Question

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?
alpha decay because alpha particles have no mass
beta decay because this kind of decay cannot change one element into another
alpha decay because it creates a new isotope of the same element
gamma decay because photons have no mass

Answer 1

Gamma decay — because gamma rays are photons (no rest mass and carry no charge), so the nucleus only loses energy and its proton and neutron numbers (and thus element and mass) remain unchanged.