Question

What supported the validity of mendeleev's periodic table

Answer 1

Mendeleev left gaps in his table to accommodate elements that hadn't been discovered yet. He reasoned that they might not be known (yet), but if they were to exist, they would have properties that put them in those spaces. Gallium and Germanium are 2 elements that he was able to pre-place on the table because of the elements that he placed around them. He predicted the existence of "eka-aluminum" based on the periodic properties that the elements shared, and he wasn't far off the mark when the element that we call gallium was eventually discovered.