1.  atom(n = noun.substance) - (physics and chemistry) the smallest component of an element having the chemical properties of the element;
is a kind of substance
is a part of chemical element, element, molecule
has parts: elementary particle, fundamental particle, nucleus
has particulars: isotope, monad, carbon atom, hydrogen atom, free radical, radical
Derived forms adjective atomic₁, verb atomise₃, verb atomise₁, verb atomize₃

2.  atom(n = noun.substance) corpuscle, molecule, mote, particle, speck - (nontechnical usage) a tiny piece of anything;
is a kind of material, stuff
has particulars: grain, grinding, chylomicron, flyspeck, identification particle
Derived forms adjective atomic₃, verb atomise₃

1.  An ultimate indivisible particle of matter. [1913 Webster]
" These three definitions correspond to different views of the nature of the ultimate particles of matter. In the case of the last two, the particles are more correctly called molecules." Dana. [1913 Webster]

2.  The smallest particle of matter that can enter into combination; one of the elementary constituents of a molecule. [1913 Webster]

3.  Anything extremely small; a particle; a whit. [1913 Webster]
"There was not an atom of water." [1913 Webster]

   To reduce to atoms. Feltham. [1913 Webster]

atom, n.
1 a the smallest particle of a chemical element that can take part in a chemical reaction. b this particle as a source of nuclear energy.
2 (usu. with neg.) the least portion of a thing or quality (not an atom of pity).

Idiom:
atom bomb a bomb involving the release of energy by nuclear fission = fission bomb. atom-smasher colloq. = ACCELERATOR 2.

Etymology:
ME f. OF atome f. L atomus f. Gk atomos indivisible

N  smallness, littleness, tenuity, paucity, fewness, meanness, insignificance (unimportance), mediocrity, moderation, small quantity, modicum, trace, hint, minimum, vanishing point, material point, atom, particle, molecule, corpuscle, point, speck, dot, mote, jot, iota, ace, minutiae, details, look, thought, idea, soupcon, dab, dight, whit, tittle, shade, shadow, spark, scintilla, gleam, touch, cast, grain, scruple, granule, globule, minim, sup, sip, sop, spice, drop, droplet, sprinkling, dash, morceau, screed, smack, tinge, tincture, inch, patch, scantling, tatter, cantlet, flitter, gobbet, mite, bit, morsel, crumb, seed, fritter, shive, snip, snippet, snick, snack, snatch, slip, scrag, chip, chipping, shiver, sliver, driblet, clipping, paring, shaving, hair, nutshell, thimbleful, spoonful, handful, capful, mouthful, fragment, fraction, drop in the ocean, animalcule, trifle, mere nothing, next to nothing, hardly anything, just enough to swear by, the shadow of a shade, finiteness, finite quantity, small, little, diminutive, minute, fine, inconsiderable, paltry, faint, slender, light, slight, scanty, scant, limited, meager, sparing, few, low, so-so, middling, tolerable, no great shakes, below par, under par, below the mark, at a low ebb, halfway, moderate, modest, tender, subtle, inappreciable, evanescent, infinitesimal, homeopathic, very small, atomic, corpuscular, microscopic, molecular, subatomic, mere, simple, sheer, stark, bare, near run, dull, petty, shallow, stolid, ungifted, unintelligent, to a small extent, on a small scale, a little bit, a wee bit, slightly, imperceptibly, miserably, wretchedly, insufficiently, imperfectly, faintly, passably, pretty well, well enough, partially, in part, in a certain degree, to a certain degree, to a certain extent, comparatively, some, rather in some degree, in some measure, something, somewhat, simply, only, purely, merely, at least, at the least, at most, at the most, ever so little, as little as may be, tant soit peu, in ever so small a degree, thus far, pro tanto, within bounds, in a manner, after a fashion, so to speak, almost, nearly, well-nigh, short of, not quite, all but, near upon, close upon, peu s'en faut, near the mark, within an ace of, within an inch of, on the brink of, scarcely, hardly, barely, only just, no more than, about, thereabouts, somewhere about, nearly, say, be the same, be little more or less, no ways, no way, no wise, not at all, not in the least, not a bit, not a bit of it, not a whit, not a jot, not a shadow, in no wise, in no respect, by no means, by no manner of means, on no account, at no hand, dare pondus idonea fumo, magno conatu magnas nugas, small sands the mountain, moments make the year.

N  inextension, nonextension, point, dot, atom.

N  littleness, smallness, exiguity, inextension, parvitude, parvity, duodecimo, Elzevir edition, epitome, microcosm, rudiment, vanishing point, thinness, dwarf, pygmy, pigmy, Liliputian, chit, pigwidgeon, urchin, elf, atomy, dandiprat, doll, puppet, Tom Thumb, Hop-o'-my-thumb, manikin, mannikin, homunculus, dapperling, cock-sparrow, animalcule, monad, mite, insect, emmet, fly, midge, gnat, shrimp, minnow, worm, maggot, entozoon, bacteria, infusoria, microzoa, phytozoaria, microbe, grub, tit, tomtit, runt, mouse, small fry, millet seed, mustard seed, barleycorn, pebble, grain of sand, molehill, button, bubble, point, atom, fragment, powder, point of a pin, mathematical point, minutiae, micrometer, vernier, scale, microphotography, photomicrography, micrography, photomicrograph, microphotograph, microscopy, microscope (optical instruments), little, small, minute, diminutive, microscopic, microzoal, inconsiderable, exiguous, puny, tiny, wee, petty, minikin, miniature, pygmy, pigmy, elfin, undersized, dwarf, dwarfed, dwarfish, spare, stunted, limited, cramp, cramped, pollard, Liliputian, dapper, pocket, portative, portable, duodecimo, dumpy, squat, short, impalpable, intangible, evanescent, imperceptible, invisible, inappreciable, insignificant, inconsiderable, trivial, infinitesimal, homoeopathic, atomic, subatomic, corpuscular, molecular, rudimentary, rudimental, embryonic, vestigial, weazen, scant, scraggy, scrubby, thin, granular, shrunk, brevipennate, in a small compass, in a nutshell, on a small scale, minutely, microscopically.

atom bomb, atom smasher, atom smashing, carbon atom, gram atom, hydrogen atom, rutherford atom