This process is experimental and the keywords may be updated as the learning algorithm improves.Īssmus, Alexi. These keywords were added by machine and not by the authors. 1 Even when accurate calculations of orbital energies have become available, the theoretical rigor of the aufbau principle-e.g., the assignment of quantum numbers to a particular electron and the explanation of the closing of electron shells-is still in question. Historians have pointed out that Bohr’s research program had distinctively a deductive and inductive character, drawing on both theoretical calculations and empirical knowledge. The quantum mechanical rationalization of the periodic table, however, does not conceal the semi-empirical origin of the aufbau principle. The electronic configuration (or electronic structure) of atoms then seemed to give a concrete quantum mechanical basis for the periodic table. Finally, in 1932, “orbits” were renamed as “orbitals” implying mathematically “one-electron wave functions,” which afforded a statistical interpretation of the electronic motion, rather than its exact trajectory.
A few years later, with the advent of quantum mechanics, the Pauli exclusion principle rationalized the pairing of electrons in the building-up process, and it turned out that four quantum numbers, not two, were needed to describe the state of electrons in an atom. Bohr denoted atomic orbits with two quantum numbers, principal and subordinate, and limited the number of electrons each orbit could accommodate, considering spectroscopic and chemical properties of elements. In practice, it meant the imaginary process in which electrons were placed, one by one, into energy levels prescribed by atomic orbits. Proposed by the Danish physicist Niels Bohr in the early 1920s, the principle was a tool for obtaining a picture of the atomic constitution, i.e., the arrangement of electrons on orbits around the nucleus. Today the aufbau (building-up) principle is best known as a theoretical ground for the electronic interpretation of the periodic table.
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