Mendeleev periodic table was that he predicted new undiscovered elements.
Step-by-step explanation:
The first periodic table was proposed by Mendeleev and he placed the elements based on the mass and recurring properties of the elements, and he also left blank spaces for unknown elements, as he predicted few more elements will be discovered. He grouped similar elements together.
He predicted the discovery of Eka-boron, Eka-aluminium, Eka-manganese, Eka-silicon which were found to be Scandium, Gallium, Technetium and Germanium respectively.