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    Question

    A nucleus with mass number A and atomic number Z

    undergoes β⁻ decay. Which of the following is correct about the daughter nucleus?
    A Same A, Z decreases by 1 Correct Answer Incorrect Answer
    B A increases by 1, Z unchanged Correct Answer Incorrect Answer
    C A decreases by 1, Z increases by 1 Correct Answer Incorrect Answer
    D Same A, Z increases by 1 Correct Answer Incorrect Answer

    Solution

    In β⁻ decay, a neutron inside the nucleus is converted into a proton, emitting a beta particle (electron) and an antineutrino. Since a neutron becomes a proton, the atomic number (Z) increases by 1, but the mass number (A) remains unchanged because both protons and neutrons contribute equally to it. So, after β⁻ decay:

    • Z → Z + 1
    • A remains the same

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