word-forming element meaning 1. "ray, ray-like" (see radius); 2. "radial, radially" (see radial (adj.)); 3. "by means of radiant energy" (see radiate (v.)); 4. "radioactive" (see radioactive); 5. "by radio" (see radio (n.)).
1913, literally "having the same place," from Greek isos "equal" (see iso-) + topos "place" (see topos); so called because, despite having different atomic weights, the various forms of an element occupy the same place on the periodic table. Introduced by British chemist Frederick Soddy (1877-1956) on suggestion of his friend, the Scottish writer and doctor Margaret Todd (c. 1859-1918). Related: Isotopic.
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<a href="https://www.etymonline.com/word/radioisotope">Etymology of radioisotope by etymonline</a>
Harper, D. (n.d.). Etymology of radioisotope. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/radioisotope