In 1869, the Russian-born scientist Dimitri Mendeleev unveiled his famous periodic table in Germany. This table was very well prepared and included all the chemical elements, which were known by then, ordering them in a table, which met the following guidelines: the elements had to be classified from left to right, always guided by horizontal lines and that those elements with similar characteristics be placed in vertical columns.
By then, 63 elements of the 118 that currently exist were recognized.
Mendeleev argued that the characteristics of the elements should respond to a periodic law that was still unknown. He was sure of his theory and that prompted him to make predictions that, perhaps for the time, were a bit risky, but which over the years proved to be true.
Some of these predictions are:
- I doubt the value of the atomic mass of certain elements, such as uranium, giving it another value, which for him was the most suitable.
- He modified the ordering of the atomic masses in certain elements, so that they could be grouped much better, with other elements with similar characteristics such as cobalt-nickel.
- He left on the table, spaces that in the future could be occupied by elements that were still unknown. Such as scandium, gallium, etc.
It should be noted that this last prediction was very useful since it predicted the existence and exact location of the elements not yet found, giving them a transitory name, such as gallium, which he called eka-aluminum, because it was located below the aluminum in the classification.
The first order made by Mendeleev was not entirely accepted, however with the passage of time and with the corresponding modifications, in 1872 he was able to publish his new periodic table, which was composed of eight columns distributed in two groups, which later with the years, they were called family A and family B.
This new periodic table, presented the universal formulas of oxides and hydrides, in each one of the groups and even the valences of the elements.
Mendeleev's periodic table has been improved and expanded over time, as a result of the discovery of new elements, coupled with the evolution of theoretical models that have emerged to explain chemical behavior.
