How did admitting mendeleev deal with elements that had not yet been discovered

Mendeleev organized his table by atomic mass, rather than atomic radius. He was able to extrapolate where certain undiscovered elements should be on the table based on that.

For example, he said that since Calcium has mass of 40g and Titanium has a mass of 48 g, there must be some element between them with atomic mass of 44 g. That was later discovers to be scandium.

Hope this helps