AIIMS 2018 Physics Radioactive Decay MCQ Question
A radioactive nucleus undergoes a series of decay according to the scheme A → A₁ → A₂ → A₃ → A₄. If the mass number and atomic number of ‘A’ are 180 and 72 respectively, then what are these numbers for A₄.
172 and 69
174 and 70
176 and 69
176 and 70
Correct Answer
Detailed Explanation
In the decay scheme A → A₁ → A₂ → A₃ → A₄, the mass number (A) decreases by 4 and the atomic number (Z) decreases by 3 for each decay step, indicating a series of alpha decays. Starting with A having a mass number of 180 and an atomic number of 72, after four decays, A₄ will have a mass number of 180 - 4 = 176 and an atomic number of 72 - 3 = 69. Therefore, A₄ has a mass number of 176 and an atomic number of 69, which matches option A.
Options B (174 and 70), C (176 and 69), and D (176 and 70) are incorrect because they either miscalculate the total decay or incorrectly account for the changes in atomic number.
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