Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: An early explanation of the periodic table: 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.

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The categories of this paper are: math-ph
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The categories of this paper are: patt-sol
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The categories of this paper are: chao-dyn
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The categories of this paper are: gr-qc
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The categories of this paper are: supr-con
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The categories of this paper are: astro-ph
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The categories of this paper are: cond-mat
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The categories of this paper are: q-alg
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The categories of this paper are: math
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The categories of this paper are: alg-geom
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The categories of this paper are: solv-int
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The categories of this paper are: hep-th
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The categories of this paper are: ao-sci
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The categories of this paper are: stat
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The categories of this paper are: acc-phys
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The categories of this paper are: hep-lat