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:

Material database: American Elements - Chromium 53 Metal Isotope
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New selenium metabolites found in human serum | EVISA's News
6. interdisziplinäres Doktorandenseminar des Deutschen Arbeitskreises für Analytische Spektroskopie (DAAS) | EVISA's Agenda of Events
Link database: Chemical Fact Sheet: Ytterbium
Chromium Isotope Ratio as a Tool to Track the Redox State of the Earth | EVISA's News
Material database: American Elements - Silicon 30 Dioxide Isotope
Material database: American Elements - Nickel 58 Metal Isotope
Material database: American Elements - Tin 117 Metal Isotope
9. Symposium Massenspektrometrische Verfahren der Elementspurenanalyse / 22. ICPMS-Anwendertreffen | EVISA's Agenda of Events
Material database: American Elements - Lithium 6 Carbonate Isotope
Material database: American Elements - Boron 10 Carbide Isotope
Link database: Chemical Fact Sheet: Niobium
Instrument database: New Wave Research, Inc. - UP266 UV YAG Laser Ablation System
Material database: National Research Council of Canada (NRC - CNRC) - SELM-1
Species transformation during speciation analysis | EVISA's News
Instrument database: Teledyne CETAC Technologies - GeoLasPro 193nm Excimer Laser Ablation System
Third Asia-Pacific Winter Conference on Plasma Spectrochemistry | EVISA's Agenda of Events
Company database: Applied Isotope Technologies
Dietary mercury exposure: Mercury isotopes can tell the source | EVISA's News