Isotopes differ from one another in the number of neutrons they possess. Because of their differing number of neutrons, isotopes also differ in their mass numbers, the total number of protons and neutrons.
Neutrons have no charge, but they do contribute mass to an atom, about the same mass a proton contributes. This difference in the number of neutrons imparts different physical properties to the isotopes. Many of the elements found in nature comprise a mixture of different isotopes.
The most stable isotopes of elements are those with roughly equal amounts of protons and neutrons. If the number of neutrons is too low or too high, the isotope becomes unstable. These unstable isotopes eventually decay into lighter elements. Any isotopes of elements heavier than the element bismuth are unstable and radioactive.
A special isotope of carbon called carbon-14 has a special use in science. All life forms contain carbon. Carbon-14 decays at a predictable rate, so scientists can measure how old dead organisms are based on the percentage of carbon-14 left in their bodies. Isotopes are used in the medical field, pest control, agriculture and smoke detectors, states the Chemistry Department at Duke University.
Radioactive isotopes are called nuclides. Some nuclides present when the solar system formed almost 5 billion years ago still exist, as stated by the University of Arizona; these nuclides include potassium-40, rubidium-87 and uranium-238.