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An atom of element A undergoes a two-stage decay: first A changes into B with emission of a helium-4 nucleus, and then B changes into C with emission of two electrons. Which statement is correct?

1. A and C are isotopes
2. A and C are isobars
3. A and B are isotopes
4. A and B are isobars

Correct answer: A and C are isotopes

Solution

Alpha emission (A -> B) reduces Z by 2 and mass by 4. Two beta- emissions (B -> C) raise Z by 2, restoring the original Z while mass stays at A-4. So A and C have the same atomic number but different mass numbers, i.e. they are isotopes.

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