late 15c., as a hypothetical indivisible extremely minute body, the building block of the universe, from Latin atomus (especially in Lucretius) "indivisible particle," from Greek atomos "uncut, unhewn; indivisible," from a- "not" (see a- (3)) + tomos "a cutting," from temnein "to cut" (from PIE root *tem- "to cut").

An ancient term of philosophical speculation (in Leucippus, Democritus); revived scientifically 1805 by British chemist John Dalton. In late classical and medieval use also a unit of time, 22,560 to the hour. Atom bomb is from 1945 as both a noun and a verb; compare atomic.

1829, "reduce to atoms;" by 1860, "reduce a liquid to a very fine mist;" a verb formed from atom + -ize. Related: Atomized; atomizing. Originally in reference to medical treatment for injured or diseased lungs. A sense of "to destroy with atomic weapons" is attested from 1945.

1590s, "atoms," also "diminutive beings," from atomy, from Latin atomi, plural of atomus (see atom), but taken as a singular in English and re-pluralized in the native way. Perhaps also in some cases a plural of atomy (from misdivision of anatomy).

also *temə-, Proto-Indo-European root meaning "to cut."

It forms all or part of: anatomy; atom; contemplate; contemplation; diatom; dichotomy; -ectomy; entomolite; entomology; entomophagous; epitome; phlebotomy; temple (n.1) "building for worship;" tmesis; tome; -tomy; tonsorial; tonsure.

It is the hypothetical source of/evidence for its existence is provided by: Greek temnein "to cut," tomos "volume, section of a book," originally "a section, piece cut off;" Old Church Slavonic tina "to cleave, split;" Middle Irish tamnaim "I cut off," Welsh tam "morsel."

"pertaining to atoms," 1670s as a philosophical term (see atomistic); scientific sense dates from 1811, from atom + -ic. Atomic number is attested from 1821; atomic mass is from 1848.

Atomic energy is recorded by 1906 in the 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 is 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. Atomic clock is from 1938. Atomical "concerned with atoms," also "very minute," is from 1640s.

also subatomic, 1874, from sub- + atomic. Sub-atom is attested from 1868.

word-forming element denoting the presence of a carbonyl group or an oxygen atom linking two other atoms; from oxygen.

"oxide with one oxygen atom in each molecule," 1840, from mono- "single" + oxide.

1869, so called because it consists of one carbon and one oxygen atom (as opposed to carbon dioxide, which has two of the latter). An older name for it was carbonic oxide gas.

mid-15c., "a derived word or form, a word formed immediately or remotely from another or a root," from derivative (adj.). General sense of "that which is derived or deduced from another" is from 1590s. Mathematical sense, "a derivative function," is from 1670s. In chemistry, "compound that can be made from a parent compound by replacement of one atom with another atom or group of atoms," by 1855.