alleryWhen a nucleus $\,^{A}_{Z}\mathrm{X}$ undergoes ALPHA decay, the daughter nucleus has
Amass number $A - 4$, atomic number $Z - 2$
Bmass number $A$, atomic number $Z + 1$
Cmass number $A - 1$, atomic number $Z + 1$
Dmass number $A + 4$, atomic number $Z + 2$
Answer & Solution
Correct answer: A. mass number $A - 4$, atomic number $Z - 2$
1. An alpha particle is a $\,^{4}_{2}\mathrm{He}$ nucleus — two protons and two neutrons.
2. Conservation of mass number: parent $A$ → daughter $A'$ + alpha $A_\alpha$. So $A' = A - 4$.
3. Conservation of charge: parent $Z$ → daughter $Z'$ + alpha $Z_\alpha$. So $Z' = Z - 2$.
4. Generic alpha-decay equation: $\,^{A}_{Z}\mathrm{X} \to \,^{A-4}_{Z-2}\mathrm{Y} + \,^{4}_{2}\mathrm{He}$.
5. Example: $\,^{238}_{92}\mathrm{U} \to \,^{234}_{90}\mathrm{Th} + \alpha$.
6. Option B is BETA-MINUS decay ($Z$ goes up by 1, $A$ unchanged). Option C is wrong on all counts. Option D would imply absorption rather than emission.
_Source: syllabus Physics Part 2, Ch 13, §13.6 (Radioactivity — alpha decay), p. 8–9._
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