- Born 27 January 1834 - Tobolsk, Russia
- Deceased 20 January 1907 - Sankt-Peterburg, Russia , age at death: 72 years old
On March 6, 1869, Mendeleev made a formal presentation to the Russian Chemical Society, entitled The Dependence between the Properties of the Atomic Weights of the Elements, which described elements according to both weight and valence. This presentation stated that
The elements, if arranged according to their atomic mass, exhibit an apparent periodicity of properties. Elements which are similar as regards to 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). The arrangement of the elements in groups of elements in the order of their atomic weights corresponds to their so-called valencies, as well as, to some extent, to their distinctive chemical properties; as is apparent among other series in that of Li, Be, B, C, N, O, and F. The elements which are the most widely diffused have small atomic weights. The magnitude of the atomic weight determines the character of the element, just as the magnitude of the molecule determines the character of a compound body. We must expect the discovery of many yet unknown elements–for example, two elements, analogous to aluminium and silicon, whose atomic weights would be between 65 and 75. The atomic weight of an element may sometimes be amended by a knowledge of those of its contiguous elements. Thus the atomic weight of tellurium must lie between 123 and 126, and cannot be 128. Here Mendeleev was wrong as the atomic mass of tellurium (127.6) remains higher than that of iodine (126.9). Certain characteristic properties of elements can be foretold from their atomic weights.