Question

An isotope undergoes radioactive decay. The new isotope that forms has an atomic number that is 2 less than the original isotope’s.

Which kind of decay has occurred, and how do you know?
alpha decay because alpha particles have a large mass
beta decay because beta particles can have negative charge
alpha decay because alpha particles have two protons and two neutrons
gamma decay because gamma rays are photons

Answer 1

Alpha decay, because an alpha particle is a helium nucleus (2 protons and 2 neutrons). Emission of an alpha particle removes two protons, so the atomic number falls by 2 (and the mass number by 4).