In 1869, he presented the findings to the Russian Chemical Society. Not only did he leave space for elements not yet discovered, but he predicted the properties of five of these elements and their compounds.

Mendeleev arranged the elements according to both atomic weight and valence. When he woke up, he immediately wrote them down on a piece of paper - only in one place did a correction seem necessary, he later said. After intensely working on this 'puzzle' for three days, Mendeleev said that he had a dream in which all of the elements fell into place as required.

This is when he noticed certain types of elements regularly appearing. After writing the properties of the elements on cards, he began ordering them by increasing atomic weight, according to the Royal Society of Chemistry.

As he was working on it, he came upon a significant discovery that would contribute greatly to the development of the current periodic table. Mendeleev ultimately wrote the definitive chemistry textbook of his time, titled 'Principles of Chemistry' (two volumes, 1868–1870), according to Khan Academy.

Period 4 elements