of a -particle (a high energy electron) 0 1 e 0 1 or I Xe e p, 78 n54 p, 77 n

24 Nuclear Chemistry Beta Decay: Loss of a -particle (a high energy electron) 0 1 e 0 1 or I Xe e p, 78 n54 p, 77 n Good for decreasing the ratio of neutrons to protons!

25 Nuclear Chemistry Positron Emission: Loss of a positron (a particle that has the same mass as but opposite charge than an electron) e 0101 C 11 6 B e 0101 Anti-matter!

26 Nuclear Chemistry Positron Emission: Loss of a positron (a particle that has the same mass as but opposite charge than an electron) e 0101 C 11 6 B e 0101 p n + e

27 Nuclear Chemistry Positron Emission: Loss of a positron (a particle that has the same mass as but opposite charge than an electron) e 0101 C 11 6 B e p, 5 n5 p, 6 n

28 Nuclear Chemistry Positron Emission: Loss of a positron (a particle that has the same mass as but opposite charge than an electron) e 0101 C 11 6 B e p, 5 n5 p, 6 n Good for increasing the ratio of neutrons to protons!

29 Nuclear Chemistry Gamma Emission: Loss of a -ray (high-energy radiation that almost always accompanies the loss of a nuclear particle) 0000

30 Nuclear Chemistry Gamma Emission: Loss of a -ray (high-energy radiation that almost always accompanies the loss of a nuclear particle) 0000 Sometimes shown to emphasize energy transfer, but not necessary for balancing.

31 Nuclear Chemistry Electron Capture (K-Capture) Addition of an electron to a proton in the nucleus As a result, a proton is transformed into a neutron. p e 0 1 n 1010

32 Nuclear Chemistry Electron Capture (K-Capture) Addition of an electron to a proton in the nucleus As a result, a proton is transformed into a neutron. p e 0 1 n 1010 Opposite of Beta Decay. Good for increasing the ratio of neutrons to protons!

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34 Nuclear Chemistry More Balancing Examples:

35 Nuclear Chemistry More Balancing Examples:

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44 Nuclear Chemistry 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.

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

46 Nuclear Chemistry For smaller nuclei (Z 20) stable nuclei have a neutron-to-proton ratio close to 1:1.

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

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

49 Nuclear Chemistry Stable Nuclei Nuclei above this belt have too many neutrons. They tend to decay by emitting beta particles.

50 Nuclear Chemistry Stable Nuclei Nuclei above this belt have too many neutrons. They tend to decay by emitting beta particles. Good for decreasing the ratio of neutrons to protons!

51 Nuclear Chemistry Stable Nuclei Nuclei below the belt have too many protons. They tend to become more stable by positron emission or electron capture.

52 Nuclear Chemistry Stable Nuclei Nuclei below the belt have too many protons. They tend to become more stable by positron emission or electron capture. Good for increasing the ratio of neutrons to protons!

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

54 Nuclear Chemistry Stable Nuclei Heavy atoms have too many protons repelling. They tend to become more stable by alpha emission. No major change in the ratio of neutrons to protons! There will be a slight increase.

55 Nuclear Chemistry Stable Nuclei Heavy atoms have too many protons repelling. They tend to become more stable by alpha emission. Example: If have 150 n and 100 p, lose an alpha particle and will have 148 n and 98 p. Ratio rises from 1.50 to 1.51.

56 Nuclear Chemistry 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).

57 Nuclear Chemistry Some Trends Nuclei with an even number of protons and neutrons tend to be more stable than nuclides that have odd numbers of these nucleons.

58 Nuclear Chemistry Some Trends Nuclei with 2, 8, 20, 28, 50, or 82 protons or 2, 8, 20, 28, 50, 82, or 126 neutrons tend to be more stable than nuclides with a different number of nucleons.

59 Nuclear Chemistry Some Trends Nuclei with 2, 8, 20, 28, 50, or 82 protons or 2, 8, 20, 28, 50, 82, or 126 neutrons tend to be more stable than nuclides with a different number of nucleons. These are called Magic Numbers!

60 Nuclear Chemistry Nuclear Fission Nuclear transformations can be induced by accelerating a particle and colliding it with the nuclide.

61 Nuclear Chemistry Particle Accelerators These particle accelerators can be enormous, having circular tracks with paths that are miles long.

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69 Nuclear Chemistry Nuclear Fusion Combine two small atoms to make a larger one. H + H He

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71 Nuclear Chemistry Kinetics of Radioactive Decay Nuclear transmutation is a first-order process. The kinetics of such a process, you will recall, obey this equation: = -kt NtN0NtN0 ln

72 Nuclear Chemistry Kinetics of Radioactive Decay Nuclear transmutation is a first-order process. The kinetics of such a process, you will recall, obey this equation: = -kt NtN0NtN0 ln Chemical Reaction Equivalent: ln [A] t [A] o = -kt

73 Nuclear Chemistry Kinetics of Radioactive Decay Nuclear transmutation is a first-order process. The kinetics of such a process, you will recall, obey this equation: = -kt NtN0NtN0 ln For the Nuclear Version, you can use the following: 1.Number of Nuclei 2.Mass of Sample 3.Disintegrations per second

74 Nuclear Chemistry Kinetics of Radioactive Decay The half-life of such a process is: = t 1/ k Comparing the amount of a radioactive nuclide present at a given point in time with the amount normally present, one can find the age of an object.

75 Nuclear Chemistry 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.

76 Nuclear Chemistry Kinetics of Radioactive Decay A wooden object from an archeological site is subjected to radiocarbon dating. The activity of the sample that is due to 14 C is measured to be 11.6 disintegrations per second. The activity of a carbon sample of equal mass from fresh wood is 15.2 disintegrations per second. The half-life of 14 C is 5715 yr. What is the age of the archeological sample?

77 Nuclear Chemistry Kinetics of Radioactive Decay First we need to determine the rate constant, k, for the process. = t 1/ k = 5715 yr k = k yr = k yr1

78 Nuclear Chemistry Kinetics of Radioactive Decay Now that we know k, we can determine t: = -kt NtN0NtN0 ln = -( yr1 ) t ln = -( yr1 ) t ln = t 2230 yr

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80 Nuclear Chemistry Energy in Nuclear Reactions There is a tremendous amount of energy stored in nuclei. Einsteins famous equation, E = mc 2, relates directly to the calculation of this energy. It would be more appropriate to think of this equation as E = m c 2.

81 Nuclear Chemistry Energy in Nuclear Reactions In the types of chemical reactions we have encountered previously, the amount of mass converted to energy has been minimal (too small to detect). However, these energies are many thousands of times greater in nuclear reactions.

82 Nuclear Chemistry Example Nuclear Decay: Loss of an -particle (a helium nucleus) He 4242 U Th He

83 Nuclear Chemistry Energy in Nuclear Reactions The mass change for the decay of 1 mol of uranium-238 is g. The change in energy, E, is then E = ( m) c 2 E = ( kg)( m/s) 2 E = J

84 Nuclear Chemistry Energy in Nuclear Reactions The mass change for the decay of 1 mol of uranium-238 is g. The change in energy, E, is then E = ( m) c 2 E = ( kg)( m/s) 2 E = J Notice mass must be in kg.

85 Nuclear Chemistry Energy in Nuclear Reactions For example, the mass change for the decay of 1 mol of uranium-238 is g. The change in energy, E, is then E = ( m) c 2 E = ( kg)( m/s) 2 E = J Compare to chemical reactions, typically a few hundred kJ.

86 Nuclear Chemistry Energy in Nuclear Reactions For example, the mass change for the decay of 1 mol of uranium-238 is g. The change in energy, E, is then E = ( m) c 2 E = ( kg)( m/s) 2 E = J In WebAssign, enter only positive energies.

87 Nuclear Chemistry How did we calculate this mass Change? It appears that mass is conserved when balancing! U Th He Total on top = 238

88 Nuclear Chemistry How did we calculate this mass Change? We need more exact masses when calculating energy changes! U Th He Total on top = g g g

89 Nuclear Chemistry How did we calculate this mass Change? We need more exact masses when calculating energy changes! U Th He Total on top = g g g m = final mass – initial mass = g

90 Nuclear Chemistry Nuclear Binding Energy A helium nucleus consists of 2 protons and 2 neutrons, so mass should be: mass He = 2(mass neutron ) + 2(mass proton ) mass He = 2( amu) + 2( amu) Predicted mass He = amu True mass He = amu

91 Nuclear Chemistry Nuclear Binding Energy Predicted mass He = amu True mass He = amu Difference = amu This difference is called the mass defect. 1 g = x amu, so: Difference = x g E = x J = Binding Energy

92 Nuclear Chemistry Nuclear Binding Energy E = x J = Binding Energy To fairly compare nuclei, want binding energy per nucleon, so x J / 4 nucleons = 1.13 x J / nucleon

93 Nuclear Chemistry Nuclear Binding Energy E = x J = Binding Energy To fairly compare nuclei, want binding energy per nucleon, so x J / 4 nucleons = 1.13 x J / nucleon Note: For 1 mole of He nuclei, binding energy = x J

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