Source: Dmitri Mendeleev,
American Institute of Physics

In 1869, Dmitri Mendeleev, a Russian scientist, organized the first periodic table of elements. At that time, there were about 60 known elements. Mendeleev studied the physical properties of the elements, such as melting point, density, color, and atomic mass. He also studied chemical properties, such as the type of compounds formed by certain types of elements.

Mendeleev noticed patterns and trends among the elements. He ordered elements according to increasing atomic masses. He left spaces in his table where he hypothesized unknown elements belonged. He even predicted the properties of the unknown elements based on where the blanks fell in the table. His system of organization was proven to be correct when other scientists later discovered the elements that fit into these spaces and had the properties that he had predicted. The image below shows Mendeleev's first periodic table as it was published in 1869.

Source: Dmitri Mendeleev 1869 Periodic Table, American Institute of Physics