March 6, 1869: Dmitri Mendeleev presents his periodic table to the Russian Chemical Society.

Dmitri Mendeleev was a Russian chemist and, from 1865 to 1890, a professor at the Saint Petersburg State University. Along with German chemist Julius Lothar Meyer, he formulated the system to classify and organize the approximately 56 known chemical elements on which the modern standard periodic table is based. Mendeleev’s system differed from previous attempts to organize the elements in that his principal organizing factor was atomic mass, which led him to logically group elements based on “an apparent periodicity of properties”. In the presentation entitled “The Dependence Between the Properties of the Atomic Weights of the Elements (March 6, 1869) in which he introduced the basic principles of his system, he noted:

Elements which are similar as regards their chemical properties have atomic weights which are either of nearly the same value (e.g., Pt, Ir, Os) or which increase regularly (e.g., K, Rb, Cs).

In addition, by noting gaps in his periodic table, he was able to predict the existence of (and leave spaces for) then unknown elements, among them gallium and germanium - which he respectively referred to as ekaaluminium and ekasilicon. Mendeleev’s accurate predictions of the existence and specific qualities of undiscovered elements based on gaps in his groups was one significant difference between his and Meyer’s table, which was otherwise similar and actually introduced earlier.

The element mendelevium (atomic number 101), discovered in 1955, was named for Mendeleev. 

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