Which decay mode reduces the atomic number by 1 via positron emission?

Which decay mode reduces the atomic number by 1 via positron emission?

• A. Beta minus decay
• B. Electron capture
• C. Gamma decay
• D. Positron emission

Answer: (D)

The key idea is how different nuclear decays change the number of protons in the nucleus. In positron emission, a proton is converted into a neutron, and a positron plus a neutrino are emitted: p → n + e+ + νe. Since a proton becomes a neutron, the atomic number (the count of protons) drops by 1, while the mass number stays essentially the same because a proton and a neutron have nearly the same mass. So this decay lowers Z by 1 and emits a positron, exactly matching the scenario described. Compare with the others to see why they don’t fit here: beta minus decay converts a neutron to a proton and emits an electron, which increases Z by 1. Gamma decay releases energy as a photon with no change in Z or A. Electron capture lowers Z by 1 but does not emit a positron; instead, it captures an orbital electron and emits a neutrino.

The key idea is how different nuclear decays change the number of protons in the nucleus. In positron emission, a proton is converted into a neutron, and a positron plus a neutrino are emitted: p → n + e+ + νe. Since a proton becomes a neutron, the atomic number (the count of protons) drops by 1, while the mass number stays essentially the same because a proton and a neutron have nearly the same mass. So this decay lowers Z by 1 and emits a positron, exactly matching the scenario described.

Compare with the others to see why they don’t fit here: beta minus decay converts a neutron to a proton and emits an electron, which increases Z by 1. Gamma decay releases energy as a photon with no change in Z or A. Electron capture lowers Z by 1 but does not emit a positron; instead, it captures an orbital electron and emits a neutrino.