On Elemental Periodicity, by Mendeleev

As later Gilbert Newton Lewis advanced the idea of atomic structure and chemical bonding, Mendeleev (1889) made the necessary passage from atomic periodicity feature to molecular composition by employing the atomic physicochemical features, eventually originating in atomic weights and number (latter with quantum mechanical relevance)  

The observations made with the spectroscope, permitted to analyze the chemical constitution by the distant lights, at the beginning, applicable to the challenge in determining the nature of the atoms themselves, but the laboratory work immediately demonstrated that the spectral 

Quantum Nanochemistry Volume II Quantum Atoms and Periodicity 

The Merignac s research (on the niobium) and those of Roscoe (on vanadium) were special moments. Sweeping analogies between vanadium and phosphorus on the one hand, and between vanadium and chromium on the other hand, which have become so apparent in the investigations connected with that element, have induced the possibility of comparison of V = 51 with Cr = 52, Nb = 94 with Mo = 96, and Ta = 192 with W = 194 on the other hand, P = 31 can be compared with S = 32, As = 75 with Se = 79, and Sb = 120 with Te = 125. From these approximations there was only a step until the discovering of the periodicity law. 

The periodicity law was, therefore, a direct consequence of a package of generalities and of the establishment of the factors accumulated until the end of the decade 1860 1870. There was a combination of data in expressions more or less systematic. In this combination was the secret of the importance assessed to the periodicity law it gave a new position to the generalizations, offering an unexpected aid of the chemist researchers, which promise to be more productive in the future. 

---