What did the periodic table scientists discover

Well, when it was first created, the person who created it (Dmitri Mendeleev?) sorted it in a way that he noticed there were some missing elements. Through the organization by atomic number and other information, he hypothesized the existence of undiscovered elements and he was eventually found to be right. Other than that, it helps with finding out how chemical bonds work and with which elements they work best with, as well as showing correlations with electronegativity, groups, etc.

Distance and velocity graph