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atomic mass unit a unit equal to one twelfth of the mass of an atom of the most abundant kind of carbon, equal to 1.6605 times 1024 gram.
atomic number the number of protons or positive electrical charges in the nucleus of one atom of a given element. [2 definitions]
atomic pile see nuclear reactor.
atomic theory any of several theories concerning the structure, properties, or behavior of atoms. [2 definitions]
atomic weight the average weight of one atom of a given element, expressed in atomic mass units.
atomize to divide into atoms. [3 definitions]
atomizer a device that converts a liquid, esp. a medicine or perfume, into a spray.
atom smasher see accelerator.
atonal in music, composed without a fixed key or scale.
at once at the same time. [2 definitions]
atone to make reparation for a sin, crime, error, or the like (usu. fol. by for). [2 definitions]
at one in a state of harmony or accord.
atonement the act of atoning; reparation. [3 definitions]
at one's elbow close by.
at one's fingertips readily available.
at one's heels in close pursuit.
at one's leisure when the time is convenient; when one has spare time.
atop at or on the highest point. [2 definitions]
-ator one that does (such) a thing or acts in (such) a way.
-atory of or pertaining to. [3 definitions]
a tough nut to crack a problem or situation that is difficult to solve or cope with. [2 definitions]