Atoms become unstable, or radioactive, when they have too much energy. Radioactive atoms become stable after releasing all of their excess energy in a process known as radioactive decay, according to the Jefferson Lab.
Atoms become unstable or radioactive when there is an imbalance in the ratio of protons to neutrons in the nucleus of the atom. The nuclear forces in unstable nuclei do not generate enough binding energy to hold the nucleus together which causes instability in the atom. The isotopes of radioactive nuclei are referred to as radioisotopes. An unstable nucleus tries to return to a balanced state by giving off a proton or neutron in a process called radioactive decay. During this radioactive decay, atoms release their excess energy in a process called ionization radiation. Ionizing radiation is energy in the form of particles that can penetrate matter and cause ionization.
Ionizing radiation has the ability to damage any living tissue in the human body, therefore, it is important to take great care when handling radioactive material. The most common health effect from radioactive exposure is cancer and stochastic effects. Cancer is caused when radiation causes uncontrolled growth of cells in the body, whereas stochastic effects occur when radiation causes changed in DNA that cause mutations.