"before the atomic age," 1914, in "World Set Free," in which H.G. Wells anticipates the word the future would use to look back from a time defined by events that hadn't yet happened in his day; from pre- + atomic.
word-forming element meaning "before," from Old French pre- and Medieval Latin pre-, both from Latin prae (adverb and preposition) "before in time or place," from PIE *peri- (source also of Oscan prai, Umbrian pre, Sanskrit pare "thereupon," Greek parai "at," Gaulish are- "at, before," Lithuanian prie "at," Old Church Slavonic pri "at," Gothic faura, Old English fore "before"), extended form of root *per- (1) "forward," hence "beyond, in front of, before."
The Latin word was active in forming verbs. Also see prae-. Sometimes in Middle English muddled with words in pro- or per-.
"pertaining to atoms," 1670s as a philosophical term (see atomistic); scientific sense dates from 1811, from atom + -ic. Atomic number is from 1821; atomic mass is from 1848. Atomic energy first recorded 1906 in modern sense (as intra-atomic energy from 1903).
March, 1903, was an historic date for chemistry. It is, also, as we shall show, a date to which, in all probability, the men of the future will often refer as the veritable beginning of the larger powers and energies that they will control. It was in March, 1903, that Curie and Laborde announced the heat-emitting power of radium. [Robert Kennedy Duncan, "The New Knowledge," 1906]
Atomic bomb first recorded 1914 in writings of H.G. Wells ("The World Set Free"), who thought of it as a bomb "that would continue to explode indefinitely."
When you can drop just one atomic bomb and wipe out Paris or Berlin, war will have become monstrous and impossible. [S. Strunsky, Yale Review, January 1917]
Atomic Age is from 1945. Atomical "concerned with atoms," also "very minute," is from 1640s. Atomic clock is from 1938.