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

The answer is alpha decay, because alpha particles have two protons and two neutrons.  If the isotope has an atomic number two less than the original, it lost two protons, the same amount as in an alpha particle.

Answer 2

alpha decay because alpha particles have two protons and two neutrons