Dmitri Mendeleev writes the definitive textbook of…

Dmitri Mendeleev writes the definitive textbook of his time: Principles of Chemistry (two volumes, 1868–1870).

In his attempts to classify the elements according to their chemical properties after becoming a teacher, he had noticed patterns that lead him to postulate his periodic table of the elements.

Mendeleev is unaware of the earlier work on periodic tables going on in the 1860s.

On March 6, 1869, Mendeleev makes a formal presentation to the Russian Chemical Society, entitled The Dependence between the Properties of the Atomic Weights of the Elements, which describes elements according to both atomic weight and valence.

This presentation states that the elements, if arranged according to their atomic weight, exhibit an apparent periodicity of properties.

Elements that are similar regarding 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 aluminum 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 seems to be wrong, as the "atomic mass" of tellurium (127.6) remains higher than that of iodine (126.9) as displayed on modern periodic tables, but this is due to the way atomic masses are calculated, based on a weighted average of all of an element's common isotopes, not just the one-to-one proton/neutron-ratio version of the element to which Mendeleev was referring.

Certain characteristic properties of elements can be foretold from their atomic weights.

Mendeleev publishes his periodic table of all known elements and predicts several new elements to complete the table.

Only a few months after, Lothar Meyer publishes a virtually identical table.

Some consider Meyer and Mendeleev the co-creators of the periodic table.

There were fifty-six known elements in 1863, with a new element being discovered at a rate of approximately one per year.

Other scientists had previously identified the periodicity of elements.

John Newlands had described a Law of Octaves, noting their periodicity according to relative atomic weight in 1864, publishing it in 1865.

His proposal had identified the potential for new elements such as germanium.

The concept had been criticized and his innovation will not be not recognized by the Society of Chemists until 1887.

Another person to propose a periodic table had been Lothar Meyer, who had published a paper in 1864 describing twenty-eight elements classified by their valence, but with no prediction of new elements.