A variant form of a chemical element that has similar structure but different mass. Some isotopes are radioactive.
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Biology Dictionary
Definition:
An alternate form of an element that has the usual number of protons but a nonstandard number of neutrons; the fewer or additional neutrons give the isotope a different atomic weight than the regular element and may make the isotope radioactive, but otherwise an isotope has the same chemical action as the regular element. Because of this, isotopes (such as radioactive carbon) are used as tracers in biological systems or processes.
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Glossary
Definition:
One of two or more forms of an element that have the same number of protons (atomic number) but differing numbers of neutrons (mass numbers). Radioactive isotopes are commonly used to make DNA probes and metabolic tracers.
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