Lars Vegard and X-ray spectroscopy
The Norwegian physicist Lars Vegard may have been the first to propose electron configurations for all the chemical elements, from hydrogen to uranium, on the basis of quantum atomic theory. This he did in papers of 1918-1919 in which he argued that the principal quantum number corresponded to the shells in Bohr’s picture of atomic structure. Vegard’s theory of the periodic system, based on the methods of theoretical X-ray spectroscopy in particular, was a significant advance but exerted little influence on the further development. It presupposed atoms made up of planar electron rings, and with the abandonment of the ring atom about 1920 this and similar models lost their credibility.