deuterium (n.)

1933, coined by U.S. chemist Harold C. Urey, with Modern Latin ending + Greek deuterion, neuter of deuterios "having second place," from deuteros "next, second," a word of uncertain origin. According to some sources from duo "two" (from PIE root *dwo- "two"), but according to Watkins the ground sense is "missing" and the Greek word is from PIE from *deu-tero-, suffixed form of *deu- (1) "to lack, be wanting." But Beekes doubts even this. So called because it is twice the mass of hydrogen.