Of or relating to constituents of the atom or forces within the atom.

Roget 32: small, little; diminutive etc. (small in size) 193; minute; fine; inconsiderable, paltry etc. (unimportant) 643; faint etc. (weak) 160; slender, light, slight, scanty, scant, limited; meager etc. (insufficient) 640; sparing; few etc. 103; 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.    ... show more

Roget 193: little; small etc. (in quantity) 32; minute, diminutive, microscopic; microzoal; inconsiderable etc. (unimportant) 643; 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 etc. 201.    impalpable, intangible, evanescent, imperceptible, invisible, inappreciable, insignificant, inconsiderable, trivial; infinitesimal, homoeopathic^†; atomic, subatomic, corpuscular, molecular; rudimentary, rudimental; embryonic, vestigial.    weazen^†, scant, scraggy, scrubby; thin etc. (narrow) 203; granular etc. (powdery) 330; shrunk etc. 195; brevipennate^†.    ... show more

Of smaller than atomic dimensions.