A form of a chemical element that has the same number of protons but a different number of neutrons in its atoms. Isotopes of the same element behave almost the same chemically but can have different weights and stability.
Coined in 1913 from Greek “isos” meaning “equal” and “topos” meaning “place.” The idea was that different isotopes share the same place in the periodic table. They are “equal in place” even though their masses differ.
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