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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.
atomism a theory that all matter is composed of small, indivisible particles called atoms, which in modern form is called atomic theory. [2 definitions]
atomistic of or relating to atoms or atomism. [2 definitions]
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]
atopic of or relating to a genetic tendency to develop allergic reactions.
atopy a genetic tendency to develop allergic reactions that is typically associated with a heightened immune response to certain allergens.