Each of two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei.
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Greek1913well-attested
Coined in 1913 by British radiochemist Frederick Soddy (Nobel Prize 1921), who built the term from classical Greek: ísos (ἴσος, equal, the same) + tópos (τόπος, place, position). Isotopes are literally same-place elements — variants of a chemical element occupying the same position in the periodic table (same atomic number, same chemical behaviour) while differing in their atomic mass due to different numbers of neutrons. Greek ísos (from PIE *i- or a non-IE Mediterranean substrate) produced isosceles (equal-legged
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'Isotope' means 'same place' — same position in the periodic table. The wordwas suggested to Frederick Soddy by Margaret Todd, a physician andwriter. Other 'iso-' words: 'isosceles' (equal legs), 'isometric' (equal measure), 'isobar' (equal weight/pressure