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From Citizendium, the Citizens' Compendium

Isotopes are forms of chemical elements which have the same atomic number but a different atomic mass, or the same number of protons in the atomic nucleus but different numbers of neutrons.

For example, carbon-12 (¹²C), carbon-13 (¹³C) and carbon-14 (¹⁴C) are three isotopes of carbon, each containing six protons and also containing six, seven or eight neutrons, respectively. 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.