In beta decay, what happens to the mass number A?

• A. Increases by 1
• B. Remains unchanged
• C. Decreases by 1
• D. Increases by 2

Answer: (B)

In beta decay, the total number of nucleons does not change. A neutron can convert into a proton (beta minus) or a proton can convert into a neutron (beta plus or via electron capture), and a beta particle and a neutrino or antineutrino are emitted. These processes swap one type of nucleon for the other, so the mass number A, which is the sum of protons and neutrons, stays the same. The nucleus becomes more proton-rich or more neutron-rich (changing Z), but the total count of nucleons remains constant. For example, carbon-14 decays to nitrogen-14 with the same mass number 14.