ANSWER:
The addition of neutrons or protons.
Explanation:
We have that an atom is made up of three different particles known as electrons, protons and neutrons.
In general terms, these particles have different masses and charges in the case of the electron has a negative charge, the proton has a positive charge and the charge of the neutron is neutral.
In the case of a radioactive atom model that happens when an unstable atom has access to energy in its center and this can be caused by the addition of neutrons or protons.