Atoms of the same element (i.e. having the same atomic number) which differ in mass number are called isotopes of that element. The isotopes of an element are identical in chemical properties, and in all physical properties except those determined by the mass of the atom. The different isotopes of an element contain different numbers of neutrons in their nuclei. Nearly all elements found in nature are mixtures of several isotopes.

(Uvarov, E. B., d. R. Chapman, and A. Isaacs. A Dictionary of Science. Third Edition. Penguin Books Ltd., Harmondsworth, England, 1966.)

The term "isotope" was found in the following pages:

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Instrument database: Agilent Technologies Inc. - 7500ce ICP-MS
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Material database: American Elements - Boron 11 Trifluoride Isotope
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Material database: American Elements - Selenium 76 Metal Isotope
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Link database: GeoReM: Geological and Environmental Reference Materials
Material database: American Elements - Ruthenium 104 Metal Isotope