The Periodic System
Classification of Mendeleev
In 1869 the distinguished Russian chemist Mendeleev, apparently unaware of the work of Newlands, discovered an important relation between the atomic weights of the elements and their other properties, both physical and chemical. He developed the whole conception in so masterly a way and so thoroughly that his classification, with some modifications, persists to the present time. Almost simultaneously and quite independently the German chemist Lothar Meyer made similar proposals as to the relation between the properties of the elements and their atomic weights.
To accomplish regularity, Mendeleev had to admit that the atomic weight of some elements was not correct. To keep the idea of periodicity, the position of some elements did not correspond to the predicted according to the atomic weight obtained by the chemists by the Canizzaro method.
Mendeleev assumed that the position of some of the elements had to be changed. For instance, tellurium should be positioned before iodine in spite of having an atomic weight of 128 against the 127 of iodine. He also pointed out the exchange of elements like argon and potassium, cobalt and nickel or thorium and protactinium.
When Mendeleev proposed the periodic table, some of the elements were not yet discovered, so he left its position free. It is the case of the elements similar to aluminum and silicon but with atomic weights of 65 and 75. These elements were later discovered and named gallium and germanium, respectively.
Mendeleev predictions were all verified. It was later verified that yttrium, indium, dysprosium, cerium, erbium, lanthanum, thorium and uranium had atomic weight miscalculated. On the other hand, all the missing elements in the periodic table of Mendeleev were discovered.
In the modern periodic table of Mendeleev, usually called the Periodic Table, each element has an order number, named atomic number and representing the number of electrons of the neutral atom which is equal to the number of nuclear protons.