Determination of Nucleidic Masses by Nuclear Reactions

A tremendous amount of information about nucleidic masses has been obtained by studying the energy released in nuclear reactions. A discussion of nuclear reactions is given in Chapter 20. The following example illustrates the general method. 

Example 4-7. The fraction 0.0118% of the element potassium in its natural occurrence is the radioactive isotope iSKi,. It undergoes beta decay  

The kinetic energy of the emitted electron (the beta ray) has been measured by use of a magnetic beta-ray spectrometer (measurement of the curvature of the path of the beta ray in a magnetic field) and found to be 1.32 MeV. The mass of Ca, which is stable and constitutes 97% of natural calcium, is 39.96259 d, as determined with the mass spectrograph. What is the mass of K  

Solution. The energy of the reaction Ca + e — gible. Hence the energy given out in the reaction 

In the early years of the atomic theory there was no secure knowledge of the true relative masses of different elements. Dalton assigned atomic masses in such a way as to lead to simple formulas for compounds. Many chemists continued to write HO for water until 1858. In that year a principle discovered much earlier (1811) by Avogadro was applied so effectively by Stanislao Cannizzaro as to convince most chemists that the atomic masses obtained by its use could be accepted as correct. This principle will be discussed in Section 4-10. 

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