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Radioisotopes are any isotope that has an unstable nucleus, admits radiation, and has a different mass compared to the isotope that's not radioactive.
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Let's use hydrogen again as an example.
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Hydrogen has a mass of 1 .01.
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Deuterium, which is hydrogen 2, remember it has one more neutron compared to hydrogen.
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It also has a mass of about 1 .01.
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So these two are not radioactive isotopes.
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However, if we look at tritium, which is hydrogen 3, it has a mass that's three times that of the other two nuclei.
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So it has a mass of 3 .02.
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So we can define this one as the radioactive isotope.
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It has an unstable nucleus...