The periodic table

Mendeleev and the periodicity of the elements

Dmitri Mendeleev He was a university professor in Russia and made an important discovery in the history of science while writing a chemistry book.

He recorded the properties of each of the known chemical elements (at the time there were 63; today there are more than 100) on paper cards, each card for one element.

Manipulating the cards in an attempt to chain ideas before writing a certain part of the work, Mendeleev realized something extraordinary.

At the time, there was scientific evidence that the atoms of each element have different masses. Mendeleev arranged the chips according to the increasing order of the mass of atoms of each element. He noted that in this sequence appeared at regular intervals elements with similar properties. There was a periodicity, a repetition, in the properties of the elements.

Among the many examples of elements with similar properties we can mention:

  • sodium (Na), potassium (K) and rubidium (Rb) - react explosively with water; combine with chlorine and oxygen to form, respectively, compounds of formulas EC1 and E2O (E represents the element);
  • Magnesium (Mg), Calcium (Ca) and Strontium (Sr) - react with water, but not so violently; combine with chlorine and oxygen to form, respectively, compounds of formulas ECl2 and EO.

Mendeleev's Periodic Table

Based on his discovery, Mendeleev was able to organize the elements into a table, in which those with similar properties appeared in the same column.

Further elaborating on his discovery, he realized that some elements seemed to be missing to complete it. Mendeleev then decided to leave some places blank in this table, thinking that someday someone would discover new chemical elements that could be fitted into these places based on their properties. He even predicted some of the properties these elements would have.

Mendeleev also realized that in some places in the table it would be better to do small inversions in the order of the elements. In 1871 he published an improved version of his work.

Before Mendeleev some scientists had realized that some groups of elements had similar properties, but the merit of the Russian chemist was to organize the elements based on their properties, make necessary minor adjustments, and leave places for elements that could exist but which had not yet been discovered.

The elements predicted by Mendeleev to exist actually exist in nature and were discovered a few years later. And the properties of these elements are equal to or very close to those predicted by it.