"Iso" means similar or same, an Isotope refers to an atom or a group of atoms of the same chemical element having the same number of protons but having an unequal number of neutrons which means that they have the same Atomic number. The name "isotope" to this unique property of the atoms of the same element was coined by Margaret Todd in the year 1914. Though she was the first one to use the term "isotope", these atoms were first discovered by J. J. Thomson in his famous experiment in 1913. Remember "The Manhattan Project " that the United States Government undertook during the World War? It was Isotopes of Uranium that caused the huge damage.
Isotopes occur naturally and can also be prepared artificially in laboratories. For example Helium has two naturally occurring isotopes that have two protons each and they differ in the number of neutrons as one of them has two neutrons and the other has only a single neutron. Depending on the number of neutrons and protons within the nucleus of the atom the atomic masses of the atoms vary. Hence isotopes have the same atomic number but different atomic Mass. Since the number of [node:679,title=" of the isotopes are the same , they have similar chemical properties but differ in their physical properties. Isotopes may be stable or unstable depending on the number of neutrons present in the nucleus of the atom.
Radioactive Isotopes are isotopes that have a highly unstable nucleus. To achieve stability these isotopes emit certain radiation in the form of alpha , beta and gamma rays till stability is attained. The isotope thus formed after the emission of these rays will be a non radioactive isotope of the previous atom of the same element. The radioactive isotopes are either primordial , in which case, they have persisted down to the present since their rate of decay is slow (Uranium-238 to Potassium -40) or they may be post primordial in which case the isotopes are formed due to cosmic ray bombardment. They may also be created by the decay of radioactive primordial isotope to a radioactive nuclide daughter (Uranium to Radium).