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atomic bomb an extremely destructive weapon, the explosive power of which derives from the immense amount of energy suddenly released when the nuclei of uranium 235 or plutonium atoms are split; atom bomb.
atomic clock an extremely accurate timekeeping device regulated by the reliably constant frequency at which atoms of certain highly reactive substances absorb or emit electromagnetic radiation.
atomic cocktail (informal) a dose of a liquid, taken orally, that contains a radioactive substance used in the diagnosis or treatment of cancer.
atomic energy the energy that is released by an atom in nuclear reactions such as fission or fusion. [2 definitions]
atomicity the number of atoms in a molecule. [2 definitions]
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.