Alpha decay involves emission of which of the following?

• A. A gamma ray is emitted.
• B. A neutron is emitted.
• C. An alpha particle consisting of two protons and two neutrons.
• D. A proton is emitted.

Answer: (C)

Alpha decay is when a heavy nucleus ejects an alpha particle, which is a tightly bound bundle of two protons and two neutrons (a helium-4 nucleus). This emission reduces the atomic number by two and the mass number by four, so the remaining nucleus is a different element located two places to the left on the periodic table. The defining feature is the release of that helium-4 nucleus as the primary emitted particle. Energy conservation shows up as the kinetic energy of the alpha particle and the recoiling daughter nucleus. Sometimes a gamma ray can accompany decays to shed excess energy, but the hallmark of alpha decay is the emission of the alpha particle itself. Emission of a neutron or a proton corresponds to other decay modes, not alpha decay.