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CHAPTER 20: NUCLEAR CHEMISTRY
Part One: Overview A. Overview of Transformation in Nature.
1. Physical changes - melting, evaporation, sublimation, etc.
2. Chemical changes - acid/base, redox.
3. Nuclear changes - change in structure and composition of nuclei in terms of its elementary particles.
B. Elementary Particles. (symbols: atomic number
mass number X ) mass number = number of nucleons (nucleon = proton or neutron)
1. Proton: 2. Neutron: 3. Electron: 4. α-particle =
5. β -particle =
6. γ radiation =
7. Atomic nuclei (nuclides):
C. Atomic Size and Mass.
1. Electron domain - determines “size” occupied by atom. (diameter ≈ 10-8 cm)
2. Nucleus - 99.99% of atomic mass. (diameter ≈ 10-12 cm - 10-13 cm)
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D. Density of Nuclear Material. 1. Typical molecule density ∝ 1 g/cm3. This is mostly empty space in which electrons
roam. 2. However, in a collapsed star, black hole, etc. which is all nuclear material (electrons
have fallen into the nucleus), nuclei can then pack together by nuclear forces: density ∝ 1014g/cm3.
E. Overview of Forces of Nature.
1. Gravitational = holds celestial bodies together; very weak; long range - over light
years distances. 2. London dispersion, hydrogen bonding, and dipole attractions = involved in physical
changes of molecules; weak, over atomic diameters.
3. Electrostatic (ionic and covalent) = “chemical bonding forces”; strong; over atomic
distances.
4. Nuclear forces = hold nuclei together; very strong; over nuclear distances (10-12 cm).
Repulsive potential energy between two protons:
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Part Two: Radioactivity A. Nuclear Stability.
1. Stable nuclei of the lighter elements (up to Calcium) tend to have: number of protons ≈ number of neutrons. For example: 2. Band of stability - see Figure 20.3.
Figure 20.3
3. Mass defect = Δm = 4. Nuclear binding energy: BE.
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5. Nuclear binding energy curve. (see Figure 20.18)
Figure 20.18
B. Radioactive Decay.
1. Neutron-rich nuclei (above the band of stability) tend to emit β particles. For example:
2. Neutron-poor nuclei (below the band of stability) tend to emit α particles. For example:
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3. Nuclei with Z > 83 are unstable and radioactive and emit by a variety of processes. 4. Nuclei with Z ≥ 92 (transuranium elements) may also decay by fission, splitting into 2
lighter nuclei. For example:
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98252 Cf → 56
142Ba + 42106Mo + 4 0
1n
5. Heavy radioactive nuclides decay into lighter species by a many step radioactive decay series:
Figure 20.5
C. Kinetics of Radioactive Decay. (Section 20.4)
1. Measured by Geiger Counter – gaseous atoms are ionized by the radiation and e- are
released. 2. Obey 1st-order kinetics. 3. Rate of decay = k[nuclide] 4. [nuclide]t = [nuclide]o e-kt
5.
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k =ln 2t 1
2
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t 12
= half life
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6.
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lnnuclide[ ]onuclide[ ] t
= kt
7. [ ] can be replaced by any kind of amount: grams, etc. 8. Example of radiocarbon dating: A once-living material contains only 69% of the 6
14C that living materials contain. How long ago did it die?
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t 12 of 6
14C = 5,730 years.
D. Uses of Radionuclides. (Section 20.5)
1. Radioactive Dating:
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14C ,
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40K (
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t 12 = 1.3 billion years)
2. Medicine:
a. radioactive tracers = very small amt of radioactive isotope added to a chemical or
biological system to study the system. b. cobalt radiation treatment.
3. Research: tracers, etc. 4. Agriculture: tracers study uptake of nutrients, etc. 5. Industrial.