Download Chapter 30: Nuclear Physics

Chapter 30: Nuclear Physics
1. (i) Consider the following three nuclei: 12C, 13N, 14O. What is the same for these three
nuclei? (a) number of protons (b) number of neutrons (c) number of nucleons. (ii)
Consider the following three nuclei: 12N, 13N, 14N. From the same list of choices, what is
the same for these three nuclei? (iii) Consider the following three nuclei: 14C, 14N, 14O.
From the same list of choices, what is the same for these three nuclei?
Answer: (i), (b). The value of N = A − Z is the same for all three nuclei. (ii), (a). The
value of Z is the same for all three nuclei because they are all nuclei of nitrogen. (iii), (c).
The value of A is the same for all three nuclei, as seen by the unchanging preceding
superscript.
2. Which do you expect to show very little variation among different isotopes of an
element? (a) atomic mass (b) nuclear spin magnetic moment (c) chemical behavior
Answer: (c). Isotopes of a given element correspond to nuclei with different numbers of
neutrons. The result is different masses of the atom, and different magnetic moments
because the neutron, despite being uncharged, has a magnetic moment. The chemical
behavior, however, is governed by the electrons. All isotopes of a given element have the
same number of electrons and therefore the same chemical behavior.
3. On your birthday, you measure the activity of a sample of 210Bi, which has a half-life
of 5.01 days. The activity you measure is 1.000 μCi. What is the activity of this sample
on your next birthday? (a) 1.000 μCi (b) 0 (c) ~0.2 μCi (d) ~0.01 μCi (e) ~10−22 μCi
Answer: (e). A year of 365 days is equivalent to 365 d/5.01 d ≈ 73 half-lives. Therefore,
the activity will be reduced after one year to approximately (1/2)73(1.000 μCi) ~ 10−22
μCi.
4. Suppose you have a pure radioactive material with a half-life of T1/2. You begin with
N0 undecayed nuclei of the material at t = 0. At t = ½ T1/2, how many of the nuclei have
decayed? (a) ¼ N0 (b) ½ N0 (c) ¾ N0 (d) 0.707N0 (e) 0.293N0
Answer: (e). The time we are interested in is half of a half-life. Therefore, the number of
remaining nuclei is (½)1/2 N0 = ( 1 / 2 )N0 = 0.707N0. The number of nuclei that have
decayed is N0 − 0.707N0 = 0.293N0.
5. Which of the following is the correct daughter nucleus associated with the alpha decay
153
153
157
of 157
72 Hf ? (a) 72 Hf (b) 70Yb (c) 70Yb
Answer: (b). In alpha decay, the atomic number decreases by two and the atomic mass
number decreases by four.
6. Which of the following is the correct daughter nucleus associated with the beta decay
183
183
184
of 184
72 Hf ? (a) 72 Hf (b) 73Ta (c) 73Ta
Answer: (c). In e− decay, the atomic number increases by one and the atomic mass
number stays fixed. None of the choices is consistent with e+ decay, so we assume that
the decay must be by e−.