(from German Aufbauprinzip, “building-up principle”), rationalization of the distribution of electrons among energy levels in the ground (most stable) states of atoms. The principle, formulated by the Danish physicist Niels Bohr about 1920, is an application of the laws of quantum mechanics to the properties of electrons subject to the electric field created by the positive charge on the nucleus of an atom and the negative charge on other electrons that are bound to the nucleus. The building-up denoted by the name of the principle is a hypothetical process in which the electrons are regarded as entering, one by one, this electric field and assuming their most stable conditions with respect to it. See also electronic configuration.