Question 25.6: Predicting Which Nuclei Are Radioactive. Which of the follow......
Predicting Which Nuclei Are Radioactive
Which of the following nuclides would you expect to be stable, and which radioactive? (a) ^{82}As; (b) ^{118}Sn; (c) ^{214}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.
Solve
(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}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}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}Sn is a stable nuclide.
(c) ^{214}Po has an atomic number of 84. All known atoms with Z > 83 are radioactive. ^{214}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.
