SEC3-NuclearStability_000 - Section 3: Nuclear Stability...

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Section 3: Nuclear Stability and Radioactive Decay Modes In a practical sense the definition of stability is a relative one. Time of observation is one factor. Sensitivity of detection devices is another. The stable nuclei are those near the peak of the binding energy curve, for which the neutron/proton combination or energy state does not change with time. Present upper limits on the measurement of nuclear lifetimes are about 10 20 years. So from a kinetics sense, anything with a longer lifetime is considered to be stable. But it is possible for nuclei to be thermodynamically unstable, but with a lifetime so long we cannot detect it. There are 266 nuclei that are considered to be stable. Every element up to and including bismuth (Z = 83) has at least one stable isotope, except technetium (Z = 43) and promethium (Z=61). These latter two elements do not exist on earth, but have been created in the laboratory and their atomic spectra are observed in young stars. Radioactive nuclei are those that SPONTANEOUSLY alter their neutron/proton composition or energy state. As such, radioactivity is a first-order rate process , identical to unimolecular decay in chemical systems A Æ B + C As described in Sec. 1, the rate of decay is characterized by a half-life, t 1/2 , which is the time required for one-half the nuclei in a sample to disintegrate (decay), i.e. for a sample
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This note was uploaded on 07/02/2011 for the course CHEM-C 460 taught by Professor Staff during the Spring '10 term at Indiana.

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SEC3-NuclearStability_000 - Section 3: Nuclear Stability...

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