When does something become radioactive?

A substance is said to have become radioactive  if it contains unstable nuclei and is able to naturally release energy in the process of shedding high speed charged particles, in an attempt to reach a stable state. With this, a non radioactive will remain intact indefinitely unless acted upon by an external force.

An element that is not normally radioactive may be irradiated with neurons to produce unstable  nuclei of the same but now heavier element. In an attempt become stable, it will lose weight by ejecting some other particle.