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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]
ATP a biochemical compound that serves to store energy in all living cells; adenosine triphosphate. (Cf. ADP.)