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isotope, one of two or more species of atoms of a chemical element with the same atomic number and position in the periodic table and nearly identical chemical behaviour but with different atomic masses and physical properties. Every chemical element has one or more isotopes

each if two or more forms of the same element that contain equal numbers of protons but different numbers if neutrons in the nuclei , and hence differ in relative atomic mass but not in chemical properties ; in particular , a radioactive form of an element .

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first one is the correct answer