isotope (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.