Isotopes are any of the different chemical species of a chemical 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.


A nuclide is any particular atomic nucleus with a specific atomic number Z and mass number A; The distinction between the terms isotope and nuclide has somewhat blurred, and they are often used interchangeably.

Isotope is better used when referring to several different nuclides of the same element; For example, it is more correct to say that an element such as fluorine consists of one stable nuclide rather than that it has one stable isotope.

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