atom

(at’om) [Gr. ἅτομος indivisible]. Any one of the ultimate particles of a molecule or of any matter. An atom is the smallest quantity of an element that can exist and still retain the chemic properties of the element. The atom is now believed to consist of a central nucleus of extremely high potential surrounded by particles of negative electricity (electrons), whose motion it controls. The various chemical elements are characterized by the number of electrons in the atom. recoil a., rest a., the portion of an atom from which an alpha particle has been given off. This remianing part recoils with a velocity inversely proportional to its ma .