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A radioactive nucleus decays through the sequence A --α--> A1 --β--> A2 --α--> A3 --γ--> A4. If nucleus A has mass number 180 and atomic number 72, what are the mass number and atomic number of A4?

1. 172 and 69
2. 174 and 70
3. 176 and 69
4. 176 and 70

Correct answer: 172 and 69

Solution

Start (180,72). After alpha (176,70), after beta (176,71), after alpha (172,69), after gamma unchanged (172,69). So A4 has mass number 172 and atomic number 69.

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