The U-238 nucleus decays to form Pb-206 by α and β decays.
-Calculate the number of α decays.
A) 2
B) 4
C) 6
D) 8
E) none of these

Question

Quizplus · Accepted Answer

8In the decay from U-238 to Pb-206, the atomic number decreases from 92 to 82, indicating 10 fewer protons. Each α decay reduces the atomic number by 2 (since an α particle is 2 protons and 2 neutrons), so to reduce the atomic number by 10, 5 α decays are needed. However, the question asks for the number of α decays, not the change in atomic number. The mass number decreases from 238 to 206, a difference of 32. Since each α decay reduces the mass number by 4 (the mass of an α particle), the number of α decays is 32/4 = 8. Beta decays do not change the mass number, only the atomic number, by converting a neutron into a proton.

The U-238 Nucleus Decays to Form Pb-206 by α and β