Answer: The strong nuclear force, also called binding energy, holds quarks together to form protons and neutrons. Residual binding energy, also called the nuclear force, holds protons and neutrons together to form the nucleus of an atom.
This holds true up to about atomic number 83 (bismuth), at which point the electromagnetic force, a repulsive force for protons, starts to overcome the distance barrier of binding energy and make the nucleus unstable. This makes the atoms starting at bismuth and above be radioactive.
Additionally, the presence or absence of extra neutrons, i.e. isotopes, even in light nuclides, can, due to the weak interaction, makes the nucleus be unstable, and radioactive.