what makes an isotope radioactive

An isotope is a form of a chemical element with the same number of protons in its atomic nucleus but differing numbers of neutrons. An isotope is considered radioactive when it has an unstable combination of neutrons and protons or excess energy in its nucleus. The unstable nucleus of a radioisotope can occur naturally or as a result of artificially altering the atom. Atoms with an unstable nucleus regain stability by shedding excess particles and energy in the form of radiation, a process called radioactive decay. The radioactive decay process for each radioisotope is unique and is measured with a time period called a half-life. One half-life is the time it takes for half of the unstable atoms to undergo radioactive decay. Therefore, the unstable combination of neutrons and protons or excess energy in the nucleus of an isotope is what makes it radioactive.