Physics .

the smallest component of an element having the chemical properties of the element, consisting of a nucleus containing combinations of neutrons and protons and one or more electrons bound to the nucleus by electrical attraction; the number of protons determines the identity of the element.

an atom with one of the electrons replaced by some other particle

muonic atom; kaonic atom.

Energy . this component as the source of nuclear energy.

a hypothetical particle of matter so minute as to admit of no division.

anything extremely small; a minute quantity.

noun

bit , crumb , dot , fragment , grain , iota , jot , minimum , mite , modicum , molecule , morsel , mote , ounce , particle , scintilla , scrap , shred , smidgen , speck , spot , tittle , trace , whitnotes:atom comes from greek atomos \'uncut , indivisible\' - based on a- \'not\' and temneir \'to cut\' and it is the smallest unit of matter that retains the characteristics of an element; a molecule is the smallest unit of matter that retains the characteristics of a compound , infinitesimal , little , molecular , monad , shade , whit