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Isotopes are forms of chemical elements which have the same atomic number Z but different atomic mass. Hence, isotopes of a single chemical element share their entry in the Periodic Table, where each entry is defined by the atomic number. This sharing of position explains the name isotope. It originates from isoos (ίσως, Greek for equal) and topos (τόπος, Greek for position). The atomic number being equal to the number of protons in the atomic nucleus of an element, isotopes are characterized by the same number of protons in the nucleus, but have different numbers of neutrons (which partly determine the atomic mass).

For example, carbon-12 (12C), carbon-13 (13C) and carbon-14 (14C) are three isotopes of carbon, each containing six protons and in addition containing six, seven, or eight neutrons, respectively. All three isotopes occupy the position Z = 6 in the Periodic Table. While carbon-12 is the most common form, 13C is magnetically active, and it is therefore useful for magnetic resonance imaging (MRI) and nuclear magnetic resonance spectroscopy. Carbon-14 is radioactive, and is therefore useful for radiation tracing and age determination.

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