After a radioactive atom decays, it is the same element that it was before with no measurable change in mass. Which kind of decay has occurred, and how do you know?

Gamma decay occurs when an excited nucleus releases energy as a photon. The photon has no rest mass and carries no charge, so the nucleus’s number of protons and its identity as an element do not change. Alpha decay emits a massive particle, while beta decay changes the number of protons and therefore changes the element. The correct choice is gamma decay because photons have no mass.

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