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Variation among atoms in terms of number of neutrons.
Those atoms that are isotopes of each other all possess the same atomic number, that is, number of protons, but differ in number of neutrons. The three somewhat stable isotopes of the element hydrogen, for example, are protium (1H; one proton, no neutrons), deuterium (2H; one proton, one neutron), and tritium (3H; one proton, two neutron). Isotopes differ in terms of their atomic mass but otherwise are chemically equivalent. Some isotopes, termed radioisotopes, are less table than others, resulting in radioactive decay into different elements. Tritium, for example, is radioactive.
The importance of isotopes to biology is mostly due to the use of radioisotopes in experimental biology, i.e., as a means of tagging molecules for subsequent detection. Isotopes and their detection also is key to radiodating, i.e., measure of the age of substances based on what isotopes are present such as is used in the absolute dating of fossils.
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