Concept:Trace the distinct changes to mass number (A) and atomic number (Z) step-by-step.
Alpha (\(\alpha\)) subtracts 4 from A, 2 from Z.
Beta (\(\beta\)) changes A by 0, adds 1 to Z.
Gamma (\(\gamma\)) changes nothing.
Solution:- Start: X (A=180, Z=72)
- Step 1 (\(\alpha\)): to \(\text{X}_1\)
A = 180 - 4 = 176
Z = 72 - 2 = 70
- Step 2 (\(\beta\)): to \(\text{X}_2\)
A = 176 - 0 = 176
Z = 70 + 1 = 71
- Step 3 (\(\alpha\)): to \(\text{X}_3\)
A = 176 - 4 = 172
Z = 71 - 2 = 69
- Step 4 (\(\gamma\)): to \(\text{X}_4\)
A = 172 - 0 = 172
Z = 69 - 0 = 69
- Final Result: Mass number 172, Atomic number 69.
Why other options are incorrect:They represent math errors or incomplete steps. Option A misses the second alpha decay. Options C and D miss the atomic number decrease from the second alpha decay.
Quality & Fidelity Assurance:
Every question on BeambePrep is rigorously curated against the official PMDC syllabus with zero filler, zero out-of-syllabus content, and zero typos. When an authentic past paper originally contained a historical mistake or ambiguity from the examining board (such as UHS or NUMS), BeambePrep faithfully reflects the original paper while detailing the nuance and scientific consensus in the autopsy above.