Question 25.6: Predicting Which Nuclei Are Radioactive Which of the followi...
Predicting Which Nuclei Are Radioactive
Which of the following nuclides would you expect to be stable, and which radioactive? (a) _{}^{82}\text{As}; (b) _{}^{118}\text{Sn}; (c) _{}^{214}\text{Po}.
Analyze
This question requires us to apply the rules of nuclear stability. We may also use Figure 25-7 to determine whether the nuclide lies within the belt of stability.
(a) Arsenic-82 has Z = 33 and N = 49. This is an odd-odd combination that is found in only four of the lighter elements. _{}^{82}\text{As} is radioactive. (Note also that this nuclide is outside the belt of stability in Figure 25-7.)
(b) Tin has an atomic number of 50—a magic number. The neutron number is 68 in the nuclide _{}^{118}\text{Sn}. This is an even-even combination, and we should expect the nucleus to be stable. Moreover, Figure 25-7 shows that this nuclide is within the belt of stability. _{}^{118}\text{Sn} is a stable nuclide.
(c) _{}^{214}\text{Po} has an atomic number of 84. All known atoms with Z > 83 are radioactive. _{}^{214}\text{Po} is radioactive.
Assess
When considering whether a nuclide is radioactive, we must remember that all elements with Z > 83 are radioactive, and those with Z < 84 are radioactive if their Z and N form odd-odd combinations. A few exceptions exist to these rules, specifically when nuclides have atomic numbers that are one of the magic numbers.