ABSTRACT
The whole realm of phenomena at the atomic or subatomic level is the special province of quantum theory. When isolated from radiation and other atoms, most atoms remain stable indefinitely: they neither collapse nor explode. When atoms are excited electrically or by collisions or otherwise, they emit radiation of discrete wavelengths characteristic of the kind of atoms excited. The simplest model of the atom is a hard, tiny, electrically neutral sphere—just the smallest possible fragment of the bulk material that possesses the identity of a given chemical element. Neils Bohr's theory was built primarily on the photon concept of radiation, which was understood as an hypothesis but was not yet unambiguously verified when Bohr did his early work. Bohr's first solution of the hydrogen-atom problem in his historic 1913 paper depended on a rather arbitrary combination of classical and quantum ideas.
