isotope

n. form of a chemical element which has the same atomic number as the other forms but a different atomic weight (Chemistry)

Isotopes are any of the several different forms of an element each having different atomic mass (mass number). Isotopes of an element have nuclei with the same number of protons (the same atomic number) but different numbers of neutrons. Therefore, isotopes have different mass numbers, which give the total number of nucleons—the number of protons plus neutrons.
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n. isotope, form of a chemical element which has the same atomic number as the other forms but a different atomic weight (Chemistry)

adj. isotopic, of an isotope, pertaining to a form of a chemical element which has the same atomic number as the other forms but a different atomic weight (Chemistry)

n. isotope, form of a chemical element which has the same atomic number as the other forms but a different atomic weight (Chemistry)

A form of a chemical element with a different atomic mass. Isotopes are used in a number of medical tests.