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An isotope is one of several kinds of atoms of the same element that have different masses. These atoms have the same number of protons in their nuclei, but different numbers of neutrons, and therefore different mass numbers. The term isotope comes from the Greek isos topos, which means same place, because isotopes of the same element have the same atomic number and therefore occupy the same place in the periodic table.

The element carbon, for example, has two stable isotopes, carbon-12 and carbon-13, symbolized as 12C and 13C. The numbers 12 and 13 are the mass numbers of the isotopes—the total numbers of the protons plus neutrons in their nuclei. Because all carbon atoms have six protons in their nuclei, 13C must have seven neutrons (13-6) in its nucleus, and 12C has six (12-6). The element carbon as we find it in nature consists of 98.89% 12C atoms and 1.11% 11C atoms. Carbon in living organisms contains also a very small amount of 14C, a radioactive isotope that is used in radiocarbon dating.

Most elements have between two and six stable isotopes (as opposed to unstable, or radioactive ones). Twenty elements, including fluorine, sodium, aluminum, phosphorus, and gold consist of only one stable isotope each. Tin, however, has ten—more than any other element. The number of stable isotopes an element has is determined by the relative stabilities of various numbers of neutrons and protons in their nuclei.

Only two isotopes have been given distinctive names, both isotopes of hydrogen. The stable isotope 2H is known as deuterium, or heavy hydrogen, and the radioactive isotope 3H is called tritium.

Because isotopes of the same element have identical chemical properties, they cannot be separated by chemical methods, but only by methods that are based on their mass differences, such as mass spectrometry. One of the extraordinary accomplishments of the Manhattan Project, which created the atomic bomb during World War II, was the successful separation of large amounts of 235U, the highly fissionable isotope of uranium, from the much more abundant 238U by allowing a gaseous uranium compound to diffuse through porous barriers. Being heavier, the 238U-containing molecules move more slowly through the barriers.

Over 1,000 radioisotopes—radioactive isotopes—either exist in nature or have been made artificially by bombarding stable isotopes in particle accelerators. They are useful in so many applications that the word isotope is commonly used to mean radioisotope, as if stable isotopes did not exist.

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