An isotope is a form of an element that has a different number of neutrons in its nucleus from the number of protons. However, the term is sometimes used more loosely to refer to any form of an element, that is, inclusive of the form with the same number of neutrons as protons.
Every element has at least one naturally occurring isotope, although the occurrence of such isotope may be extremely rare. There is a total of about stable 250 isotopes of the approximately ninety naturally occurring elements, for a total of about 340 versions of these elements. More than 3,000 additional isotopes have been created artificially, most of which are radioactive.
The element with the largest number of stable (never decay into other isotopes or elements, or only decay extremely slowly) isotopes is tin, which has ten, in addition to another 29 known unstable isotopes. This large number is due to both the size of the tin atom, with 50 protons, and its stable structure. Its most common isotope has 60 neutrons and accounts for about 36.6 percent of all tin.
Hydrogen, which has only a single proton, has two naturally occurring isotopes, with zero and two neutrons, and also has four additional isotopes that are manmade and highly unstable. Carbon, with six protons, has two isotopes, with seven and eight neutrons; its non-isotopic form accounts for more than 99 percent of all carbon on the earth. Oxygen, with eight protons, also has two stable isotopes with nine and ten neutrons; its non-isotopic form accounts for more than 99.7 percent of all oxygen.
The chemical properties of isotopes of any element and its non-isotopic form are identical or nearly identical, with the exception of hydrogen because the number of its neutrons has such a large effect on the size of its nucleus. However, the physical properties of isotopes and the non-isotopic form differ from each other because such properties often depend on mass. This difference in mass is used to separate isotopes of an element from the non-isotopic form and from each other by any of several methods.
The unique properties of isotopes make them useful for a variety of applications, including nuclear medicine, the dating of organic remains and archaeologic artifacts, oil and gas exploration, national security, and basic research. For example, carbon-14, which has six protons and eight neutrons, is unstable and undergoes radioactive decay with a half-life of about 5,730 years (meaning that half of the material will be gone after 5,730 years). This decay allows the proportion of carbon-14 remaining in an organic material or artifact to serve as a clock that reveals its age in a process called "carbon dating."