Alpha decay effect: the emission of an alpha (α) particle from a nucleus changes the daughter nucleus in what way?
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AMore atomic weight and less atomic number
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BLess atomic weight and less atomic number
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CLess atomic weight and more atomic number
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DNone of these
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EUnchanged atomic number and weight
Answer
Correct Answer: Less atomic weight and less atomic number
Explanation
Introduction / Context:Alpha decay is a common mode of radioactive decay among heavy nuclei. An alpha particle is a helium-4 nucleus consisting of 2 protons and 2 neutrons, so its emission alters both atomic number (Z) and mass number (A) of the parent nucleus.
Given Data / Assumptions:
- α particle = ^4He nucleus = 2p + 2n.
- Mass number A decreases by 4; atomic number Z decreases by 2 with each α emission.
Concept / Approach:Conservation of nucleon number and charge dictates the daughter’s numbers: A_daughter = A_parent − 4; Z_daughter = Z_parent − 2. Hence both the atomic mass (proportional to A) and atomic number decrease.
Step-by-Step Solution:
1) Subtract the α particle nucleons: A → A − 4.2) Subtract the α particle protons: Z → Z − 2.3) Therefore, the daughter has less atomic weight and less atomic number.Verification / Alternative check:Example: ^238U → ^234Th + α. A: 238 → 234 (−4), Z: 92 → 90 (−2), consistent with the rule.
Why Other Options Are Wrong:
- More weight with less Z or less weight with more Z contradict conservation with α emission.
- Unchanged values or ”none of these” do not represent α decay.
Common Pitfalls:Confusing α with β decay (β changes Z by ±1 with nearly unchanged A); overlooking that α removes two protons and two neutrons.
Final Answer: Less atomic weight and less atomic number