Dmitri Mendeleev presented his periodic table in 1869. He arranged the elements by their atomic weight, showing how elements with similar chemical properties had either similar atomic weights or they were incremented at regular intervals.
He also left spaces for elements that were not known at the time but that had to exist to maintain the regularities of the table. For example, he predicted the existence of Gallium and Germanium: two elements similar to Aluminium and Silicon, but with higher atomic weights. The predicted elements were observed in 1875 and 1886.