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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.