Isotopes are atoms that differ only in the number of neutrons of their nuclei. Isotopes have the same chemical and atomic properties. On the contrary, the nuclear properties are different because the number of neutrons affects the stability of the nucleus. For example, the carbon atom with 6 neutrons is stable, while the carbon atom with 8 neutrons is radioactive. The majority of atoms found in nature are stable isotopes. Radioactive isotopes, much more numerous, have usually disappeared because of their instability. They can be recreated artificially, for example for medical applications.