The strong nuclear force, also called binding energy, holdsquarks together to form protons and neutrons. Residual bindingenergy, also called the nuclear force, holds protons and neutronstogether to form the nucleus of an atom.This holds true up to about atomic number 83 (bismuth), at whichpoint the electromagnetic force, a repulsive force for protons,starts to overcome the distance barrier of binding energy and makethe nucleus unstable. This makes the atoms starting at bismuth andabove 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.