Since the properties of an element are mostly determined by its electron configuration, the properties of the elements likewise show recurring patterns or periodic behaviour, some examples of which are shown in the diagrams below for atomic radii, ionization energy and electron affinity. It is this periodicity of properties, manifestations of which were noticed well before the underlying theory was developed , that led to the establishment of the periodic law (the properties of the elements recur at varying intervals) and the formulation of the first periodic tables.  
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A. W. von Schlegel: Indische Bibliothek. Bd. 1 (1823). Inhaltsverzeichnis.
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